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Planet Nine is a
hypothetical planet Various unknown astronomical objects have been hypothesized throughout recorded history. For example, in the 5th century BCE, the philosopher Philolaus defined a hypothetical astronomical object which he called the "Central Fire", around whic ...
in the
outer region of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. Its gravitational effects could explain the peculiar clustering of
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
s for a group of
extreme trans-Neptunian object An extreme trans-Neptunian object (ETNO) is a trans-Neptunian object orbiting the Sun well beyond Neptune (30  AU) in the outermost region of the Solar System. An ETNO has a large semi-major axis of at least 150–250 AU. Its orbit is m ...
s (ETNOs), bodies beyond
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
that orbit the Sun at distances averaging more than 250 times that of the Earth. These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
objects. Nonetheless, some astronomers question this conclusion and instead assert that the clustering of the ETNOs orbits is due to observational biases, resulting from the difficulty of discovering and tracking these objects during much of the year. Based on earlier considerations, this hypothetical
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
-sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times as far from the Sun as the Earth. The orbit estimation was refined in 2021, resulting in a somewhat smaller semimajor axis of 380 AU. This was shortly thereafter updated to 460 AU.
Konstantin Batygin Konstantin Batygin (russian: Константи́н Юрьевич Батыгин) is an American astronomer and Professor of Planetary Sciences at Caltech. Early life Konstantin Batygin was born in Moscow, Soviet Union. His father, Yuri Konsta ...
and
Michael E. Brown Michael E. Brown (born June 5, 1965) is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), including ...
suggested that Planet Nine may be the
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
of a
giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...
that was ejected from its original orbit by Jupiter during the
genesis Genesis may refer to: Bible * Book of Genesis, the first book of the biblical scriptures of both Judaism and Christianity, describing the creation of the Earth and of mankind * Genesis creation narrative, the first several chapters of the Book of ...
of the Solar System. Others proposed that the planet was captured from another star, was once a
rogue planet A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star. Although sky surveys such as
Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
(WISE) and
Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical c ...
did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine was dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using
NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
and the 8-meter
Subaru Telescope is the telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades. It had the largest monolithic primary mirror in the wo ...
. Unless Planet Nine is observed, its existence remains purely conjectural. Several alternative hypotheses have been proposed to explain the observed clustering of
trans-Neptunian objects A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
(TNOs).


History

Following the
discovery of Neptune The planet Neptune was mathematically predicted before it was directly observed. With a prediction by Urbain Le Verrier, telescopic observations confirming the existence of a major planet were made on the night of September 23–24, 1846, at the ...
in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
and
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
. After extensive calculations
Percival Lowell Percival Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, and furthered theories of a ninth planet within the Solar System. ...
predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object ''
Planet X Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...
'', a name previously used by Gabriel Dallet.
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
continued Lowell's search and in 1930 discovered Pluto, but it was soon determined to be too small to qualify as Lowell's Planet X. After
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
's flyby of Neptune in 1989, the difference between Uranus' predicted and observed orbit was determined to have been due to the use of a previously inaccurate mass of Neptune. Attempts to detect
planets beyond Neptune Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...
by indirect means such as orbital perturbation date back to before the discovery of Pluto. Among the first was George Forbes who postulated the existence of two trans-Neptunian planets in 1880. One would have an average distance from the Sun, or
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
, of 100
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
s (AU), 100 times that of the Earth. The second would have a semi-major axis of 300 AU. His work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the clustering of
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
distances of periodic
comets A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
near 100 and 300 AU. This is similar to how the aphelion distances of Jupiter-family comets cluster near its orbit. The discovery of Sedna's peculiar orbit in 2004 led to speculation that it had encountered a massive body other than one of the known planets. Sedna's orbit is
detached A stand-alone house (also called a single-detached dwelling, detached residence or detached house) is a free-standing residential building. It is sometimes referred to as a single-family home, as opposed to a multi-family residential dwelli ...
, with a
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
distance of 76 AU that is too large to be due to gravitational interactions with Neptune. Several authors proposed that Sedna entered this orbit after encountering a massive body such as an unknown planet on a distant orbit, a member of the
open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
that formed with the Sun, or another star that later passed near the Solar System. The announcement in March 2014 of the discovery of a second
sednoid A sednoid is a trans-Neptunian object with a perihelion well beyond the Kuiper cliff at . Only four objects are known from this population: 90377 Sedna, , 541132 Leleākūhonua (), and , but it is suspected that there are many more. All four ha ...
with a perihelion distance of 80 AU, , in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System. At a conference in 2012, Rodney Gomes proposed that an undetected planet was responsible for the orbits of some ETNOs with detached orbits and the large semi-major axis
Centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as being ...
,
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as fol ...
that cross the orbits of the giant planets. The proposed Neptune-massed planet would be in a distant (1500 AU), eccentric (
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
0.4), and inclined (
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
°) orbit. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna. An article by Gomes, Soares, and Brasser was published in 2015, detailing their arguments. In 2014, astronomers
Chad Trujillo Chadwick A. Trujillo (born November 22, 1973) is an American astronomer, discoverer of minor planets and the co-discoverer of Eris, the most massive dwarf planet known in the Solar System. Trujillo works with computer software and has examined ...
and
Scott S. Sheppard Scott Sander Sheppard (born 1977) is an American astronomer and a discoverer of numerous moons, comets and minor planets in the outer Solar System. He is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Scie ...
noted the similarities in the orbits of Sedna and and several other ETNOs. They proposed that an unknown planet in a circular orbit between 200 and 300 AU was perturbing their orbits. Later that year, Raúl and Carlos de la Fuente Marcos argued that two massive planets in
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
were necessary to produce the similarities of so many orbits, 13 known at the time. Using a larger sample of 39 ETNOs, they estimated that the nearer planet had a semi-major axis in the range 300–400 AU, a relatively low eccentricity, and an inclination of nearly 14 degrees.


Batygin and Brown hypothesis

In early 2016,
California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
's Batygin and Brown described how the similar orbits of six ETNOs could be explained by Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain ETNOs with orbits
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
to the
inner planets The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
and others with extreme inclinations, and had been offered as an explanation of the
tilt Tilt may refer to: Music * Tilt (American band), a punk rock group, formed in 1992 * Tilt (British band), an electronic music group, formed in 1993 * Tilt (Polish band), a rock band, formed in 1979 Albums * ''Tilt'' (Cozy Powell album), 1981 * ...
of the Sun's
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...
.


Orbit

Planet Nine was initially hypothesized to follow an
elliptical orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it ...
around the Sun with an eccentricity of , and its
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
was estimated to be , roughly 13 to 26 times the distance from Neptune to the Sun. It would take the planet between 10,000 and 20,000 years to make one full orbit around the Sun, and its inclination to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
, the plane of the Earth's orbit, was projected to be . The aphelion, or farthest point from the Sun, would be in the general direction of the
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...
of
Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (astrology), the astrological sign * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological characters named Taurus * '' Bos tauru ...
, whereas the perihelion, the nearest point to the Sun, would be in the general direction of the southerly areas of
Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...
(Caput),
Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constella ...
, and
Libra Libra generally refers to: * Libra (constellation), a constellation * Libra (astrology), an astrological sign based on the star constellation Libra may also refer to: Arts and entertainment * ''Libra'' (novel), a 1988 novel by Don DeLillo Musi ...
. Brown thinks that if Planet Nine exists, a probe could reach it in as little as 20 years by using a powered slingshot
trajectory A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete traj ...
around the Sun.


Mass and radius

The planet is estimated to have 5 to 10 times the mass of Earth and a radius of 2 to 4 times Earth's. Brown thinks that if Planet Nine exists, its mass is sufficient to clear its orbit of large bodies in 4.5 billion years, the age of the Solar System, and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Astronomer
Jean-Luc Margot Jean-Luc Margot (born 1969) is a Belgian-born astronomer and a UCLA professor who specializes in planetary sciences. Career Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries include ( ...
has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.


Origin

Several possible origins for Planet Nine have been examined including its ejection from the neighborhood of the known giant planets, capture from another star, and ''
in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
'' formation. In their initial article, Batygin and Brown proposed that Planet Nine formed closer to the Sun and was ejected into a distant eccentric orbit following a close encounter with Jupiter or
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
during the nebular epoch. The gravity of a nearby star, or drag from the gaseous remnants of the
Solar nebula The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
, then reduced the eccentricity of its orbit. This raised its perihelion, leaving it in a very wide but stable orbit beyond the influence of the other planets. The odds of this occurring has been estimated at a few percent. Had it not been flung into the Solar System's farthest reaches, Planet Nine could have accreted more mass from the proto-planetary disk and developed into the core of a
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
. Instead, its growth was halted early, leaving it with a lower mass than Uranus or Neptune.
Dynamical friction In astrophysics, dynamical friction or Chandrasekhar friction, sometimes called ''gravitational drag'', is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first d ...
from a massive belt of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s could also enable Planet Nine's capture in a stable orbit. Recent models propose that a 60–130 Earth mass disk of planetesimals could have formed as the gas was cleared from the outer parts of the proto-planetary disk. As Planet Nine passed through this disk its gravity would alter the paths of the individual objects in a way that reduced Planet Nine's velocity relative to it. This would lower the eccentricity of Planet Nine and stabilize its orbit. If this disk had a distant inner edge, 100–200 AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine, with the observed clustering more likely if the inner edge is at 200 AU. Unlike the gas nebula, the planetesimal disk is likely to have been long lived, potentially allowing a later capture. An encounter with another star could also alter the orbit of a distant planet, shifting it from a circular to an eccentric orbit. The ''in situ'' formation of a planet at this distance would require a very massive and extensive disk, or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years. If a planet formed at such a great distance while the Sun was in its original cluster, the probability of it remaining bound to the Sun in a highly eccentric orbit is roughly 10%. An extended disk would have been subject to gravitational disruption by passing stars and by mass loss due to photoevaporation while the Sun remained in the open cluster where it formed, however. Planet Nine could have been captured from outside the Solar System during a close encounter between the Sun and another star. If a planet was in a distant orbit around this star,
three-body Three body may refer to: ;Science *Three-body problem, a problem in physics and classical mechanics *Euler's three-body problem, a problem in physics and astronomy *Three-body force, a force appearing in a three-body system ;Science fiction * ''Th ...
interactions during the encounter could alter the planet's path, leaving it in a stable orbit around the Sun. A planet originating in a system without Jupiter-massed planets could remain in a distant eccentric orbit for a longer time, increasing its chances of capture. The wider range of possible orbits would reduce the odds of its capture in a relatively low inclination orbit to 1–2%. Amir Siraj and
Avi Loeb Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He h ...
found that the odds of the Sun capturing Planet Nine increases by a factor of 20 if the Sun once had a distant, equal-mass binary companion. This process could also occur with rogue planets, but the likelihood of their capture is much smaller, with only 0.05–0.10% being captured in orbits similar to that proposed for Planet Nine.


Evidence

The gravitational influence of Planet Nine would explain four peculiarities of the Solar System: * the clustering of the orbits of ETNOs; * the high perihelia of objects like 90377 Sedna that are
detached A stand-alone house (also called a single-detached dwelling, detached residence or detached house) is a free-standing residential building. It is sometimes referred to as a single-family home, as opposed to a multi-family residential dwelli ...
from Neptune's influence; * the high inclinations of ETNOs with orbits roughly perpendicular to the orbits of the eight known planets; * high-inclination
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
s (TNOs) with semi-major axis less than 100 AU. Planet Nine was initially proposed to explain the clustering of orbits, via a mechanism that would also explain the high perihelia of objects like Sedna. The evolution of some of these objects into perpendicular orbits was unexpected, but found to match objects previously observed. The orbits of some objects with perpendicular orbits were later found to evolve toward smaller semi-major axes when the other planets were included in simulations. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four. The gravity of Planet Nine would also increase the inclinations of other objects that cross its orbit, however, which could leave the scattered disk objects, bodies orbiting beyond Neptune with semi-major axes greater than 50 AU, and short-period comets with a broader inclination distribution than is observed. Previously Planet Nine was hypothesized to be responsible for the 6 degree tilt of the Sun's axis relative to the orbits of the planets, but recent updates to its predicted orbit and mass limit this shift to ~1 degree.


Observations: Orbital clustering of high perihelion objects

The clustering of the orbits of TNOs with large semi-major axes was first described by Trujillo and Sheppard, who noted similarities between the orbits of Sedna and . Without the presence of Planet Nine, these orbits should be distributed randomly, without preference for any direction. Upon further analysis, Trujillo and Sheppard observed that the arguments of perihelion of 12 TNOs with perihelia greater than and semi-major axes greater than were clustered near zero degrees, meaning that they rise through the ecliptic when they are closest to the Sun. Trujillo and Sheppard proposed that this alignment was caused by a massive unknown planet beyond Neptune via the
Kozai mechanism In celestial mechanics, the Kozai mechanism is a dynamical phenomenon affecting the orbit of a binary system perturbed by a distant third body under certain conditions. It is also known as the von Zeipel-Kozai-Lidov, Lidov–Kozai mechanism, Kozai ...
. For objects with similar semi-major axes the Kozai mechanism would confine their arguments of perihelion to near 0 or 180 degrees. This confinement allows objects with eccentric and inclined orbits to avoid close approaches to the planet because they would cross the plane of the planet's orbit at their closest and farthest points from the Sun, and cross the planet's orbit when they are well above or below its orbit. Trujillo and Sheppard's hypothesis about how the objects would be aligned by the Kozai mechanism has been supplanted by further analysis and evidence. Batygin and Brown, looking to refute the mechanism proposed by Trujillo and Sheppard, also examined the orbits of the TNOs with large semi-major axes. After eliminating the objects in Trujillo and Sheppard's original analysis that were unstable due to close approaches to Neptune or were affected by Neptune's
mean-motion resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s, Batygin and Brown determined that the arguments of perihelion for the remaining six objects (Sedna, , , , , and ) were clustered around . This finding did not agree with how the Kozai mechanism would tend to align orbits with arguments of perihelion at 0° or 180°. Batygin and Brown also found that the orbits of the six ETNOs with semi-major axis greater than 250 AU and perihelia beyond 30 AU (Sedna, , , , , and ) were aligned in space with their perihelia in roughly the same direction, resulting in a clustering of their longitudes of perihelion, the location where they make their closest approaches to the Sun. The orbits of the six objects were also tilted with respect to that of the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
and approximately
coplanar In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. Howe ...
, producing a clustering of their longitudes of ascending nodes, the directions where they each rise through the ecliptic. They determined that there was only a 0.007% likelihood that this combination of alignments was due to chance. These six objects had been discovered by six different surveys on six telescopes. That made it less likely that the clumping might be due to an observation bias such as pointing a telescope at a particular part of the sky. The observed clustering should be smeared out in a few hundred million years due to the locations of the perihelia and the ascending nodes changing, or precessing, at differing rates due to their varied semi-major axes and eccentricities. This indicates that the clustering could not be due to an event in the distant past, for example a passing star, and is most likely maintained by the gravitational field of an object orbiting the Sun. Two of the six objects ( and ) also have very similar orbits and spectra. This has led to the suggestion that they were a
binary object Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...
disrupted near aphelion during an encounter with a distant object. The disruption of a binary would require a relatively close encounter, which becomes less likely at large distances from the Sun. In a later article Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of the TNOs with semi-major axes greater than 150 AU. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280 and 360°, and those with longitude of perihelion between ° and ° have arguments of perihelion between ° and °. The statistical significance of this correlation was 99.99%. They suggested that the correlation is due to the orbits of these objects avoiding close approaches to a massive planet by passing above or below its orbit. A 2017 article by Carlos and Raúl de la Fuente Marcos noted that distribution of the distances to the ascending nodes of the ETNOs, and those of centaurs and comets with large semi-major axes, may be
bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...
. They suggest it is due to the ETNOs avoiding close approaches to a planet with a semi-major axis of 300–400 AU. With more data (40 objects), the distribution of mutual nodal distances of the ETNOs shows a statistically significant asymmetry between the shortest mutual ascending and descending nodal distances that may not be due to observational bias but likely the result of external perturbations.


Simulations: Observed clustering reproduced

The clustering of the orbits of ETNOs and raising of their perihelia is reproduced in simulations that include Planet Nine. In simulations conducted by Batygin and Brown, swarms of scattered disk objects with semi-major axes up to 550 AU that began with random orientations were sculpted into roughly
collinear In geometry, collinearity of a set of points is the property of their lying on a single line. A set of points with this property is said to be collinear (sometimes spelled as colinear). In greater generality, the term has been used for aligned ...
and coplanar groups of spatially confined orbits by a massive distant planet in a highly eccentric orbit. This left most of the objects' perihelia pointed in similar directions and the objects' orbits with similar tilts. Many of these objects entered high-perihelion orbits like Sedna and, unexpectedly, some entered perpendicular orbits that Batygin and Brown later noticed had been previously observed. In their original analysis Batygin and Brown found that the distribution of the orbits of the first six ETNOs was best reproduced in simulations using a 10 Earth mass planet in the following orbit: *
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
''a'' ≈ (
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
7001.5=18,520 years) *
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
''e'' ≈ 0.6, (
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
≈ ,
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
≈ ) *
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
''i'' ≈ 30° to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
*
longitude of the ascending node The longitude of the ascending node (☊ or Ω) is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a specified reference direction, called the ''origin of longitude'', to the direction of the asce ...
''Ω'' ≈ . *
argument of perihelion The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ''ω'', is one of the orbital elements of an orbiting body. Parametrically, ''ω'' is the angle from the body's ascending node to its periapsi ...
''ω'' ≈ 140° and
longitude of perihelion In celestial mechanics, the longitude of the periapsis, also called longitude of the pericenter, of an orbiting body is the longitude (measured from the point of the vernal equinox) at which the periapsis (closest approach to the central body) woul ...
''ϖ'' = These parameters for Planet Nine produce different simulated effects on TNOs. Objects with semi-major axis greater than 250 AU are strongly anti-aligned with Planet Nine, with perihelia opposite Planet Nine's perihelion. Objects with semi-major axes between 150 AU and 250 AU are weakly aligned with Planet Nine, with perihelia in the same direction as Planet Nine's perihelion. Little effect is found on objects with semi-major axes less than 150 AU. The simulations also revealed that objects with semi-major axis greater than could have stable, aligned orbits if they had lower eccentricities. These objects have yet to be observed. Other possible orbits for Planet Nine were also examined, with semi-major axes between and , eccentricities up to 0.8, and a wide range of inclinations. These orbits yield varied results. Batygin and Brown found that orbits of the ETNOs were more likely to have similar tilts if Planet Nine had a higher inclination, but anti-alignment also decreased. Simulations by Becker et al. showed that their orbits were more stable if Planet Nine had a smaller eccentricity, but that anti-alignment was more likely at higher eccentricities. Lawler et al. found that the population captured in orbital resonances with Planet Nine was smaller if it had a circular orbit, and that fewer objects reached high inclination orbits. Investigations by Cáceres et al. showed that the orbits of the ETNOs were better aligned if Planet Nine had a lower perihelion orbit, but its perihelion would need to be higher than 90 AU. Later investigations by Batygin et al. found that higher eccentricity orbits reduced the average tilts of the ETNOs orbits. While there are many possible combinations of orbital parameters and masses for Planet Nine, none of the alternative simulations were better at predicting the observed alignment the original ETNOs. The discovery of additional distant Solar System objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet. These may also provide further support for, or refutation of, the Planet Nine hypothesis. Simulations that included the migration of giant planets resulted in a weaker alignment of the ETNOs orbits. The direction of alignment also switched, from more aligned to anti-aligned with increasing semi-major axis, and from anti-aligned to aligned with increasing perihelion distance. The latter would result in the sednoids' orbits being oriented opposite most of the other ETNOs.


Dynamics: How Planet Nine modifies the orbits of ETNOs

Planet Nine modifies the orbits of ETNOs via a combination of effects. On very long timescales Planet Nine exerts a
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
on the orbits of the ETNOs that varies with the alignment of their orbits with Planet Nine's. The resulting exchanges of
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
cause the perihelia to rise, placing them in Sedna-like orbits, and later fall, returning them to their original orbits after several hundred million years. The motion of their directions of perihelion also reverses when their eccentricities are small, keeping the objects anti-aligned, see blue curves on diagram, or aligned, red curves. On shorter timescales mean-motion resonances with Planet Nine provides phase protection, which stabilizes their orbits by slightly altering the objects' semi-major axes, keeping their orbits synchronized with Planet Nine's and preventing close approaches. The gravity of Neptune and the other giant planets, and the inclination of Planet Nine's orbit, weaken this protection. This results in a
chaotic Chaotic was originally a Danish trading card game. It expanded to an online game in America which then became a television program based on the game. The program was able to be seen on 4Kids TV (Fox affiliates, nationwide), Jetix, The CW4Kid ...
variation of semi-major axes as objects hop between resonances, including high-order resonances such as 27:17, on million-year timescales. The mean-motion resonances may not be necessary for the survival of ETNOs if they and Planet Nine are both on inclined orbits. The orbital poles of the objects precess around, or circle, the pole of the Solar System's
Laplace plane The Laplace plane or Laplacian plane of a planetary satellite, named after its discoverer Pierre-Simon Laplace (1749–1827), is a mean or reference plane about whose axis the instantaneous orbital plane of that satellite precesses. Laplace's ...
. At large semi-major axes the Laplace plane is warped toward the plane of Planet Nine's orbit. This causes orbital poles of the ETNOs on average to be tilted toward one side and their longitudes of ascending nodes to be clustered.


Objects in perpendicular orbits with large semi-major axis

Planet Nine can deliver ETNOs into orbits roughly perpendicular to the ecliptic. Several objects with high inclinations, greater than 50°, and large semi-major axes, above 250 AU, have been observed. These orbits are produced when some low inclination ETNOs enter a
secular resonance A secular resonance is a type of orbital resonance between two bodies with synchronized precessional frequencies. In celestial mechanics, secular refers to the long-term motion of a system, and resonance is periods or frequencies being a simple nu ...
with Planet Nine upon reaching low eccentricity orbits. The resonance causes their eccentricities and inclinations to increase, delivering the eTNOs into perpendicular orbits with low perihelia where they are more readily observed. The ETNOs then evolve into retrograde orbits with lower eccentricities, after which they pass through a second phase of high eccentricity perpendicular orbits, before returning to low eccentricity and inclination orbits. The secular resonance with Planet Nine involves a linear combination of the orbit's arguments and longitudes of perihelion: Δϖ – 2ω. Unlike the Kozai mechanism this resonance causes objects to reach their maximum eccentricities when in nearly perpendicular orbits. In simulations conducted by Batygin and Morbidelli this evolution was relatively common, with 38% of stable objects undergoing it at least once. The arguments of perihelion of these objects are clustered near or opposite Planet Nine's and their longitudes of ascending node are clustered around 90° in either direction from Planet Nine's when they reach low perihelia. This is in rough agreement with observations with the differences attributed to distant encounters with the known giant planets.


Orbits of high-inclination objects

A population of high-inclination TNOs with semi-major axes less than 100 AU may be generated by the combined effects of Planet Nine and the other giant planets. The ETNOs that enter perpendicular orbits have perihelia low enough for their orbits to intersect those of Neptune or the other giant planets. An encounter with one of these planets can lower an ETNO's semi-major axis to below 100 AU, where the object's orbits is no longer controlled by Planet Nine, leaving it in an orbit like . The predicted orbital distribution of the longest lived of these objects is nonuniform. Most would have orbits with perihelia ranging from 5 AU to 35 AU and inclinations below 110°; beyond a gap with few objects are would be others with inclinations near 150° and perihelia near 10 AU. Previously it was proposed that these objects originated in the
Oort Cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2 ...
, a theoretical cloud of icy planetesimals surrounding the Sun at distances of 2,000 to 200,000 AU. In simulations without Planet Nine an insufficient number are produced from the Oort cloud relative to observations, however. A few of the high-inclination TNOs may become retrograde Jupiter Trojans.


Oort cloud and comets

Planet Nine would alter the source regions and the inclination distribution of comets. In simulations of the migration of the giant planets described by the
Nice model The Nice () model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant p ...
fewer objects are captured in the
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2 ...
when Planet Nine is included. Other objects would be captured in a cloud of objects dynamically controlled by Planet Nine. This Planet Nine cloud, made up of the ETNOs and the perpendicular objects, would extend from semi-major axes of 200 AU to 3000 AU and contain roughly 0.3–0.4 Earth masses. When the perihelia of objects in the Planet Nine cloud drop low enough for them to encounter the other planets some would be scattered into orbits that enter the inner Solar System where they could be observed as comets. If Planet Nine exists these would make up roughly one third of the
Halley-type comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
s. Interactions with Planet Nine would also increase the inclinations of the scattered disk objects that cross its orbit. This could result in more with moderate inclinations of 15–30 degrees than are observed. The inclinations of the Jupiter-family comets derived from that population would also have a broader inclination distribution than is observed. Recent estimates of a smaller mass and eccentricity for Planet Nine would reduce its effect on these inclinations.


2019 estimate

In February 2019, the total of ETNOs that fit the original hypothesis of having semi-major axis of over 250 AU had increased to 14 objects. The orbit parameters for Planet Nine favored by Batygin and Brown after an analysis using these objects were: *semi-major axis of 400–500 AU; *orbital eccentricity of 0.15–0.3; *orbital inclination around 20°; *mass of about 5 Earth masses.


2021 estimate

In August 2021, Batygin and Brown reanalyzed the data related to ETNO observations while accounting for observational biases, they found that observations were more likely in some directions than others. They stated that the orbital clustering observed "remains significant at a 99.6% confidence level." Combining observational biases with numerical simulations, they predicted the characteristics of Planet Nine: *semi-major axis of 380 AU (300–520 AU); *perihelion of 300 AU; *orbital inclination of 16°; *mass of 6.2 Earth masses.


Reception

Batygin was cautious in interpreting the results of the simulation developed for his and Brown's research article, saying, "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo." In 2016, Brown put the odds for the existence of Planet Nine at about 90%.
Greg Laughlin Gregory Haines Laughlin (born January 21, 1942) is a politician from Texas. He is a former member of the United States House of Representatives. Early life and education Laughlin was born in Bay City, Texas, and was raised in West Columbia, T ...
, one of the few researchers who knew in advance about this article, gives an estimate of 68.3%. Other skeptical scientists demand more data in terms of additional KBOs to be analyzed or final evidence through photographic confirmation. Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet. The Planet Nine hypothesis is supported by several astronomers and academics. In January 2016 Jim Green, director of NASA's
Science Mission Directorate The Science Mission Directorate (SMD) of the National Aeronautics and Space Administration (NASA) engages the United States’ science community, sponsors scientific research, and develops and deploys satellites and probes in collaboration with NA ...
, said, "the evidence is stronger now than it's been before". But Green also cautioned about the possibility of other explanations for the observed motion of distant ETNOs and, quoting
Carl Sagan Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is research on ext ...
, he said, "extraordinary claims require extraordinary evidence."
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...
Professor Tom Levenson concluded that, for now, Planet Nine seems the only satisfactory explanation for everything now known about the outer regions of the Solar System. Astronomer Alessandro Morbidelli, who reviewed the research article for ''
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
'', concurred, saying, "I don't see any alternative explanation to that offered by Batygin and Brown." Astronomer Renu Malhotra remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of ETNOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she said. "To me, it's the most intriguing evidence for Planet Nine I've run across so far." Other experts have varying degrees of skepticism. American astrophysicist
Ethan Siegel Ethan R. Siegel is an American theoretical astrophysicist and science writer, who studies the Big Bang theory. In the past he has been a professor at Lewis & Clark College and a blogger at ''Starts With a Bang'', on ScienceBlogs and also on For ...
, who previously speculated that planets may have been ejected from the Solar System during an early dynamical instability, is skeptical of the existence of an undiscovered planet in the Solar System. In a 2018 article discussing a survey that did not find evidence of clustering of the ETNOs' orbits he suggests the previously observed clustering could have been the result of observational bias and claims most scientists think Planet Nine does not exist. Planetary scientist
Hal Levison Harold F. (Hal) Levison (born 1959) is an American planetary scientist specializing in planetary dynamics. He currently works at the Southwest Research Institute, Boulder, Colorado, and studies planetary orbits and their evolution through Solar S ...
thinks that the chance of an ejected object ending up in the inner Oort cloud is only about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit. Further skepticism about the Planet Nine hypothesis arose in 2020, based on results from the
Outer Solar System Origins Survey The Outer Solar System Origins Survey (OSSOS) is an astronomical survey and observing program aimed at discovering and tracking trans-Neptunian objects located in the outermost regions of the Solar System beyond the orbit of Neptune. OSSOS is d ...
and the
Dark Energy Survey The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, visible, and near-infrared to measure the expansion of the universe using Type Ia supernov ...
, with the OSSOS documenting over 800 trans-Neptunian objects and the DES discovering 316 new ones. Both surveys adjusted for observational bias and concluded that of the objects observed there was no evidence for clustering. The authors go further to explain that practically all objects' orbits can be explained by physical phenomena rather than a ninth planet as proposed by Brown and Batygin. An author of one of the studies, Samantha Lawler, said the hypothesis of Planet Nine proposed by Brown and Batygin "does not hold up to detailed observations" pointing out the much larger sample size of 800 objects compared to the much smaller 14 and that conclusive studies based on said objects were "premature". She went further to explain the phenomenon of these extreme orbits could be due to gravitational occultation from Neptune when it migrated outwards earlier in the Solar System's history.


Alternative hypotheses


Temporary or coincidental clustering

The results of the Outer Solar System Survey (OSSOS) suggest that the observed clustering is the result of a combination of observational bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight objects with semi-major axis > 150 AU with orbits oriented in a wide range of directions. After accounting for the observational biases of the survey, no evidence for the arguments of perihelion (ω) clustering identified by Trujillo and Sheppard was seen, and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with being random. Pedro Bernardinelli and his colleagues also found that the orbital elements of the ETNOs found by the Dark Energy Survey showed no evidence of clustering. However, they also noted that the sky coverage and number of objects found were insufficient to show that there was no Planet Nine. A similar result was found when these two surveys were combined with a survey by Trujillo and Sheppard. These result differed from an analysis of discovery biases in the previously observed ETNOs by Mike Brown. He found that after observation biases were accounted for, the clustering of longitudes of perihelion of 10 known ETNOs would be observed only 1.2% of the time if their actual distribution was uniform. When combined with the odds of the observed clustering of the arguments of perihelion, the probability was 0.025%. A later analysis of the discovery biases of 14 ETNOs by Brown and Batygin determined the probability of the observed clustering of the longitudes of perihelion and the orbital pole locations to be 0.2%. Simulations of 15 known objects evolving under the influence of Planet Nine also revealed differences from observations. Cory Shankman and his colleagues included Planet Nine in a simulation of many clones (objects with similar orbits) of 15 objects with semi-major axis > 150 AU and perihelion > 30 AU. While they observed alignment of the orbits opposite that of Planet Nine's for the objects with semi-major axis greater than 250 AU, clustering of the arguments of perihelion was not seen. Their simulations also showed that the perihelia of the ETNOs rose and fell smoothly, leaving many with perihelion distances between 50 AU and 70 AU where none had been observed, and predicted that there would be many other unobserved objects. These included a large reservoir of high-inclination objects that would have been missed due to most observations being at small inclinations, and a large population of objects with perihelia so distant that they would be too faint to observe. Many of the objects were also ejected from the Solar System after encountering the other giant planets. The large unobserved populations and the loss of many objects led Shankman et al. to estimate that the mass of the original population was tens of Earth masses, requiring that a much larger mass had been ejected during the early Solar System. Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing ETNOs is a temporary phenomenon that will disappear as more objects are detected.


Inclination instability in a massive disk

Ann-Marie Madigan and Michael McCourt postulate that an inclination instability in a distant massive belt is responsible for the alignment of the arguments of perihelion of the ETNOs. An inclination instability could occur in a disk of particles with high eccentricity orbits ( ''e'' > 0.6) around a central body, such as the Sun. The self-gravity of this disk would cause its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane. This process would require an extended time and significant mass of the disk, on the order of a billion years for a 1–10 Earth-mass disk. Mike Brown considers Planet Nine a more probable explanation, noting that current surveys have not revealed a large enough scattered-disk to produce an "inclination instability". In Nice model simulations of the Solar System that included the self-gravity of the planetesimal disk an inclination instability did not occur. Instead, the simulation produced a rapid precession of the objects' orbits and most of the objects were ejected on too short of a timescale for an inclination instability to occur. In 2020 Madigan and colleagues showed that the inclination instability would require 20 Earth masses in a disk of objects with semi-major axes of a few hundred AU. An inclination instability in this disk could also reproduce the observed gap in the perihelion distances of the extreme TNOs, and the observed apsidal alignment following the inclination instability given sufficient time.


Shepherding by a massive disk

Antranik Sefilian and Jihad Touma propose that a massive disk of moderately eccentric TNOs is responsible for the clustering of the longitudes of perihelion of the ETNOs. This disk would contain 10 Earth-mass of TNOs with aligned orbits and eccentricities that increased with their semi-major axes ranging from zero to 0.165. The gravitational effects of the disk would offset the forward precession driven by the giant planets so that the orbital orientations of its individual objects are maintained. The orbits of objects with high eccentricities, such as the observed ETNOs, would be stable and have roughly fixed orientations, or longitudes of perihelion, if their orbits were anti-aligned with this disk. Although Brown thinks the proposed disk could explain the observed clustering of the ETNOs, he finds it implausible that the disk could survive over the age of the Solar System. Batygin thinks that there is insufficient mass in the Kuiper belt to explain the formation of the disk and asks "why would the protoplanetary disk end near 30 AU and restart beyond 100 AU?"


Planet in lower eccentricity orbit

The Planet Nine hypothesis includes a set of predictions about the mass and orbit of the planet. An alternative hypothesis predicts a planet with different orbital parameters. Renu Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond and semi-major axes greater than , are in ''n'':1 or ''n'':2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with eccentricity ''e'' < 0.18 and inclination ''i'' ≈ 11°. The eccentricity is limited in this case by the requirement that close approaches of to the planet be avoided. If the ETNOs are in periodic orbits of the third kind, with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with ''i'' ≈ 48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.


Alignment due to the Kozai mechanism

Trujillo and Sheppard argued in 2014 that a massive planet in a circular orbit with an average distance between and was responsible for the clustering of the arguments of perihelion of twelve TNOs with large semi-major axes. Trujillo and Sheppard identified a clustering near zero degrees of the arguments of perihelion of the orbits of twelve TNOs with perihelia greater than and semi-major axes greater than . After numerical simulations showed that the arguments of perihelion should circulate at varying rates, leaving them randomized after billions of years, they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering. This massive planet would cause the arguments of perihelion of the TNOs to librate about 0° or 180° via the
Kozai mechanism In celestial mechanics, the Kozai mechanism is a dynamical phenomenon affecting the orbit of a binary system perturbed by a distant third body under certain conditions. It is also known as the von Zeipel-Kozai-Lidov, Lidov–Kozai mechanism, Kozai ...
so that their orbits crossed the plane of the planet's orbit near perihelion and aphelion, the closest and farthest points from the planet. In numerical simulations including a 2–15 Earth mass body in a circular low-inclination orbit between and the arguments of perihelia of Sedna and librated around 0° for billions of years (although the lower perihelion objects did not) and underwent periods of libration with a Neptune mass object in a high inclination orbit at 1,500 AU. Another process such as a passing star would be required to account for the absence of objects with arguments of perihelion near 180°. These simulations showed the basic idea of how a single large planet can shepherd the smaller TNOs into similar types of orbits. They were basic proof of concept simulations that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have. Thus they did not fully formulate a model that successfully incorporated all the clustering of the ETNOs with an orbit for the planet. But they were the first to notice there was a clustering in the orbits of TNOs and that the most likely reason was from an unknown massive distant planet. Their work is very similar to how
Alexis Bouvard Alexis Bouvard (, 27 June 1767 – 7 June 1843) was a French astronomer. He is particularly noted for his careful observations of the irregularities in the motion of Uranus and his hypothesis of the existence of an eighth planet in the Solar ...
noticed Uranus' motion was peculiar and suggested that it was likely gravitational forces from an unknown 8th planet, which led to the discovery of Neptune. Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance. An analysis by Carlos and Raúl de la Fuente Marcos with Sverre J. Aarseth confirmed that the observed alignment of the arguments of perihelion could not be due to observational bias. They speculated that instead it was caused by an object with a mass between that of Mars and Saturn that orbited at some from the Sun. Like Trujillo and Sheppard they theorized that the TNOs are kept bunched together by a Kozai mechanism and compared their behavior to that of Comet 96P/Machholz under the influence of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. They also struggled to explain the orbital alignment using a model with only one unknown planet, and therefore suggested that this planet is itself in resonance with a more-massive world about from the Sun. In their article, Brown and Batygin noted that alignment of arguments of perihelion near 0° or 180° via the Kozai mechanism requires a ratio of the semi-major axes nearly equal to one, indicating that multiple planets with orbits tuned to the data set would be required, making this explanation too unwieldy.


Primordial black hole

In 2019, Jakub Scholtz and James Unwin proposed that a
primordial black hole Primordial black holes (also abbreviated as PBH) are hypothetical black holes that formed soon after the Big Bang. Due to the extreme environment of the newly born universe, extremely dense pockets of sub-atomic matter had been tightly packed to ...
was responsible for the clustering of the orbits of the ETNOs. Their analysis of
OGLE Ogle may refer to: Places * Ogle County, Illinois, United States * Original name of Ashton, Illinois, a village * Ogle, Kentucky, United States, an unincorporated community * Ogle Township, Somerset County, Pennsylvania, United States * Ogle, N ...
gravitational lensing data revealed a population of planetary mass objects in the direction of the galactic bulge more numerous than the local population of stars. They propose that instead of being free floating planets, these objects are primordial black holes. Since their estimate of the size of this population is greater than the estimated population of free floating planets from planetary formation models they argue that the capture of a hypothetical primordial black hole would be more probable than the capture of a free floating planet. This could also explain why an object responsible for perturbing the orbits of the ETNOs, if it exists, has yet to be seen. A detection method was proposed in the paper, stating that the black hole is too cold to be detected over the CMB, but interaction with surrounding
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
would produce
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s detectable by the FERMILAT. Konstantin Batygin commented on this, saying while it is possible for Planet Nine to be a primordial black hole, there is currently not enough evidence to make this idea more plausible than any other alternative.
Edward Witten Edward Witten (born August 26, 1951) is an American mathematical and theoretical physicist. He is a Professor Emeritus in the School of Natural Sciences at the Institute for Advanced Study in Princeton. Witten is a researcher in string theory, q ...
proposed a fleet of probes accelerated by radiation pressure that could discover a Planet Nine primordial black hole's location, however Thiem Hoang and
Avi Loeb Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He h ...
showed that any signal would be dominated by noise from the
interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
. Amir Siraj and
Avi Loeb Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He h ...
proposed a method for the
Vera C. Rubin Observatory The Vera C. Rubin Observatory, previously referred to as the Large Synoptic Survey Telescope (LSST), is an astronomical observatory currently under construction in Chile. Its main task will be carrying out a synoptic astronomical survey, the L ...
to detect flares from any low-mass black hole in the outer Solar System, including a possible Planet Nine primordial black hole.


Detection attempts


Visibility and location

Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings. It is expected to have an
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
fainter than 22, making it at least 600 times fainter than
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
. If Planet Nine exists and is close to perihelion, astronomers could identify it based on existing images. At aphelion, the largest telescopes would be required, but if the planet is currently located in between, many
observatories An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
could spot Planet Nine. Statistically, the planet is more likely to be close to its aphelion at a distance greater than 600 AU. This is because objects move more slowly when near their aphelion, in accordance with
Kepler's second law In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits ...
. A 2019 study estimated that Planet Nine, if it exists, may be smaller and closer than originally thought. This would make the hypothetical planet brighter and easier to spot, with an apparent magnitude of 21–22. According to
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $10.3 billion (2021) , endowment = $17 billion (2021)As o ...
professor Fred Adams, by 2035, Planet Nine will either be observable or enough data will have been gathered to rule out its existence.


Searches of existing data

The search of databases of stellar objects by Batygin and Brown has already excluded much of the sky along Planet Nine's predicted orbit. The remaining regions include the direction of its aphelion, where it would be too faint to be spotted by these surveys, and near the plane of the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
, where it would be difficult to distinguish from the numerous stars. This search included the archival data from the
Catalina Sky Survey Catalina Sky Survey (CSS; obs. code: 703) is an astronomical survey to discover comets and asteroids. It is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States. CSS focuses on the search ...
to magnitude 21–22,
Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical c ...
to magnitude 21.5, and infrared data from the
Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
(WISE) satellite. In 2021, they also searched the first three years of data from the
Zwicky Transient Facility The Zwicky Transient Facility (ZTF, obs. code: I41) is a wide-field sky astronomical survey using a new camera attached to the Samuel Oschin Telescope at the Palomar Observatory in California, United States. Commissioned in 2018, it supersedes ...
(ZTF) without identifying Planet Nine. The search of the ZTF data alone has ruled out 56% of the
parameter space The parameter space is the space of possible parameter values that define a particular mathematical model, often a subset of finite-dimensional Euclidean space. Often the parameters are inputs of a function, in which case the technical term for the ...
for possible Planet Nine positions. As a result of ruling out mostly objects with small semi-major axes, the expected orbit of Planet Nine was pushed slightly further away. Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the
Dark Energy Survey The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, visible, and near-infrared to measure the expansion of the universe using Type Ia supernov ...
, claims that software designed to identify distant Solar System objects such as could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky. Michael Medford and Danny Goldstein, graduate students at the
University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...
, are also examining archived data using a technique that combines images taken at different times. Using a
supercomputer A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly measured in floating-point operations per second ( FLOPS) instead of million instructions ...
they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image. A search combining multiple images collected by WISE and NEOWISE data has also been conducted without detecting Planet Nine. This search covered regions of the sky away from the
galactic plane The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usuall ...
at the "W1" wavelength (the 3.4 μm wavelength used by WISE) and is estimated to be able to detect a 10-Earth mass object out to 800–900 AU. Malena Rice and Gregory Laughlin applied a targeted shift-stacking search algorithm to analyze data from
TESS Tess or TESS may refer to: Music * Tess (band), a Spanish pop band active from 2000 to 2005 * TESS (musician), a UK musician Film and theatre * ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles'' * ''Tess'' (2016 film ...
sectors 18 and 19 looking for Planet Nine and candidate outer Solar System objects. Their search generated no serious evidence for the presence of a distant planet, but it produced 17 new outer Solar System body candidates located at geocentric distances in the range 80–200 AU, that need follow-up observations with ground-based telescope resources for confirmation. Early results from a survey with WHT aimed at recovering these distant TNO candidates have failed to confirm two of them. By 2022, a comparison between
IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...
and
AKARI Akari (ASTRO-F) was an infrared astronomy satellite developed by Japan Aerospace Exploration Agency, in cooperation with institutes of Europe and Korea. It was launched on 21 February 2006, at 21:28 UTC (06:28, 22 February JST) by M-V rocke ...
data yielded no Planet Nine detection. It was noted that far-infrared data in the major portion of the sky are heavily contaminated by the emission from the galactic nebulae, making detection of Planet Nine thermal emission problematic close to the galactic plane or bulge.


Ongoing searches

Because the planet is predicted to be visible in the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, the primary search is expected to be carried out using the
Subaru Telescope is the telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades. It had the largest monolithic primary mirror in the wo ...
, which has both an
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...
large enough to see faint objects and a wide field of view to shorten the search. Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams expect the search to take up to five years. Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000
square degree __NOTOC__ A square degree (deg2) is a non- SI unit measure of solid angle. Other denotations include ''sq. deg.'' and (°)2. Just as degrees are used to measure parts of a circle, square degrees are used to measure parts of a sphere. Analogous to ...
s of sky near Orion, a swath of space that Batygin thinks could be covered in about 20 nights by the Subaru Telescope. Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky. In December 2018, they spent four half–nights and three full nights observing with the Subaru Telescope. Due to the elusiveness of the hypothetical planet, it has been proposed that different detection methods be used when looking for a
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
mass planet ranging from using differing telescopes to using multiple spacecraft. In late April and early May 2020, Scott Lawrence and Zeeve Rogoszinski proposed the latter method for finding it as multiple spacecraft would have advantages that land-based telescopes do not have.


Radiation

Although a distant planet such as Planet Nine would reflect little light, due to its large mass it would still be radiating the heat from its formation as it cools. At its estimated temperature of the peak of its emissions would be at
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths. This radiation signature could be detected by Earth-based submillimeter telescopes, such as
ALMA Alma or ALMA may refer to: Arts and entertainment * ''Alma'' (film), a 2009 Spanish short animated film * ''Alma'' (Oswald de Andrade novel), 1922 * ''Alma'' (Le Clézio novel), 2017 * ''Alma'' (play), a 1996 drama by Joshua Sobol about Alma ...
, and a search could be conducted by
cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
experiments operating at mm wavelengths. A search of part of the sky using archived data of the Atacama Cosmology Telescope has not detected Planet Nine. Jim Green of NASA's Science Mission Directorate is optimistic that it could be observed by the
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...
, the successor to the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
.


Citizen science

The Zooniverse
Backyard Worlds Backyard Worlds: Planet 9 is a NASA-funded citizen science project which is part of the Zooniverse web portal. It aims to discover new brown dwarfs, faint objects that are less massive than stars, some of which might be among the nearest neighb ...
project, originally started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will also search for substellar objects like
brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s in the neighborhood of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. 32,000 animations of four images each, which constitute 3% of the WISE spacecraft's data, have been uploaded to the Backyard Worlds website. In April 2017, using data from the
SkyMapper SkyMapper is a fully automated 1.35 m (4.4 ft) wide-angle optical telescope at Siding Spring Observatory in northern New South Wales, Australia. It is one of the telescopes of the Research School of Astronomy and Astrophysics of the ...
telescope at
Siding Spring Observatory Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics (RSAA) at the Australian National University (ANU), incorporates the Anglo-Australian Telescope along with a coll ...
,
citizen scientist Citizen science (CS) (similar to community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is scientific research conducted with participation from the public (who are sometimes re ...
s on the
Zooniverse Zooniverse is a citizen science web portal owned and operated by the Citizen Science Alliance. It is home to some of the Internet's largest, most popular and most successful citizen science projects. The organization grew from the original Gal ...
platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals. The Zooniverse Catalina Outer Solar System Survey project, started in August 2020, is using archived data from the
Catalina Sky Survey Catalina Sky Survey (CSS; obs. code: 703) is an astronomical survey to discover comets and asteroids. It is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States. CSS focuses on the search ...
to search for TNOs. By looking for moving objects in animations, and depending on the size, distance and magnitude, citizen scientists might be able to find Planet Nine.


Attempts to predict location


''Cassini'' measurements of Saturn's orbit

Precise observations of Saturn's orbit using data from ''Cassini'' suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position. This data neither proves nor disproves that Planet Nine exists. An initial analysis by Fienga, Laskar, Manche, and Gastineau using Cassini data to search for Saturn's orbital residuals, small differences with its predicted orbit due to the Sun and the known planets, was inconsistent with Planet Nine being located with a
true anomaly In celestial mechanics, true anomaly is an angular parameter that defines the position of a body moving along a Keplerian orbit. It is the angle between the direction of periapsis and the current position of the body, as seen from the main focus ...
, the location along its orbit relative to perihelion, of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of . At this location, Planet Nine would be approximately from the Sun, with
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth. When paired w ...
close to 2h and
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
close to −20°, in
Cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the sky that contains other water- ...
. In contrast, if the putative planet is near aphelion it would be located near right ascension 3.0h to 5.5h and declination −1° to 6°. A later analysis of ''Cassini'' data by astrophysicists Matthew Holman and Matthew Payne tightened the constraints on possible locations of Planet Nine. Holman and Payne developed a more efficient model that allowed them to explore a broader range of parameters than the previous analysis. The parameters identified using this technique to analyze the Cassini data was then intersected with Batygin and Brown's dynamical constraints on Planet Nine's orbit. Holman and Payne concluded that Planet Nine is most likely to be located within 20° of RA = 40°, Dec = −15°, in an area of the sky near the constellation Cetus. William Folkner, a planetary scientist at the
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
(JPL), has stated that the ''Cassini'' spacecraft was not experiencing unexplained deviations in its orbit around Saturn. An undiscovered planet would affect the orbit of Saturn, not ''Cassini''. This could produce a signature in the measurements of ''Cassini'', but JPL has seen no unexplained signatures in ''Cassini'' data.


Analysis of Pluto's orbit

An analysis in 2016 of Pluto's orbit by Holman and Payne found perturbations much larger than predicted by Batygin and Brown's proposed orbit for Planet Nine. Holman and Payne suggested three possible explanations: systematic errors in the measurements of Pluto's orbit; an unmodeled mass in the Solar System, such as a small planet in the range of 60– (potentially explaining the
Kuiper cliff The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
); or a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.


Orbits of nearly parabolic comets

An analysis of the orbits of comets with nearly
parabolic orbit In astrodynamics or celestial mechanics a parabolic trajectory is a Kepler orbit with the eccentricity equal to 1 and is an unbound orbit that is exactly on the border between elliptical and hyperbolic. When moving away from the source it is ca ...
s identifies five new comets with
hyperbolic orbit In astrodynamics or celestial mechanics, a hyperbolic trajectory or hyperbolic orbit is the trajectory of any object around a central body with more than enough speed to escape the central object's gravitational pull. The name derives from the fa ...
s that approach the nominal orbit of Planet Nine described in Batygin and Brown's initial article. If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°. Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.


Occultations by Jupiter trojans

Malena Rice and Gregory Laughlin have proposed that a network of telescopes be built to detect
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s by
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s. The timing of these occultations would provide precise astrometry of these objects enabling their orbits to be monitored for variations due to the tide from Planet Nine.


Possible encounter with interstellar meteor

In May 2022, it was suggested that the peculiar meteor
CNEOS 2014-01-08 CNEOS 2014-01-08 is an interstellar object reported in June 2019 by astronomers Amir Siraj and Avi Loeb, Abraham Loeb, and confirmed by the United States Space Command in April 2022. The discovery was publicized in 2019 in a preprint announcing a ...
may have entered Earth-crossing orbit after a swing-by of Planet Nine. If that hypothesis is true, the trajectory back-tracing of
CNEOS 2014-01-08 CNEOS 2014-01-08 is an interstellar object reported in June 2019 by astronomers Amir Siraj and Avi Loeb, Abraham Loeb, and confirmed by the United States Space Command in April 2022. The discovery was publicized in 2019 in a preprint announcing a ...
means Planet Nine may be currently located in the constellation of
Aries Aries may refer to: *Aries (astrology), an astrological sign *Aries (constellation), a constellation of stars in the zodiac Arts, entertainment and media * ''Aries'' (album), by Luis Miguel, 1993 * ''Aries'' (EP), by Alice Chater, 2020 * "Aries" ...
, at right ascension 50°, and declination 11.8°.


Attempts to predict the semi-major axis

An analysis by Sarah Millholland and Gregory Laughlin identified a pattern of commensurabilities (ratios between orbital periods of pairs of objects consistent with both being in resonance with another object) of the ETNOs. They identify five objects that would be near resonances with Planet Nine if it had a semi-major axis of 654 AU: Sedna (3:2), 474640 Alicanto (3:1), (4:1), (5:1), and (5:1). They identify this planet as Planet Nine but propose a different orbit with an eccentricity ''e'' ≈ 0.5, inclination ''i'' ≈ 30°, argument of perihelion ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°). Carlos and Raúl de la Fuente Marcos also note commensurabilities among the known ETNOs similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that some of these objects would be in 5:3 and 3:1 resonances with a planet that had a semi-major axis of ≈700 AU. Three objects with smaller semi-major axes near 172 AU (, and ) have also been proposed to be in resonance with Planet Nine. These objects would be in resonance and anti-aligned with Planet Nine if it had a semi-major axis of 315 AU, below the range proposed by Batygin and Brown. Alternatively, they could be in resonance with Planet Nine, but have orbital orientations that circulate instead of being confined by Planet Nine if it had a semi-major axis of 505 AU. A later analysis by Elizabeth Bailey, Michael Brown and Konstantin Batygin found that if Planet Nine is in an eccentric and inclined orbit the capture of many of the ETNOs in higher-order resonances and their chaotic transfer between resonances prevent the identification of Planet Nine's semi-major axis using current observations. They also determined that the odds of the first six objects observed being in N/1 or N/2 period ratios with Planet Nine are less than 5% if it has an eccentric orbit. In late 2020 it was determined
HD 106906 b HD 106906 b is a directly imaged planetary-mass companion and candidate exoplanet orbiting the star , in the constellation Crux at about from Earth. It is estimated to be about eleven times the mass of Jupiter and is located about 738  AU ...
, a candidate exoplanet, had an eccentric orbit that took it outside the debris disk of its binary host stars. Its orbit appears to be similar to the predictions made for Planet Nine's semi-major axis and it may serve as a proxy for Planet Nine that helps explain how such planetary orbits evolve, although this exoplanet is well over ten times as massive as Jupiter.


Naming

Planet Nine does not have an official name and will not receive one unless its existence is confirmed via imaging. Only two planets, Uranus and Neptune, have been discovered in the Solar System during recorded history. However, many
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s, including
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
s such as Pluto,
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s, and comets have been discovered and named. Consequently, there is a well-established process for naming newly discovered Solar System objects. If Planet Nine is observed, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from
Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter ...
or
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
. In their original article, Batygin and Brown simply referred to the object as "perturber", and only in later press releases did they use "Planet Nine". They have also used the names "
Jehoshaphat Jehoshaphat (; alternatively spelled Jehosaphat, Josaphat, or Yehoshafat; ; el, Ἰωσαφάτ, Iosafát; la, Josaphat), according to 1 Kings 22:41, was the son of Asa, and the fourth king of the Kingdom of Judah, in succession to his father ...
" and "George" (a reference to
William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
's proposed name for
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
) for Planet Nine. Brown has stated: "We actually call it Phattie when we're just talking to each other." In 2018, Batygin has also informally suggested, based on a petition on
Change.org Change.org is a worldwide nonprofit petition website, based in California, US, operated by the San Francisco-based company of the same name, which has over 400 million users and offers the public the ability to promote the petitions they care abo ...
, to name the planet after singer
David Bowie David Robert Jones (8 January 194710 January 2016), known professionally as David Bowie ( ), was an English singer-songwriter and actor. A leading figure in the music industry, he is regarded as one of the most influential musicians of the ...
, and to name any potential moons of the planet after characters from Bowie's song catalogue, such as Ziggy Stardust or
Starman ''StarMan'' is a 1996 fantasy novel by Australian writer Sara Douglass. It follows the second book in the series, '' Enchanter'', with Axis marching north with his army to confront a formidable enemy. Background ''StarMan'' was first published ...
. Jokes have been made connecting "Planet Nine" to
Ed Wood Edward Davis Wood Jr. (October 10, 1924 – December 10, 1978) was an American filmmaker, actor, and pulp novel author. In the 1950s, Wood directed several low-budget science fiction, crime and horror films that later became cult cla ...
's 1959 science-fiction horror film ''
Plan 9 from Outer Space ''Plan 9 from Outer Space'' is a 1957 American independent science fiction-horror film produced, written, directed, and edited by Ed Wood. The film was shot in black-and-white in November 1956 and had a theatrical preview screening on March 15 ...
''. In connection with the Planet Nine hypothesis, the film title recently found its way into academic discourse. In 2016, an article titled ''Planet Nine from Outer Space'' about the hypothesized planet in the
outer region of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
was published in ''
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
''. Several conference talks since then have used the same
word play Word play or wordplay (also: play-on-words) is a literary technique and a form of wit in which words used become the main subject of the work, primarily for the purpose of intended effect or amusement. Examples of word play include puns, phonet ...
, as did a lecture by Mike Brown given in 2019.
Persephone In ancient Greek mythology and religion, Persephone ( ; gr, Περσεφόνη, Persephónē), also called Kore or Cora ( ; gr, Κόρη, Kórē, the maiden), is the daughter of Zeus and Demeter. She became the queen of the underworld after ...
, the wife of the deity Pluto, had been a popular name commonly used in
science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel unive ...
for a planet beyond Neptune (see
Fictional planets of the Solar System The fictional portrayal of the Solar System has often included planets, moons, and other celestial objects which do not actually exist in reality. Some of these objects were, at one time, seriously considered as List of hypothetical Solar System o ...
). However, it is unlikely that Planet Nine or any other conjectured planet beyond Neptune will be given the name Persephone once its existence is confirmed, as it is already the name for asteroid 399 Persephone. In 2018, planetary scientist
Alan Stern Sol Alan Stern (born November 22, 1957) is an American engineer and planetary scientist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved in 24 suborbital ...
objected to the name ''Planet Nine'', saying, "It is an effort to erase
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
's legacy and it's frankly insulting", suggesting the name Planet X until its discovery. He signed a statement with 34 other scientists saying, "We further believe the use of this term lanet Nineshould be discontinued in favor of culturally and
taxonomically In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
neutral terms for such planets, such as Planet X, Planet Next, or Giant Planet Five." According to Brown,
Planet X Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...
' is not a generic reference to some unknown planet, but a specific prediction of Lowell's which led to the (accidental) discovery of Pluto. Our prediction is not related to this prediction."


See also

*
Planets beyond Neptune Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...
* Hypothetical planets of the Solar System *
Tyche (hypothetical planet) Tyche is a hypothetical gas giant located in the Solar System's Oort cloud, first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette. They argued that evidence of ...
*
Nemesis (hypothetical star) Nemesis is a hypothetical red dwarf or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in t ...


Notes


References

{{Reflist, 30em, refs= {{cite journal , last1=Brown , first1=Michael E. , last2=Batygin , first2=Konstantin , title=Observational Constraints on the Orbit and Location of Planet Nine in the Outer Solar System , journal=
The Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery H ...
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The Washington Post ''The Washington Post'' (also known as the ''Post'' and, informally, ''WaPo'') is an American daily newspaper published in Washington, D.C. It is the most widely circulated newspaper within the Washington metropolitan area and has a large nati ...
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Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
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The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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Astronomy & Astrophysics ''Astronomy & Astrophysics'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a re ...
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Physics Reports ''Physics Reports'' is a peer-reviewed scientific journal, a review section of ''Physics Letters'' that has been published by Elsevier since 1971. The journal publishes long and deep reviews on all aspects of physics. In average, the length of thes ...
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National Geographic ''National Geographic'' (formerly the ''National Geographic Magazine'', sometimes branded as NAT GEO) is a popular American monthly magazine published by National Geographic Partners. Known for its photojournalism, it is one of the most widely ...
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The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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The New Yorker ''The New Yorker'' is an American weekly magazine featuring journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. Founded as a weekly in 1925, the magazine is published 47 times annually, with five of these issues ...
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The Conversation ''The Conversation'' is a 1974 American mystery thriller film written, produced, and directed by Francis Ford Coppola and starring Gene Hackman, John Cazale, Allen Garfield, Cindy Williams, Frederic Forrest, Harrison Ford, Teri Garr, and Rober ...
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Isis Isis (; ''Ēse''; ; Meroitic: ''Wos'' 'a''or ''Wusa''; Phoenician: 𐤀𐤎, romanized: ʾs) was a major goddess in ancient Egyptian religion whose worship spread throughout the Greco-Roman world. Isis was first mentioned in the Old Kingd ...
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Space.com Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
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The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ...
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The Guardian ''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers ''The Observer'' and ''The Guardian Weekly'', ''The Guardian'' is part of the Gu ...
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{{cite web , last=Wolchover , first=Natalie , date=25 May 2012 , title=Planet X? New Evidence of an Unseen Planet at Solar System's Edge , url=http://www.livescience.com/20583-planet-edge-solar-system.html , website=
LiveScience Live Science is a science news website run by Future via Purch, which it purchased from Imaginova in 2009. Stories and editorial commentary are typically syndicated to major news outlets, such as Yahoo!, MSNBC, AOL, and Fox News.{{fact, date=Mar ...
, access-date=7 February 2016 , quote=More work is needed to determine whether Sedna and the other scattered disc objects were sent on their circuitous trips round the Sun by a star that passed by long ago, or by an unseen planet that exists in the solar system right now. Finding and observing the orbits of other distant objects similar to Sedna will add more data points to astronomers' computer models. , archive-url=https://web.archive.org/web/20160130192931/http://www.livescience.com/20583-planet-edge-solar-system.html , archive-date=30 January 2016 , url-status=live
{{cite web , last=Lovett , first=Richard A. , date=12 May 2012 , title=New Planet Found in Our Solar System? , url=http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/ , website=
National Geographic News The National Geographic Society (NGS), headquartered in Washington, D.C., United States, is one of the largest non-profit scientific and educational organizations in the world. Founded in 1888, its interests include geography, archaeology, and ...
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{{cite journal , last1=Gomes , first1=Rodney , date=2015 , title=The Observation of Large Semi-Major Axis Centaurs: Testing for the Signature of a Planetary-Mass Solar Companion , journal=
Icarus In Greek mythology, Icarus (; grc, Ἴκαρος, Íkaros, ) was the son of the master craftsman Daedalus, the architect of the labyrinth of Crete. After Theseus, king of Athens and enemy of Minos, escaped from the labyrinth, King Minos suspe ...
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{{cite journal , last1=de la Fuente Marcos , first1=C. , last2=de la Fuente Marcos , first2=R. , title=Memories of past close encounters in extreme trans-Neptunian space: Finding unseen planets using pure random searches , journal=Astronomy and Astrophysics Letters , volume=646 , page=L14 (9 pp) , date=18 February 2021 , doi=10.1051/0004-6361/202140311 , arxiv=2102.02220 , bibcode=2021A&A...646L..14D , s2cid=231802033 , url=https://www.aanda.org/articles/aa/full_html/2021/02/aa40311-21/aa40311-21.html , access-date=18 February 2021 , archive-date=18 February 2021 , archive-url=https://web.archive.org/web/20210218162631/https://www.aanda.org/articles/aa/full_html/2021/02/aa40311-21/aa40311-21.html , url-status=live {{cite web, url=http://www.findplanetnine.com/p/blog-page.html, title=Where is Planet Nine?, date=20 January 2016, website=The Search for Planet Nine, type=Blog, archive-url=https://web.archive.org/web/20160130091255/http://www.findplanetnine.com/p/blog-page.html, archive-date=30 January 2016, url-status=live {{Cite web, url=https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth, title=Planet X, website=NASA Solar System Exploration, access-date=2019-05-14, archive-date=19 October 2019, archive-url=https://web.archive.org/web/20191019012033/https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/, url-status=live {{cite web , author=Michael E. Brown , date=3 March 2017 , title=Planet Nine , url=https://www.youtube.com/watch?v=v-ktWBtt7sc&t=1147 , at=19:06 , via=
YouTube YouTube is a global online video platform, online video sharing and social media, social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by ...
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{{cite web , last1=Batygin , first1=Konstantin , last2=Brown , first2=Mike , date=20 January 2016 , title=Where is Planet Nine? , url=http://www.findplanetnine.com/p/blog-page.html , at=RA/Dec chart, website=The Search for Planet Nine, publisher=
Michael E. Brown Michael E. Brown (born June 5, 1965) is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), including ...
and
Konstantin Batygin Konstantin Batygin (russian: Константи́н Юрьевич Батыгин) is an American astronomer and Professor of Planetary Sciences at Caltech. Early life Konstantin Batygin was born in Moscow, Soviet Union. His father, Yuri Konsta ...
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Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
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{{cite web , last1=Becker , first1=Adam , last2=Grossman , first2=Lisa , last3=Aron , first3=Jacob , date=22 January 2016 , title=How Planet Nine May Have Been Exiled to Solar System's Edge , url=https://www.newscientist.com/article/2074961-how-planet-nine-may-have-been-exiled-to-solar-systems-edge/ , website=
New Scientist ''New Scientist'' is a magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organisation publishe ...
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{{cite web , last=Margot , first=Jean-Luc , date=22 January 2016 , title=Would Planet Nine Pass the Planet Test? , url=http://mel.epss.ucla.edu/jlm/epo/planet/nine.html , website=
University of California at Los Angeles The University of California, Los Angeles (UCLA) is a public university, public Land-grant university, land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a Normal school, teachers colle ...
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The New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid ...
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{{cite news , first=Sanden , last=Totten , date=20 January 2016 , title=Caltech Researchers Answer Skeptics' Questions about Planet 9 , url=http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ , publisher= 89.3 KPCC , access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160706171901/http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ , archive-date=6 July 2016 , url-status=live {{cite journal , last1=Bailey , first1=Nora , last2=Fabrycky , first2=Daniel , title=Stellar Flybys Interrupting Planet-Planet Scattering Generates Oort Planets , journal=The Astronomical Journal , year=2019 , volume=158 , issue=2 , pages=94 , doi=10.3847/1538-3881/ab2d2a , arxiv=1905.07044, bibcode=2019AJ....158...94B , s2cid=158047152 {{cite book, last=D'Angelo, first=G., author2=Lissauer, J.J., chapter=Formation of Giant Planets , bibcode=2018haex.bookE.140D, title=Handbook of Exoplanets , publisher=
Springer International Publishing AG Springer Nature or the Springer Nature Group is a German-British academic publishing company created by the May 2015 merger of Springer Science+Business Media and Holtzbrinck Publishing Group's Nature Publishing Group, Palgrave Macmillan, and Macm ...
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Astronomy & Astrophysics ''Astronomy & Astrophysics'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a re ...
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The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ...
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Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
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The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ...
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{{cite journal , last1=Mustill , first1=Alexander J. , last2=Raymond , first2=Sean N. , last3=Davies , first3=Melvyn B. , date=21 July 2016 , title=Is there an exoplanet in the Solar System? , journal= Monthly Notices of the Royal Astronomical Society: Letters , volume=460 , issue=1 , pages=L109–L113 , arxiv=1603.07247 , bibcode=2016MNRAS.460L.109M , doi=10.1093/mnrasl/slw075, s2cid=119229382 {{cite journal , last1=Li , first1=Gongjie , last2=Adams , first2=Fred C. , date=2016 , title=Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine , journal=
The Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery H ...
, volume=823 , issue=1 , page=L3 , arxiv=1602.08496 , bibcode=2016ApJ...823L...3L , doi=10.3847/2041-8205/823/1/L3, s2cid=15890864
{{Cite journal, last1=Siraj, first1=Amir, last2=Loeb, first2=Abraham, date=2020-08-18, title=The Case for an Early Solar Binary Companion, url=https://iopscience.iop.org/article/10.3847/2041-8213/abac66, journal=The Astrophysical Journal, language=en, volume=899, issue=2, pages=L24, doi=10.3847/2041-8213/abac66, issn=2041-8213, arxiv=2007.10339, bibcode=2020ApJ...899L..24S, s2cid=220665422 {{cite web , last1=Rabie , first1=Passant , title=Did the Sun Have a Twin? New Study Rewrites the Star's Early History , url=https://www.inverse.com/science/long-lost-twin-sun-study , website=Inverse , access-date=28 August 2020 , archive-date=22 August 2020 , archive-url=https://web.archive.org/web/20200822033224/https://www.inverse.com/science/long-lost-twin-sun-study , url-status=live {{cite journal, last1=Parker, first1=Richard J., last2=Lichtenberg, first2=Tim, last3=Quanz, first3=Sascha P., date=2017, title=Was Planet 9 Captured in the Sun's Natal Star-Forming Region?, journal= Monthly Notices of the Royal Astronomical Society: Letters, volume=472, issue=1, pages=L75–L79, arxiv=1709.00418, doi=10.1093/mnrasl/slx141, bibcode=2017MNRAS.472L..75P, s2cid=10792152 {{cite web , last1=Brennan , first1=Pat , title=The Super-Earth that Came Home for Dinner , url=https://www.jpl.nasa.gov/news/news.php?release=2017-259 , website=
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
, access-date=13 October 2017 , archive-url=https://web.archive.org/web/20171016045431/https://www.jpl.nasa.gov/news/news.php?release=2017-259 , archive-date=16 October 2017 , url-status=live
{{cite journal , last1=Nesvorny , first1=D. , last2=Vokrouhlicky , first2=D. , last3=Dones , first3=L. , last4=Levison , first4=H.F. , last5=Kaib , first5=N. , last6=Morbidelli , first6=A. , journal=
The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ...
, volume=845 , issue=1 , page=27 , title=Origin and Evolution of Short-Period Comets , arxiv=1706.07447 , date=2017 , doi=10.3847/1538-4357/aa7cf6 , bibcode=2017ApJ...845...27N, s2cid=119399322
{{cite web , last1=Stirone , first1=Shannon , title=Planet Nine May Be Responsible for Tilting the Sun , url=http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun , website=
Astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, access-date=29 July 2017 , archive-url=https://web.archive.org/web/20170810194731/http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun , archive-date=10 August 2017 , url-status=live
{{cite web , first=Barbara , last=Koponyás , date=10 April 2010 , title=Near-Earth Asteroids and the Kozai-Mechanism , url=http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf , website=5th Austrian Hungarian Workshop in Vienna , access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160314110041/http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf , archive-date=14 March 2016 , url-status=live {{cite news , last=McDonald , first=Bob , date=24 January 2016 , title=How Did We Miss Planet 9? , url=https://www.cbc.ca/news/technology/planet-9-bob-macdonald-1.3414268 , publisher=
CBC News CBC News is a division of the Canadian Broadcasting Corporation responsible for the news gathering and production of news programs on the corporation's English-language operations, namely CBC Television, CBC Radio, CBC News Network, and CBC.ca. ...
, access-date=18 July 2016 , quote="It's like seeing a disturbance on the surface of water but not knowing what caused it. Perhaps it was a jumping fish, a whale or a seal. Even though you didn't actually see it, you could make an informed guess about the size of the object and its location by the nature of the ripples in the water." , archive-url=https://web.archive.org/web/20160205174831/https://www.cbc.ca/news/technology/planet-9-bob-macdonald-1.3414268 , archive-date=5 February 2016 , url-status=live
{{cite web , first=Emily , last=Lakdawalla , date=20 January 2016 , title=Theoretical Evidence for an Undiscovered Super-Earth at the Edge of Our Solar System , url=http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html , website=
The Planetary Society The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy Space advocacy is supporting or advocating for a human use of out ...
, access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160423222919/http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html , archive-date=23 April 2016 , url-status=live
{{cite journal , last1=Hands , first1=T. O. , last2=Dehnen , first2=W. , last3=Gration , first3=A. , last4=Stadel , first4=J. , last5=Moore , first5=B. , title=The fate of planetesimal discs in young open clusters: implications for 1I/'Oumuamua, the Kuiper belt, the Oort cloud and more , journal=Monthly Notices of the Royal Astronomical Society , pages=21–36 , date=2019 , volume=490 , issue=1 , doi=10.1093/mnras/stz1069 , arxiv=1901.02465, bibcode=2019MNRAS.490...21H , s2cid=118597453 {{Cite journal , first1=Julia , last1=de León , first2=Carlos , last2=de la Fuente Marcos , last3=de la Fuente Marcos , first3=Raúl , year=2017 , title=Visible Spectra of (474640) 2004 VN112-2013 RF98 with OSIRIS at the 10.4 M GTC: Evidence for Binary Dissociation near Aphelion Among the Extreme Trans-Neptunian Objects , journal= Monthly Notices of the Royal Astronomical Society: Letters , volume=467 , issue=1 , pages=L66–L70 , arxiv=1701.02534 , bibcode=2017MNRAS.467L..66D , doi=10.1093/mnrasl/slx003 , s2cid=119419889 {{cite web , author=Instituto de Astrofísica de Canarias (IAC), title=New Data About Two Distant Asteroids Give a Clue to the Possible 'Planet Nine' , url=https://www.sciencedaily.com/releases/2017/02/170221120816.htm , website=
ScienceDaily ''Science Daily'' is an American website launched in 1995 that aggregates press releases and publishes lightly edited press releases (a practice called churnalism) about science, similar to Phys.org and EurekAlert!. The site was founded by mar ...
, access-date=29 July 2017 , archive-url=https://web.archive.org/web/20170729225604/https://www.sciencedaily.com/releases/2017/02/170221120816.htm , archive-date=29 July 2017 , url-status=live
{{cite journal , title=Binary Stripping as a Plausible Origin of Correlated Pairs of Extreme Trans-Neptunian Objects , first1=C. , last1=de la Fuente Marcos , first2=R. , last2=de la Fuente Marcos , first3=S.J. , last3=Aarseth , date=1 November 2017 , journal=
Astrophysics and Space Science ''Astrophysics and Space Science'' is a bimonthly peer-reviewed scientific journal covering astronomy, astrophysics, and space science and astrophysical aspects of astrobiology. It was established in 1968 and is published by Springer Science+Busin ...
, volume=362 , issue=11 , page=198 , arxiv=1709.06813 , bibcode=2017Ap&SS.362..198D , doi=10.1007/s10509-017-3181-1 , s2cid=118890903
{{cite journal , last1=Sheppard, Scott S. , first1=Scott S. , last2=Trujillo , first2=Chadwick , title=New Extreme Trans-Neptunian Objects: Toward a Super-Earth in the Outer Solar System , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=2016 , volume=152 , issue=6 , page=221 , doi=10.3847/1538-3881/152/6/221 , arxiv=1608.08772 , bibcode=2016AJ....152..221S , s2cid=119187392
{{cite journal , last1=de la Fuente Marcos , first1=Carlos , last2=de la Fuente Marcos , first2=Raúl , year=2017 , title=Evidence for a Possible Bimodal Distribution of the Nodal Distances of the Extreme Trans-Neptunian Objects: Avoiding a Trans-Plutonian Planet or Just Plain Bias? , journal= Monthly Notices of the Royal Astronomical Society Letters , volume=471 , issue=1 , pages=L61–L65 , arxiv=1706.06981 , bibcode=2017MNRAS.471L..61D , doi=10.1093/mnrasl/slx106 , s2cid=55469849 {{cite web , author=Spanish Foundation for Science and Technology (FECYT) , title=New Evidence in Support of the Planet Nine Hypothesis , url=https://phys.org/news/2017-07-evidence-planet-hypothesis.html , website=phys.org , access-date=29 July 2017 , archive-url=https://web.archive.org/web/20170730014118/https://phys.org/news/2017-07-evidence-planet-hypothesis.html , archive-date=30 July 2017 , url-status=live {{cite journal , last1=de la Fuente Marcos , first1=Carlos , last2=de la Fuente Marcos , first2=Raúl , title=Peculiar orbits and asymmetries in extreme trans-Neptunian space , journal=
Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
, url=https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 , volume=506 , issue=1 , pages=633–649 , arxiv=2106.08369 , bibcode=2021MNRAS.506..633D , doi=10.1093/mnras/stab1756 , date=1 September 2021 , access-date=8 July 2021 , archive-date=19 October 2021 , archive-url=https://web.archive.org/web/20211019195919/https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 , url-status=live
{{cite journal , last1=de la Fuente Marcos , first1=Carlos , last2=de la Fuente Marcos , first2=Raúl , title=Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed , journal=Monthly Notices of the Royal Astronomical Society Letters , url=https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 , volume=512 , issue=1 , pages=L6–L10 , arxiv=2202.01693 , bibcode=2022MNRAS.512L...6D , doi=10.1093/mnrasl/slac012 , date=1 May 2022 {{cite web , last1=Brown , first1=Michael E. , title=Planet Nine: Where Are You? (Part 1) , url=http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html , website=The Search for Planet Nine , publisher=
Michael E. Brown Michael E. Brown (born June 5, 1965) is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), including ...
and
Konstantin Batygin Konstantin Batygin (russian: Константи́н Юрьевич Батыгин) is an American astronomer and Professor of Planetary Sciences at Caltech. Early life Konstantin Batygin was born in Moscow, Soviet Union. His father, Yuri Konsta ...
, access-date=19 October 2017 , archive-url=https://web.archive.org/web/20171020083937/http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html , archive-date=20 October 2017 , url-status=live
{{cite journal , last1=Becker , first1=Juliette C. , last2=Adams , first2=Fred C. , last3=Khain , first3=Tali , last4=Hamilton , first4=Stephanie J. , last5=Gerdes , first5=David , title=Evaluating the Dynamical Stability of Outer Solar System Objects in the Presence of Planet Nine , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=2017 , volume=154 , issue=2 , page=61 , doi=10.3847/1538-3881/aa7aa2 , arxiv=1706.06609 , bibcode=2017AJ....154...61B, s2cid=111384673
{{cite journal , last1=Lawler , first1=S.M. , last2=Shankman , first2=C. , last3=Kaib , first3=N. , last4=Bannister , first4=M.T. , last5=Gladman , first5=B. , last6=Kavelaars , first6=J.J. , orig-year=21 May 2016 , title=Observational Signatures of a Massive Distant Planet on the Scattering Disk , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=29 December 2016 , volume=153 , issue=1 , page=33 , bibcode=2017AJ....153...33L , doi=10.3847/1538-3881/153/1/33 , arxiv=1605.06575, s2cid=54854087
{{cite journal , last1=Cáceres , first1=Jessica , last2=Gomes , first2=Rodney , title=The Influence of Planet 9 on the Orbits of Distant TNOs: The Case for a Low Perihelion Planet , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, volume=156 , issue=4 , page=157 , date=2018 , arxiv=1808.01248, doi=10.3847/1538-3881/aad77a , bibcode=2018AJ....156..157C , s2cid=119064276
{{cite magazine , last=Scharping , first=Nathaniel , date=20 January 2016 , title=Planet Nine: A New Addition to the Solar System? , url=http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ , magazine=
Discover Discover may refer to: Art, entertainment, and media * ''Discover'' (album), a Cactus Jack album * ''Discover'' (magazine), an American science magazine Businesses and brands * DISCover, the ''Digital Interactive Systems Corporation'' * D ...
, access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160716105822/http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ , archive-date=16 July 2016 , url-status=live
{{cite news , last=Allen , first=Kate , date=20 January 2016 , title=Is a Real Ninth Planet out There Beyond Pluto? , url=https://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html , newspaper=
Toronto Star The ''Toronto Star'' is a Canadian English-language broadsheet daily newspaper. The newspaper is the country's largest daily newspaper by circulation. It is owned by Toronto Star Newspapers Limited, a subsidiary of Torstar Corporation and part ...
, access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160417232029/http://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html , archive-date=17 April 2016 , url-status=live
{{cite journal , last1=Li , first1=Gongjie , last2=Hadden , first2=Samuel , last3=Payne , first3=Matthew , last4=Holman , first4=Matthew J. , title=The Secular Dynamics of TNOs and Planet Nine Interactions , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=2018 , volume=156 , issue=6 , page=263 , doi=10.3847/1538-3881/aae83b , arxiv=1806.06867, bibcode=2018AJ....156..263L , s2cid=118898729
{{cite journal , last1=Batygin , first1=Konstantin , last2=Morbidelli , first2=Alessandro , title=Dynamical Evolution Induced by Planet Nine , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=2017 , volume=154 , issue=6 , page=229 , doi=10.3847/1538-3881/aa937c , arxiv=1710.01804 , bibcode=2017AJ....154..229B, s2cid=119704953
{{cite web , last=Hruska , first=Joel , date=20 January 2016 , title=Our Solar System May Contain a Ninth Planet, Far beyond Pluto , url=http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto , website=
ExtremeTech ExtremeTech is a technology weblog, launched in June 2001, which focuses on hardware, computer software, science and other technologies. Between 2003 and 2005, ExtremeTech was also a print magazine and the publisher of a popular series of how-t ...
, access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160728074926/http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto , archive-date=28 July 2016 , url-status=live
{{cite web , title=MPC list of ''a'' > 250, ''i'' > 40, and ''q'' > 6 , url=http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 , website=
Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...
, access-date=4 February 2016 , archive-url=https://web.archive.org/web/20170802041218/http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 , archive-date=2 August 2017 , url-status=live
{{cite journal , last1=Batygin , first1=Konstantin , last2=Brown , first2=Michael E. , title=Generation of Highly Inclined Trans-Neptunian Objects by Planet Nine , date=2016 , journal=
The Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery H ...
, volume=833 , issue=1 , page=L3 , arxiv=1610.04992 , bibcode=2016ApJ...833L...3B , doi=10.3847/2041-8205/833/1/L3, s2cid=6751947
{{cite journal , last1=Brasser , first1=R. , last2=Schwamb , first2=M.E. , last3=Lykawka , first3=P.S. , last4=Gomes , first4=R.S. , title=An Oort Cloud Origin for the High-Inclination, High-Perihelion Centaurs , journal=
Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
, date=2012 , volume=420 , issue=4 , pages=3396–3402 , doi=10.1111/j.1365-2966.2011.20264.x , arxiv=1111.7037 , bibcode=2012MNRAS.420.3396B, s2cid=56403467
{{cite web , last1=Williams , first1=Matt , title=What is the Oort Cloud? , url=https://www.universetoday.com/32522/oort-cloud/ , website=Universe Today , date=10 August 2015 , access-date=25 February 2019 , archive-date=23 January 2018 , archive-url=https://web.archive.org/web/20180123211102/https://www.universetoday.com/32522/oort-cloud/ , url-status=live {{cite journal , last1=Köhne , first1=Tobias , last2=Batygin , first2=Konstantin , title=On the Dynamical Origins of Retrograde Jupiter Trojans and their Connection to High-Inclination TNOs , journal=Celestial Mechanics and Dynamical Astronomy , year=2020 , volume=132 , issue=9 , page=44 , doi=10.1007/s10569-020-09985-1 , arxiv=2008.11242, bibcode=2020CeMDA.132...44K , s2cid=221319701 {{cite journal, last1=Kaib , first1=Nathan A. , last2=Pike , first2=Rosemary , last3=Lawler , first3=Samantha , last4=Kovalik , first4=Maya , last5=Brown , first5=Christopher , last6=Alexandersen , first6=Mike , last7=Bannister , first7=Michele T. , last8=Gladman , first8=Brett J. , last9=Petit , first9=Jean-Marc , title=OSSOS XV: Probing the Distant Solar System with Observed Scattering TNOs , date=2019 , arxiv=1905.09286, doi=10.3847/1538-3881/ab2383 , journal=The Astronomical Journal , volume=158 , issue=1 , page=43 , pmid=31379385 , pmc=6677154 , bibcode=2019AJ....158...43K {{cite magazine , last=Levenson , first=Thomas , date=25 January 2016 , title=A New Planet or a Red Herring? , url=https://www.theatlantic.com/science/archive/2016/01/a-new-planet-or-a-red-herring/42681 , magazine=
The Atlantic ''The Atlantic'' is an American magazine and multi-platform publisher. It features articles in the fields of politics, foreign affairs, business and the economy, culture and the arts, technology, and science. It was founded in 1857 in Boston, ...
, access-date=18 July 2016 , quote={{-'We plotted the real data on top of the model' Batyagin recalls, and they fell 'exactly where they were supposed to be.' That was, he said, the epiphany. 'It was a dramatic moment. This thing I thought could disprove it turned out to be the strongest evidence for Planet Nine.{{'- , archive-date=19 October 2021 , archive-url=https://web.archive.org/web/20211019195840/https://www.theatlantic.com/politics/archive/2007/06/the-capitolist/42681/ , url-status=live
{{cite news , last=Grush , first=Loren , date=20 January 2016 , title=Our Solar System May Have a Ninth Planet After All – but Not All Evidence Is in (We Still Haven't Seen It Yet) , url=https://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence , newspaper=
The Verge ''The Verge'' is an American technology news website operated by Vox Media, publishing news, feature stories, guidebooks, product reviews, consumer electronics news, and podcasts. The website launched on November 1, 2011, and uses Vox Media' ...
, access-date=18 July 2016 , quote=The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. ... 'When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have,' says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. ... But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion—by researching observed anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University. , archive-url=https://web.archive.org/web/20160729141120/http://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence , archive-date=29 July 2016 , url-status=live
{{cite magazine , last=Crocket , first=Christopher , date=31 January 2016 , title=Computer Simulations Heat up Hunt for Planet Nine , url=https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine , magazine=
Science News ''Science News (SN)'' is an American bi-weekly magazine devoted to articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. History ''Science News'' has been published since 1 ...
, access-date=7 February 2016 , quote={{-'It's exciting and very compelling work,' says Meg Schwamb, a planetary scientist at Academia Sinica in Taipei, Taiwan. But only six bodies lead the way to the putative planet. 'Whether that's enough is still a question.{{'- , archive-url=https://web.archive.org/web/20160206211941/https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine , archive-date=6 February 2016 , url-status=live
{{cite news , date=22 January 2016 , title=We Can't See This Possible 9th Planet, but We Feel Its Presence , url=https://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ , newspaper=
PBS NewsHour ''PBS NewsHour'' is an American evening television news program broadcast on over 350 PBS The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virg ...
, access-date=18 July 2016 , quote={{-'Right now, any good scientist is going to be skeptical, because it's a pretty big claim. And without the final evidence that it's real, there is always that chance that it's not. So, everybody should be skeptical. But I think it's time to mount this search. I mean, we like to think of it as, we have provided the treasure map of where this ninth planet is, and we have done the starting gun, and now it's a race to actually point your telescope at the right spot in the sky and make that discovery of planet nine.' —Mike Brown , archive-url=https://web.archive.org/web/20160722042941/http://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ , archive-date=22 July 2016 , url-status=live
{{cite web , last=Fecht , first=Sarah , date=22 January 2016 , title=Can There Really Be a Planet in Our Solar System That We Don't Know About? , url=http://www.popsci.com/could-there-really-be-9th-planet-that-we-didnt-know-about , website=
Popular Science ''Popular Science'' (also known as ''PopSci'') is an American digital magazine carrying popular science content, which refers to articles for the general reader on science and technology subjects. ''Popular Science'' has won over 58 awards, incl ...
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{{cite web , author-link=Ethan Siegel , last1=Siegel , first1=Ethan , date=20 January 2016 , title=Not So Fast: Why There Likely Isn't A Large Planet Beyond Pluto , url=https://www.forbes.com/sites/startswithabang/2016/01/20/not-so-fast-why-there-likely-isnt-a-large-planet-beyond-pluto/#749e30245af4 , website=
Forbes ''Forbes'' () is an American business magazine owned by Integrated Whale Media Investments and the Forbes family. Published eight times a year, it features articles on finance, industry, investing, and marketing topics. ''Forbes'' also re ...
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Forbes ''Forbes'' () is an American business magazine owned by Integrated Whale Media Investments and the Forbes family. Published eight times a year, it features articles on finance, industry, investing, and marketing topics. ''Forbes'' also re ...
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{{cite news , author-link=Ethan Siegel , last1=Siegel , first1=Ethan , date=14 September 2018 , title=This Is Why Most Scientists Think Planet Nine Doesn't Exist , url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/ , work=
Forbes ''Forbes'' () is an American business magazine owned by Integrated Whale Media Investments and the Forbes family. Published eight times a year, it features articles on finance, industry, investing, and marketing topics. ''Forbes'' also re ...
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Sky & Telescope ''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following: *current events in astronomy and space exploration; *events in the amateur astronomy community; *reviews of astronomic ...
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{{Cite journal , doi = 10.3847/1538-4365/ab6bd8, title = Trans-Neptunian Objects Found in the First Four Years of the Dark Energy Survey, year = 2020, last1 = Bernardinelli, first1 = Pedro H., last2 = Bernstein, first2 = Gary M., last3 = Sako, first3 = Masao, last4 = Liu, first4 = Tongtian, last5 = Saunders, first5 = William R., last6 = Khain, first6 = Tali, last7 = Lin, first7 = Hsing Wen, last8 = Gerdes, first8 = David W., last9 = Brout, first9 = Dillon, last10 = Adams, first10 = Fred C., last11 = Belyakov, first11 = Matthew, last12 = Somasundaram, first12 = Aditya Inada, last13 = Sharma, first13 = Lakshay, last14 = Locke, first14 = Jennifer, last15 = Franson, first15 = Kyle, last16 = Becker, first16 = Juliette C., last17 = Napier, first17 = Kevin, last18 = Markwardt, first18 = Larissa, last19 = Annis, first19 = James, last20 = Abbott, first20 = T. M. C., last21 = Avila, first21 = S., last22 = Brooks, first22 = D., last23 = Burke, first23 = D. L., last24 = Rosell, first24 = A. Carnero, last25 = Kind, first25 = M. Carrasco, last26 = Castander, first26 = F. J., last27 = Costa, first27 = L. N. da, last28 = Vicente, first28 = J. De, last29 = Desai, first29 = S., last30 = Diehl, first30 = H. T., s2cid = 202537605, journal = The Astrophysical Journal Supplement Series, volume = 247, issue = 1, page = 32, arxiv = 1909.01478, bibcode = 2020ApJS..247...32B, display-authors = 29 {{Cite web, url=https://www.sciencealert.com/astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system, title=Many Astronomers Now Think Planet Nine Might Not Exist After All, Here's Why, first=Samantha Lawler, The, last=Conversation, website=ScienceAlert, date=26 May 2020 , access-date=27 May 2020, archive-date=30 May 2020, archive-url=https://web.archive.org/web/20200530185950/https://www.sciencealert.com/astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system, url-status=live {{Cite web, url=http://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598, title=Why astronomers now doubt there is an undiscovered 9th planet in our solar system, first=Samantha, last=Lawler, website=The Conversation, access-date=19 October 2021, archive-date=7 September 2021, archive-url=https://web.archive.org/web/20210907121445/https://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598, url-status=live {{Cite web, url=https://www.universetoday.com/146283/maybe-the-elusive-planet-9-doesnt-exist-after-all/, title=Maybe the Elusive Planet 9 Doesn't Exist After All, date=29 May 2020, access-date=9 June 2020, archive-date=9 June 2020, archive-url=https://web.archive.org/web/20200609023753/https://www.universetoday.com/146283/maybe-the-elusive-planet-9-doesnt-exist-after-all/, url-status=live {{cite journal , last1=Shankman , first1=Cory , display-authors=etal , title=OSSOS. VI. Striking Biases in the Detection of Large Semimajor Axis Trans-Neptunian Objects , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
, date=2017 , volume=154 , issue=2 , page=50 , doi=10.3847/1538-3881/aa7aed , arxiv=1706.05348 , bibcode=2017AJ....154...50S , url=https://pure.qub.ac.uk/portal/en/publications/ossos-vi-striking-biases-in-the-detection-of-large-semimajor-axis-transneptunian-objects(027b7a9c-a171-42b4-9449-9355b0a255d4).html , hdl=10150/625487 , s2cid=3535702 , access-date=3 September 2018 , archive-date=14 January 2019 , archive-url=https://web.archive.org/web/20190114092120/https://pure.qub.ac.uk/portal/en/publications/ossos-vi-striking-biases-in-the-detection-of-large-semimajor-axis-transneptunian-objects(027b7a9c-a171-42b4-9449-9355b0a255d4).html , url-status=live
{{cite web , last1=Siegel , first1=Ethan , title=This Is Why Most Scientists Think Planet Nine Doesn't Exist , url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/#cd530456e77c , department=Starts with a Bang , website=
Forbes ''Forbes'' () is an American business magazine owned by Integrated Whale Media Investments and the Forbes family. Published eight times a year, it features articles on finance, industry, investing, and marketing topics. ''Forbes'' also re ...
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{{cite web , last1=Ratner , first1=Paul , title=New study deepens the controversy over Planet Nine's existence , url=https://bigthink.com/surprising-science/planet-nine , website=Big Think , access-date=25 April 2020 , archive-date=30 April 2020 , archive-url=https://web.archive.org/web/20200430165959/https://bigthink.com/surprising-science/planet-nine , url-status=live {{cite journal , last1=Bernardelli , first1=Pedro , display-authors= etal , title=Testing the isotropy of the Dark Energy Survey's extreme trans-Neptunian objects , journal=The Planetary Science Journal , year=2020 , volume=1 , issue=2 , page=28 , doi=10.3847/PSJ/ab9d80 , arxiv=2003.08901, bibcode=2020PSJ.....1...28B , s2cid=213175490 {{cite journal , last1=Napier , first1=J. K. , display-authors=etal , title=No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects , journal=The Planetary Science Journal , year=2021 , volume=2 , issue=2 , page=59 , doi=10.3847/PSJ/abe53e , arxiv=2102.05601, bibcode=2021PSJ.....2...59N , s2cid=231861808 {{cite journal , last1=Brown , first1=Michael E. , title=Observational Bias and the Clustering of Distant Eccentric Kuiper Belt Objects , date=2017 , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite journal , last1=Brown , first1=Michael E. , last2=Batygin , first2=Konstantin , title=Orbital Clustering in the Distant Solar System , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite web , last=Choi , first=Charles Q. , date=25 October 2016 , title=Closing in on a Giant Ghost Planet , url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ , website=
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
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The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite journal , last1=Madigan , first1=Ane-Marie , last2=McCourt , first2=Michael , title=A New Inclination Instability Reshapes Keplerian Discs into Cones: Application to the Outer Solar System , journal= Monthly Notices of the Royal Astronomical Society: Letters , date=2016 , volume=457 , issue=1 , pages=L89–93 , doi=10.1093/mnrasl/slv203 , arxiv=1509.08920 , bibcode=2016MNRAS.457L..89M, s2cid=119181329 {{cite journal , last1=Madigan , first1=Ann-Marie , last2=Zderic , first2=Alexander , last3=McCourt , first3=Michael , last4=Fleisig , first4=Jacob , title=On the Dynamics of the Inclination Instability , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite web , last=Wall , first=Mike , date=4 February 2016 , title='Planet Nine'? Cosmic objects' strange orbits may have a different explanation , url=http://www.space.com/31817-planet-nine-existence-question.html , website=
Space.com Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
, quote=We need more mass in the outer solar system," she (Madigan) said. "So it can either come from having more minor planets, and their self-gravity will do this to themselves naturally, or it could be in the form of one single massive planet—a Planet Nine. So it's a really exciting time, and we're going to discover one or the other., access-date=8 February 2016 , archive-url=https://web.archive.org/web/20160208095024/http://www.space.com/31817-planet-nine-existence-question.html , archive-date=8 February 2016 , url-status=live
{{cite web , last=Snell , first=Jason , date=5 February 2016 , title=This Week in Space: Weird Pluto and No Plan for Mars , url=https://www.yahoo.com/tech/this-week-in-space-weird-pluto-and-no-plan-for-183233179.html , website=
Yahoo! Tech Yahoo! Tech is a technology news web site operated by Yahoo!. Former Yahoo! Tech The site, which was the first new product from the Santa Monica, California-based Yahoo! Media Group, featured a selection of original, licensed, and user-generated ...
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{{cite journal , last1=Fan , first1=Siteng , last2=Batygin , first2=Konstantin , title=Simulations of the Solar System's early dynamical evolution with a self-gravitating planetesimal disk , journal=
The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ...
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{{Cite web, url=https://www.scientificamerican.com/article/planet-nine-could-be-a-mirage/, title=Planet Nine Could Be a Mirage, first=Ramin, last=Skibba, website=Scientific American, access-date=7 May 2020, archive-date=7 May 2020, archive-url=https://web.archive.org/web/20200507025819/https://www.scientificamerican.com/article/planet-nine-could-be-a-mirage/, url-status=live {{cite journal , last1=Zderic , first1=Alexander , last2=Collier , first2=Angela , last3=Tiongco , first3=Maria , last4=Madigan , first4=Ann-Marie , title=Apsidal Clustering following the Inclination Instability , journal=The Astrophysical Journal , date=2020 , volume=895 , issue=2 , pages=L27 , doi=10.3847/2041-8213/ab91a0 , arxiv=2004.01198, bibcode=2020ApJ...895L..27Z , s2cid=214794969 {{cite journal , last1=Zderic , first1=Alexander , last2=Madigan , first2=Ann-Marie , title=Giant Planet Influence on the Collective Gravity of a Primordial Scattered Disk , journal=The Astronomical Journal , date=2020 , volume=160 , issue=1 , page=50 , doi=10.3847/1538-3881/ab962f , arxiv=2004.00037, bibcode=2020AJ....160...50Z , s2cid=214743005 {{cite journal , last1=Zderic , first1=Alexander , last2=Tiongco , first2=Maria , last3=Collier , first3=Angela , last4=Wernke , first4=Heather , last5=Generozov , first5=Aleksey , last6=Madigan , first6=Ann-Marie , title=A Lopsided Outer Solar System? , journal=The Astronomical Journal , year=2021 , volume=162 , issue=6 , page=278 , doi=10.3847/1538-3881/ac2def , arxiv=2106.09739, bibcode=2021AJ....162..278Z , s2cid=235485138 {{cite journal , last1=Sefilian , first1=Antranik A. , last2=Touma , first2=Jihad R. , title=Shepherding in a Self-gravitating Disk of Trans-Neptunian Objects , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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Popular Science ''Popular Science'' (also known as ''PopSci'') is an American digital magazine carrying popular science content, which refers to articles for the general reader on science and technology subjects. ''Popular Science'' has won over 58 awards, incl ...
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Gizmodo ''Gizmodo'' ( ) is a design, technology, science and science fiction website. It was originally launched as part of the Gawker Media network run by Nick Denton, and runs on the Kinja platform. ''Gizmodo'' also includes the subsite ''io9'', whic ...
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The Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery H ...
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{{cite web , last1=Malhotra , first1=Renu , title=The search for Planet Nine , date=15 April 2018 , url=https://www.youtube.com/watch?v=MptrypvBTag , via=
YouTube YouTube is a global online video platform, online video sharing and social media, social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by ...
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Science News ''Science News (SN)'' is an American bi-weekly magazine devoted to articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. History ''Science News'' has been published since 1 ...
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Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
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Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
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Universe Today Universe Today (U.T.) is a popular North American-based non-commercial space and astronomy news website. The domain was registered on December 30, 1998, and the website went live in March 1999, founded by Canadian Fraser Cain. The ''Universe Today ...
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{{cite web , last1=O'Connor , first1=J.J. , last2=Robertson , first2=E.F. , title=Alexis Bouvard , url=http://www-history.mcs.st-and.ac.uk/Biographies/Bouvard.html , website=
MacTutor History of Mathematics archive The MacTutor History of Mathematics archive is a website maintained by John J. O'Connor and Edmund F. Robertson and hosted by the University of St Andrews in Scotland. It contains detailed biographies on many historical and contemporary mathemati ...
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{{cite journal , last1=de la Fuente Marcos , first1=Carlos , last2=de la Fuente Marcos , first2=Raúl , year=2014 , title=Extreme Trans-Neptunian Objects and the Kozai Mechanism: Signalling the Presence of Trans-Plutonian Planets , journal= Monthly Notices of the Royal Astronomical Society Letters , volume=443 , issue=1 , pages=L59–L63 , arxiv=1406.0715 , bibcode=2014MNRAS.443L..59D , doi=10.1093/mnrasl/slu084, s2cid=118622180 {{cite magazine , last=Lemonick , first=Michael D. , date=19 January 2015 , title=There May Be 'Super Earths' at the Edge of Our Solar System , url=http://time.com/3673527/super-earths-possibly-discovered/ , magazine=
Time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...
, access-date=7 February 2016 , archive-url=https://web.archive.org/web/20160128125505/http://time.com/3673527/super-earths-possibly-discovered/ , archive-date=28 January 2016 , url-status=live
{{cite journal , last1 = de la Fuente Marcos , first1 = Carlos , last2 = de la Fuente Marcos , first2 = Raúl , last3 = Aarseth , first3 = Sverre J. , year = 2015 , title = Flipping Minor Bodies: What Comet 96P/Machholz 1 Can Tell Us About the Orbital Evolution of Extreme Trans-Neptunian Objects and the Production of Near-Earth Objects on Retrograde Orbits , journal =
Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
, volume = 446 , issue = 2 , pages = 1867–187 , arxiv = 1410.6307 , bibcode = 2015MNRAS.446.1867D , doi = 10.1093/mnras/stu2230 , s2cid = 119256764
{{cite web , last=Atkinson , first=Nancy , date=15 January 2015 , title=Astronomers Are Predicting at Least Two More Large Planets in the Solar System , url=http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ , website=
Universe Today Universe Today (U.T.) is a popular North American-based non-commercial space and astronomy news website. The domain was registered on December 30, 1998, and the website went live in March 1999, founded by Canadian Fraser Cain. The ''Universe Today ...
, access-date=7 February 2016 , archive-url=https://web.archive.org/web/20160206145105/http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ , archive-date=6 February 2016 , url-status=live
{{Cite journal, last1=Scholtz, first1=Jakub, last2=Unwin, first2=James, date=29 July 2020, title=What if Planet 9 is a Primordial Black Hole?, journal=Physical Review Letters, volume=125, issue=5, pages=051103, doi=10.1103/PhysRevLett.125.051103, pmid=32794880, arxiv=1909.11090, bibcode=2020PhRvL.125e1103S, issn=0031-9007, doi-access=free {{cite news , last=Overbye , first=Dennis , author-link=Dennis Overbye , title=Is There a Black Hole in Our Backyard? – Astrophysicists have recently begun hatching plans to find out just how weird Planet Nine might be. , url=https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html , date=11 September 2020 , work=
The New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid ...
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{{Cite web , last=Parks , first=Jake , title=Planet Nine may be a black hole the size of a baseball , url=https://astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball , date=1 October 2019 , work=
Astronomy magazine ''Astronomy'' is a monthly United States, American magazine about astronomy. Targeting amateur astronomy, amateur astronomers, it contains columns on sky viewing, reader-submitted astrophotography, astrophotographs, and articles on astronomy and ...
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{{Cite web, first=Rafi, last=Letzter, title=Renowned string theorist proposes new way to hunt our solar system's mysterious 'Planet 9', url=https://www.livescience.com/planet-9-laser-breakthrough-starshot-ed-witten.html, date=May 2020, access-date=12 November 2020, website=livescience.com, language=en, archive-date=12 November 2020, archive-url=https://web.archive.org/web/20201112192454/https://www.livescience.com/planet-9-laser-breakthrough-starshot-ed-witten.html, url-status=live {{Cite journal, last1=Hoang, first1=Thiem, last2=Loeb, first2=Abraham, date=2020-05-29, title=Can Planet Nine Be Detected Gravitationally by a Subrelativistic Spacecraft?, journal=The Astrophysical Journal, language=en, volume=895, issue=2, pages=L35, doi=10.3847/2041-8213/ab92a7, arxiv=2005.01120, bibcode=2020ApJ...895L..35H, issn=2041-8213, doi-access=free {{Cite news, last=Overbye, first=Dennis, date=2020-09-11, title=Is There a Black Hole in Our Backyard?, language=en-US, work=The New York Times, url=https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html, access-date=2020-11-12, issn=0362-4331, archive-date=11 September 2020, archive-url=https://web.archive.org/web/20200911143517/https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html, url-status=live {{Cite journal, last1=Siraj, first1=Amir, last2=Loeb, first2=Abraham, date=2020-07-16, title=Searching for Black Holes in the Outer Solar System with LSST, url=https://iopscience.iop.org/article/10.3847/2041-8213/aba119, journal=The Astrophysical Journal, language=en, volume=898, issue=1, pages=L4, doi=10.3847/2041-8213/aba119, issn=2041-8213, arxiv=2005.12280, bibcode=2020ApJ...898L...4S, s2cid=218889510, access-date=12 November 2020, archive-date=16 September 2021, archive-url=https://web.archive.org/web/20210916224806/https://iopscience.iop.org/article/10.3847/2041-8213/aba119, url-status=live {{cite magazine , date=24 July 2006 , title=What is the faintest object imaged by ground-based telescopes? , url=http://www.skyandtelescope.com/astronomy-resources/astronomy-questions-answers/what-is-the-faintest-object-imaged-by-ground-based-telescopes/ , magazine=
Sky & Telescope ''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following: *current events in astronomy and space exploration; *events in the amateur astronomy community; *reviews of astronomic ...
, access-date=18 July 2016 , archive-date=2 February 2016 , archive-url=https://web.archive.org/web/20160202021559/http://www.skyandtelescope.com/astronomy-resources/astronomy-questions-answers/what-is-the-faintest-object-imaged-by-ground-based-telescopes/ , url-status=live
{{cite web , last1=Illingworth , first1=G. , last2=Magee , first2=D. , last3=Oesch , first3=P. , last4=Bouwens , first4=R. , date=25 September 2012 , title=Hubble goes to the extreme to assemble the deepest ever view of the universe , url=https://www.spacetelescope.org/news/heic1214/ , website=
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
, access-date=7 February 2016 , archive-url=https://web.archive.org/web/20160201104727/https://www.spacetelescope.org/news/heic1214/ , archive-date=1 February 2016 , url-status=live
{{cite journal , first=Eric , last=Hand , date=20 January 2016 , title=Astronomers say a Neptune-sized planet lurks beyond Pluto , url=http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system , journal=
Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence for ...
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{{cite web , author=Deep Astronomy , date=19 February 2016 , title=Ninth Planet Beyond Neptune? , via=
YouTube YouTube is a global online video platform, online video sharing and social media, social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by ...
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{{cite web , url=https://phys.org/news/2019-02-planet.html , title=More support for Planet Nine , publisher=Phys.org , date=27 February 2019 , access-date=26 June 2019 , archive-date=26 June 2019 , archive-url=https://web.archive.org/web/20190626032137/https://phys.org/news/2019-02-planet.html , url-status=live {{Cite web , url=https://www.techradar.com/news/are-we-getting-closer-to-finding-planet-nine , title=Are we getting closer to finding 'Planet Nine'? , department=Future tech , first=Jamie , last=Carter , date=2019-03-25 , website=TechRadar , language=en , access-date=2019-05-14 , archive-date=14 May 2019 , archive-url=https://web.archive.org/web/20190514091808/https://www.techradar.com/news/are-we-getting-closer-to-finding-planet-nine , url-status=live {{cite web, url=https://earthsky.org/space/review-paper-physics-reports-planet-9-feb-2019, title=Planet 9 hypothesis gets a boost, author=Paul Scott Anderson, publisher=EarthSky, date=3 March 2019, access-date=26 June 2019, archive-date=26 June 2019, archive-url=https://web.archive.org/web/20190626032132/https://earthsky.org/space/review-paper-physics-reports-planet-9-feb-2019, url-status=live {{cite journal , last1=Brown , first1=Michael E. , last2=Batygin , first2=Konstantin , title=A search for Planet Nine using the Zwicky Transient Facility public archive , journal=The Astronomical Journal , date=31 January 2022 , volume=163 , issue=2 , page=102 , arxiv=2110.13117 , bibcode=2022AJ....163..102B , doi=10.3847/1538-3881/ac32dd , s2cid=239768690 {{cite web , title=A Friend For Pluto: Astronomers Find New Dwarf Planet in Our Solar System , publisher=
NPR National Public Radio (NPR, stylized in all lowercase) is an American privately and state funded nonprofit media organization headquartered in Washington, D.C., with its NPR West headquarters in Culver City, California. It differs from other ...
, first=Joe , last=Palka , website=NPR.org , url=https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system , access-date=5 April 2018 , archive-url=https://web.archive.org/web/20180405032925/https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system , archive-date=5 April 2018 , url-status=live
{{cite web , last=Hall , first=Shannon , date=20 April 2016 , title=We Are Closing in on Possible Whereabouts of Planet Nine , url=https://www.newscientist.com/article/2084924-we-are-closing-in-on-possible-whereabouts-of-planet-nine/ , website=
New Scientist ''New Scientist'' is a magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organisation publishe ...
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{{cite web , last1=Gibbs , first1=W. Wayt , title=Is There a Giant Planet Lurking Beyond Pluto? , url=https://spectrum.ieee.org/aerospace/satellites/is-there-a-giant-planet-lurking-beyond-pluto , website=
IEEE Spectrum ''IEEE Spectrum'' is a magazine edited by the Institute of Electrical and Electronics Engineers. The first issue of ''IEEE Spectrum'' was published in January 1964 as a successor to ''Electrical Engineering''. The magazine contains peer-revie ...
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{{cite journal , last1=Meisner , first1=Aaron M. , last2=Bromley , first2=Benjamin B. , last3=Nugent , first3=Peter E. , last4=Schlegel , first4=David J , last5=Kenyon , first5=Scott J. , last6=Schlafly , first6=Edward F. , last7=Dawson , first7=Kyle S. , title=Searching for Planet Nine with Coadded WISE and NEOWISE-Reactivation Images , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite web , first=Nicole , last=Mortillaro , date=9 February 2016 , title=Meet Mike Brown: Pluto Killer and the Man Who Brought Us Planet 9 , url=http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ , website=
Global News Global News is the news and current affairs division of the Canadian Global Television Network. The network is owned by Corus Entertainment, which oversees all of the network's national news programming as well as local news on its 21 owned-and ...
, quote={{-'It was that search for more objects like Sedna ... led to the realization ... that they're all being pulled off in one direction by something. And that's what finally led us down the hole that there must be a big planet out there.' —Mike Brown , access-date=10 February 2016 , archive-url=https://web.archive.org/web/20160210095238/http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ , archive-date=10 February 2016 , url-status=live
{{cite web , last1=Wall , first1=Mike , date=21 January 2016 , title=How Astronomers Could Actually See 'Planet Nine' , url=http://www.space.com/31677-astronomers-could-see-planet-nine.html , website=
Space.com Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
, access-date=24 January 2016 , archive-url=https://web.archive.org/web/20160123220605/http://www.space.com/31677-astronomers-could-see-planet-nine.html , archive-date=23 January 2016 , url-status=live
{{cite magazine , first=Christopher , last=Crockett , date=5 July 2016 , title=New Clues in Search for Planet Nine , url=https://www.sciencenews.org/article/new-clues-search-planet-nine , magazine=
Science News ''Science News (SN)'' is an American bi-weekly magazine devoted to articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. History ''Science News'' has been published since 1 ...
, access-date=6 July 2016 , archive-url=https://web.archive.org/web/20160705130303/https://www.sciencenews.org/article/new-clues-search-planet-nine , archive-date=5 July 2016 , url-status=live
{{cite web , last=Choi , first=Charles C. , url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ , title=Closing in on a Giant Ghost Planet , website=
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
, date=25 October 2016 , access-date=26 October 2016 , archive-url=https://web.archive.org/web/20161026023621/https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ , archive-date=26 October 2016 , url-status=live
{{cite web , last1=Stirone , first1=Shannon , title=The Hunt for Planet Nine , url=https://longreads.com/2019/01/22/the-hunt-for-planet-nine/ , website=Longreads , date=22 January 2019 , access-date=22 January 2019 , archive-date=22 January 2019 , archive-url=https://web.archive.org/web/20190122200521/https://longreads.com/2019/01/22/the-hunt-for-planet-nine/ , url-status=live {{Cite arXiv, eprint = 2004.14980, last1 = Lawrence, first1 = Scott, last2 = Rogoszinski, first2 = Zeeve, title = The Brute-Force Search for Planet Nine, year = 2020, class = astro-ph.EP {{cite journal , last1=Linder , first1=Esther F. , last2=Mordasini , first2=Christoph , date=2016 , title=Evolution and Magnitudes of Candidate Planet Nine , journal=
Astronomy & Astrophysics ''Astronomy & Astrophysics'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a re ...
, volume=589 , issue= 134, pages=A134 , arxiv=1602.07465 , bibcode=2016A&A...589A.134L , doi=10.1051/0004-6361/201628350, s2cid=53702941
{{cite magazine , last=Powel , first=Corey S. , date=22 January 2016 , title=A Little Perspective on the New "9th Planet" (and the 10th, and the 11th) , url=http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/#.VqYOnZp97iw , magazine=
Discover Discover may refer to: Art, entertainment, and media * ''Discover'' (album), a Cactus Jack album * ''Discover'' (magazine), an American science magazine Businesses and brands * DISCover, the ''Digital Interactive Systems Corporation'' * D ...
, access-date=18 July 2016 , archive-url=https://web.archive.org/web/20160714182831/http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/#.VqYOnZp97iw#.VqYOnZp97iw , archive-date=14 July 2016 , url-status=live
{{cite journal , last1=Cowan , first1=Nicolas B. , last2=Holder , first2=Gil , last3=Kaib , first3=Nathan A. , date=2016 , title=Cosmologists in Search of Planet Nine: the Case for CMB Experiments , journal=
The Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery H ...
, volume=822 , issue=1 , pages=L2 , arxiv=1602.05963 , bibcode=2016ApJ...822L...2C , doi=10.3847/2041-8205/822/1/L2, s2cid=119308822
{{cite web , last1=Aron , first1=Jacob , date=24 February 2016 , title=Planet Nine Hunters Enlist Big Bang Telescopes and Saturn Probe , url=https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ , website=
New Scientist ''New Scientist'' is a magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organisation publishe ...
, access-date=27 February 2016 , archive-url=https://web.archive.org/web/20160225105639/https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ , archive-date=25 February 2016 , url-status=live
{{cite news , title=Is there a mysterious Planet Nine lurking in our Solar system beyond Neptune? , newspaper=
The Washington Post ''The Washington Post'' (also known as the ''Post'' and, informally, ''WaPo'') is an American daily newspaper published in Washington, D.C. It is the most widely circulated newspaper within the Washington metropolitan area and has a large nati ...
, first=Charlie , last=Wood , date=2 September 2018 , url=https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html , access-date=17 January 2019 , archive-url=https://web.archive.org/web/20180902220558/https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html , archive-date=2 September 2018 , url-status=live
{{cite web , last=Kohler , first=Susanna , date=25 April 2016 , title=Can CMB experiments find Planet Nine? , url=http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ , website=AAS Nova , publisher=
American Astronomical Society The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC. The primary objective of the AAS is to promote the adv ...
, access-date=29 April 2016 , archive-url=https://web.archive.org/web/20160531185715/http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ , archive-date=31 May 2016 , url-status=live
{{cite journal , last1=Naess , first1=Sigurd , display-authors=et al , title=The Atacama Cosmology Telescope: A Search for Planet 9 , journal=The Astrophysical Journal , date=2021 , volume=923 , issue=2 , page=224 , doi=10.3847/1538-4357/ac2307 , arxiv=2104.10264, bibcode=2021ApJ...923..224N , s2cid=233324478 {{cite news , url=http://earthsky.org/space/help-astronomers-look-for-planet-9 , title=Help Astronomers Look for Planet 9 , work=
EarthSky ''Earth & Sky'' was a daily radio series that presented information about science and nature. It began broadcasting in 1991 and ceased operations in 2013. ''EarthSky'' is the ongoing website, serving 21 million users in 2019, according to Google ...
, first1=Deborah , last1=Byrd , first2=Eleanor , last2=Imster , date=20 February 2017 , access-date=9 April 2017 , archive-url=https://web.archive.org/web/20170410062214/http://earthsky.org/space/help-astronomers-look-for-planet-9 , archive-date=10 April 2017 , url-status=live
{{cite news , url=http://www.csmonitor.com/Science/Spacebound/2017/0217/Hunt-for-Planet-9-how-you-can-help-NASA-search-for-brown-dwarfs-and-low-mass-stars , title=Hunt for Planet 9: How You Can Help NASA Search for Brown Dwarfs and Low-Mass Stars , work=
The Christian Science Monitor ''The Christian Science Monitor'' (''CSM''), commonly known as ''The Monitor'', is a nonprofit news organization that publishes daily articles in electronic format as well as a weekly print edition. It was founded in 1908 as a daily newspaper ...
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{{cite news , url=http://earthsky.org/space/anu-citizen-science-search-for-planet-9 , title=Another Planet 9 Search! You Can Help , work=
EarthSky ''Earth & Sky'' was a daily radio series that presented information about science and nature. It began broadcasting in 1991 and ceased operations in 2013. ''EarthSky'' is the ongoing website, serving 21 million users in 2019, according to Google ...
, first=Deborah , last=Byrd , date=27 March 2017 , access-date=8 April 2017 , archive-url=https://web.archive.org/web/20170409131625/http://earthsky.org/space/anu-citizen-science-search-for-planet-9 , archive-date=9 April 2017 , url-status=live
{{cite news , url=http://www.space.com/36322-planet-nine-candidates-citizen-science.html , title=Where's Planet Nine? Citizen Scientists Spot 4 Possible Candidates , work=
Space.com Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
, first=Mike , last=Wall , date=3 April 2017 , access-date=8 April 2017 , archive-url=https://web.archive.org/web/20170409112829/http://www.space.com/36322-planet-nine-candidates-citizen-science.html , archive-date=9 April 2017 , url-status=live
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The Sydney Morning Herald ''The Sydney Morning Herald'' (''SMH'') is a daily compact newspaper published in Sydney, New South Wales, Australia, and owned by Nine. Founded in 1831 as the ''Sydney Herald'', the ''Herald'' is the oldest continuously published newspaper ...
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{{Cite web, title=Catalina Outer Solar System Survey – About, url=https://www.zooniverse.org/projects/fulsdavid/catalina-outer-solar-system-survey/about/faq, access-date=2020-09-01, website=Catalina Outer Solar System Survey, archive-date=17 September 2021, archive-url=https://web.archive.org/web/20210917122910/https://www.zooniverse.org/projects/fulsdavid/catalina-outer-solar-system-survey/about/faq, url-status=live {{Cite web, date=2020-08-11, title=Comb the Edges of the Solar System with the Catalina Outer Solar System Survey, url=https://science.nasa.gov/citizenscience/news/catalina-outer-solar-system-survey, access-date=2020-09-01, website=NASA Science, archive-date=29 September 2020, archive-url=https://web.archive.org/web/20200929165411/https://science.nasa.gov/citizenscience/news/catalina-outer-solar-system-survey, url-status=live {{cite journal , last1=Fienga , first1=A. , last2=Laskar , first2=J. , last3=Manche , first3=H. , last4=Gastineau , first4=M. , date=2016 , title=Constraints on the Location of a Possible 9th Planet Derived from the Cassini Data , journal=
Astronomy & Astrophysics ''Astronomy & Astrophysics'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a re ...
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{{cite journal , first1=Carlos , last1=de la Fuente Marcos , first2=Raúl , last2=de la Fuente Marcos , year=2016 , journal= Monthly Notices of the Royal Astronomical Society Letters , volume=459 , issue=1 , pages=L66–L70 , title=Finding Planet Nine: a Monte Carlo Approach , arxiv=1603.06520 , bibcode=2016MNRAS.459L..66D , doi=10.1093/mnrasl/slw049, s2cid=118433545 {{cite journal , last1=de la Fuente Marcos , first1=Carlos , last2=de la Fuente Marcos , first2=Raúl , year=2016 , title=Finding Planet Nine: Apsidal Anti-Alignment Monte Carlo Results , journal=
Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...
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{{cite journal , last1=Holman , first1=Matthew J. , last2=Payne , first2=Matthew J. , title=Observational Constraints on Planet Nine: Cassini Range Observations , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite web , date=8 April 2016 , title=Saturn Spacecraft Not Affected by Hypothetical Planet 9 , url=http://www.jpl.nasa.gov/news/news.php?feature=6200 , website=
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
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Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
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{{cite journal , last1=Holman , first1=Matthew J. , last2=Payne , first2=Matthew J. , date=9 September 2016 , title=Observational Constraints on Planet Nine: Astrometry of Pluto and Other Trans-Neptunian Objects , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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Astronomy Letters ''Astronomy Letters'' (Russian: ''Pis’ma v Astronomicheskii Zhurnal'') is a Russian peer-reviewed scientific journal. The journal covers research on all aspects of astronomy and astrophysics, including high energy astrophysics, cosmology, spa ...
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{{cite journal , last1=Rice , first1=Malena , last2=Laughlin , first2=Gregory , title=The Case for a Large-Scale Occultation Network , journal=The Astronomical Journal , volume=158 , issue=1 , pages=19 , date=2019 , arxiv=1905.06354, doi=10.3847/1538-3881/ab21df , bibcode=2019AJ....158...19R , s2cid=155099837 {{cite journal , last1=Millholland , first1=Sarah , last2=Laughlin , first2=Gregory , title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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{{cite journal , last1=Millholland , first1=Sarah , last2=Laughlin , first2=Gregory , title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances , journal=
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...
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GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continuous ...
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Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
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{{Citation, author=Lemonick, M. D., year=2016, title=Planet Nine from Outer Space, journal=Scientific American, volume=314, issue=5, page=36, doi=10.1038/scientificamerican0516-36, pmid=27100252, bibcode=2016SciAm.314e..36L {{Citation, title=Planet Nine from Outer Space, author=Batygin, Konstantin, journal = American Astronomical Society Meeting Abstracts , year=2017, volume = 230, pages = 211.01, bibcode=2017AAS...23021101B {{Citation, title=Planet Nine from Outer Space, author1=Batygin, Konstantin, author2=Brown, Michael, journal=42nd Cospar Scientific Assembly, year=2018, volume=42, pages=PIR.1–14–18, bibcode=2018cosp...42E.229B {{Citation, title=Planet Nine from Outer Space, author=Brown, Mike, publisher=Caltech Astro, date=15 March 2019, access-date=8 April 2019, url=https://www.youtube.com/watch?v=Zw5MDh_wpnI?t=1, archive-date=28 December 2020, archive-url=https://web.archive.org/web/20201228100238/https://www.youtube.com/watch?v=Zw5MDh_wpnI, url-status=live {{cite web, title = Planet X Marks the Spot, work = TechRepublic, date = 2006, url = http://www.sunflower-astronomy.com/KCKCC_Docs/NewsArticles/Geek_Trivia_Planet_X_marks_the_spot.pdf, access-date = 13 July 2008, archive-url = https://web.archive.org/web/20080910074246/http://www.sunflower-astronomy.com/KCKCC_Docs/NewsArticles/Geek_Trivia_Planet_X_marks_the_spot.pdf, archive-date = 10 September 2008, url-status = live {{cite web , url=http://www.businessinsider.com/planet-nine-x-name-argument-2018-6 , title = Is It Planet 9 or Planet X? Scientists Spar over What to Call the Solar System's Hypothetical Missing World , website =
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The Planetary Science Journal ''The Planetary Science Journal'' is a peer-reviewed open access scientific journal of astrophysics and astronomy, established in 2020. It is published by IOP Publishing on behalf of the American Astronomical Society. The founding editor-in-ch ...
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External links

* {{Commons category-inline
The Search for Planet Nine
– Blog by Brown and Batygin
Hypothetical Planet X
– NASA Planetary Science Division {{Solar system {{2016 in space {{Portal bar, Astronomy, Outer space, Solar system, Physics 2016 in space Astronomical events of the Solar System Hypothetical planets Hypothetical trans-Neptunian objects