planetary-mass object
A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...
that is in direct orbit of the Sun, smaller than any of the eight classical
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s but still a world in its own right. The prototypical dwarf planet is
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 ...
. The interest of dwarf planets to planetary geologists is that they may be geologically active bodies, an expectation that was borne out in 2015 by the ''
Dawn
Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's horizo ...
'' mission to and the ''
New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' mission to
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 ...
.
Astronomers are in general agreement that at least the nine largest candidates are dwarf planets: Pluto, , , , , , , , and . Of these and the tenth-largest candidate , all but Sedna have either been visited by spacecraft (Pluto and Ceres) or have at least one known moon (Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds. Some astronomers include many smaller bodies as well, but there is no consensus that these are likely to be dwarf planets.
The term ''dwarf planet'' was coined by 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 ...
as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets, dwarf planets, and
satellite planet
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede ...
s. Dwarf planets were thus conceived of as a category of planet. However, in 2006, the concept was adopted by 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 ...
(IAU) as a category of ''sub''-planetary objects, part of a three-way recategorization of bodies orbiting the Sun: planets, dwarf planets and
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 ...
. Thus Stern and other planetary geologists consider dwarf planets and large satellites to be planets, but since 2006 the IAU and perhaps the majority of astronomers have excluded them from the roster of planets.
History of the concept
Starting in 1801, astronomers discovered
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
and other bodies between
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word
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.
...
for the smaller bodies and began to distinguish them as ''minor planets'' rather than ''major planets''.
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major planets, along with thousands of significantly smaller bodies (
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
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 ar ...
s). For almost 50 years,
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 ...
was thought to be larger than
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, but with the discovery in 1978 of Pluto's moon
Charon
In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, Ceres, it had only one-fifth the mass of Earth's
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. Furthermore, having some unusual characteristics, such as large
orbital eccentricity
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
and a high
orbital inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Earth ...
, it became evident that it was a different kind of body from any of the other planets.
In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the
Kuiper belt
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 ...
), and some even farther away.
Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, the
plutino
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the lar ...
s. It became clear that either the larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids.
This led some astronomers to stop referring to Pluto as a planet. Several terms, including ''subplanet'' and ''planetoid'', started to be used for the bodies now known as dwarf planets.
Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain classified as a planet.
Eris (then known as ) was discovered in January 2005; it was thought to be slightly larger than Pluto, and some reports informally referred to it as the ''
tenth planet
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 ...
''. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers
Julio Ángel Fernández
Julio Ángel Fernández Alves (born 5 April 1946) is a Uruguayan astronomer and teacher, member of the department of astronomy at the Universidad de la República in Montevideo. He is also a member of PEDECIBA, (the program for development of b ...
and
Gonzalo Tancredi
Gonzalo Tancredi (born 8 March 1963) is an Uruguayan astronomer and full professor in the Department of Astronomy at the University of the Republic in Montevideo, Uruguay. He is an active member of the International Astronomical Union (IAU) and i ...
: they proposed an intermediate category for objects large enough to be round but which had not cleared their orbits 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. Beside dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.
Although concerns were raised about the classification of planets orbiting other stars, the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed.
In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts. Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.
NASA announced in 2006 that it would use the new guidelines established by the IAU.
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 ...
, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets. Thus, in 2011, he still referred to Pluto as a planet, and accepted other likely dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets. Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets".
Name
Names for large subplanetary bodies include ''dwarf planet'', ''planetoid'' (more general term), '' meso-planet'' (narrowly used for sizes between Mercury and Ceres), ''quasi-planet'' and (in the transneptunian region) ''plutoid''. ''Dwarf planet'', however, was originally coined as a term for the smallest planets, not the largest sub-planets, and is still used that way by many planetary astronomers.
Alan Stern coined the term ''dwarf planet'', analogous to the term ''
dwarf star
A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants which are ...
'', as part of a three-fold classification of planets, and he and many of his colleagues continue to classify dwarf planets as a class of planets. The IAU decided that dwarf planets are not to be considered planets, but kept Stern's term for them. Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include ''quasi-planet''
and the older term ''planetoid'' ("having the form of a planet").
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 ...
stated that ''planetoid'' is "a perfectly good word" that has been used for these bodies for years, and that the use of the term ''dwarf planet'' for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. Indeed, the draft of Resolution 5A had called these median bodies ''planetoids,'' but the plenary session voted unanimously to change the name to ''dwarf planet.'' The second resolution, 5B, defined ''dwarf planets'' as a subtype of ''planet'', as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight ''classical planets'' and four ''dwarf planets''. Resolution 5B was defeated in the same session that 5A was passed. Because of the semantic inconsistency of a ''dwarf planet'' not being a planet due to the failure of Resolution 5B, alternative terms such as ''nanoplanet'' and ''subplanet'' were discussed, but there was no consensus among the CSBN to change it.
In most languages equivalent terms have been created by translating ''dwarf planet'' more-or-less literally: French ', Spanish ', German ', Russian ''karlikovaya planeta'' (), Arabic ''kaukab qazm'' (), Chinese ''ǎixíngxīng'' (), Korean ''waesohangseong'' () or ''waehangseong'' (), but in Japanese they are called ''junwakusei'' (), meaning "quasi-planets" or "peneplanets" ( pene- meaning "almost").
IAU Resolution 6a of 2006 recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as ''plutons'' and ''plutonian objects'' but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their ''
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
''.
On June 11, 2008, the IAU Executive Committee announced a new term, ''plutoid'', and a definition: all trans-Neptunian dwarf planets are plutoids. However, other departments of the IAU have rejected the term:
The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system, part of a conception of a threefold division of the Solar System into inner ''
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
s'', central ''
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" ...
s'' and outer ''ice dwarfs'', of which Pluto was the principal member. 'Ice dwarf' however also saw some use as an umbrella term for all trans-Neptunian
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, or for the ice
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 of the outer Solar System; one attempted definition was that an ice dwarf "is larger than the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucle ...
of a normal
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 ar ...
and icier than a typical asteroid."
Since the ''Dawn'' mission, it has been recognized that Ceres is an icy body more similar to the icy moons of the outer planets and to TNOs such as Pluto than it is to the terrestrial planets, blurring the distinction,
and Ceres has since been called an ice dwarf as well.
Criteria
The category ''dwarf planet'' arose from a conflict between dynamical and geophysical ideas of what a useful conception of a planet would be. In terms of the dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those which do not (such as the asteroids and Kuiper belt objects). However, a celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight, which results in the body acquiring a round shape. Because this requires a much lower mass than gravitationally dominating the region of space near their orbit, there are a population of objects that are massive enough to have a world-like appearance and planetary geology, but not massive enough to clear their neighborhood. Examples are Ceres in the asteroid belt and Pluto in the Kuiper belt.
Dynamicists usually prefer using gravitational dominance as the threshold for planethood, because from their perspective smaller bodies are better grouped with their neighbours, e.g. Ceres as simply a large asteroid and Pluto as a large Kuiper belt object. However, geoscientists usually prefer roundness as the threshold, because from their perspective the internally driven geology of a body like Ceres makes it more similar to a classical planet like Mars, than to a small asteroid that lacks internally driven geology. This necessitated the creation of the category of ''dwarf planets'' to describe this intermediate class.Emily Lakdawalla et al. What Is A Planet? The Planetary Society, 21 April 2020
Orbital dominance
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 ...
and
Harold F. 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 ...
introduced a parameter Λ (
lambda
Lambda (}, ''lám(b)da'') is the 11th letter of the Greek alphabet, representing the voiced alveolar lateral approximant . In the system of Greek numerals, lambda has a value of 30. Lambda is derived from the Phoenician Lamed . Lambda gave rise ...
), expressing the likelihood of an encounter resulting in a given deflection of orbit. The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. A gap of five orders of magnitude in Λ was found between the smallest
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
s and the largest asteroids and Kuiper belt objects.
Using this parameter,
Steven Soter
Steven Soter is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural Hi ...
and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so. Soter went on to propose a parameter he called the ''planetary discriminant'', designated with the symbol µ ( mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared.
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 ( ...
refined Stern and Levison's concept to produce a similar parameter Π ( Pi). It is based on theory, avoiding the empirical data used by Λ. Π > 1 indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.
There are several other schemes that try to differentiate between planets and dwarf planets, but the 2006 definition uses this concept.
Hydrostatic equilibrium
Enough internal pressure, caused by the body's gravitation, will turn a body
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
, and enough plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape and may be rubble piles. Larger objects, where gravity is significant but not dominant, are potato-shaped; the more massive the body, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is enough to overcome its
compressive strength
In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...
and it achieves
hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
. Then, a body is as round as it is possible to be, given its rotation and tidal effects, and is an
ellipsoid
An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a surface that may be defined as the ...
in shape. This is the defining limit of a dwarf planet.
If an object is in hydrostatic equilibrium, a global layer of liquid on its surface would form a surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. The body will have a spherical shape if it doesn't rotate and an ellipsoidal one if it does. The faster it rotates, the more
oblate
In Christianity (especially in the Roman Catholic, Orthodox, Anglican and Methodist traditions), an oblate is a person who is specifically dedicated to God or to God's service.
Oblates are individuals, either laypersons or clergy, normally livi ...
or even scalene it becomes. If such a rotating body were heated until it melts, its shape would not change. The extreme example of a body that may be scalene due to rapid rotation is , which is twice as long on its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces gradually slow its rotation until it is tidally locked; that is, it always presents the same face to its companion. Tidally locked bodies are also scalene, though sometimes only slightly so. Earth's
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
is tidally locked, as are all the rounded satellites of the gas giants. Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia.
There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out the Solar system that is more massive than
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
might not have had time to clear its neighbourhood; such a body would fit the definition of dwarf planet rather than planet. The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium and depends on its composition and thermal history, not simply its mass. An IAU question-and-answer press release from 2006 estimated that objects with mass above and radius greater than 400 km would "normally" be in hydrostatic equilibrium ("the shape ... would normally be determined by self-gravity"), but that "all borderline cases would need to be determined by observation." This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with (''r'' = , ''m'' = ) and possibly (''r'' = , ''m'' unknown) being borderline cases both for the 2006 Q&A expectations and in more recent evaluations, and with being just above the expected limit. No other body with a measured mass is close to the expected mass limit, though several without a measured mass approach the expected size limit.
Population of dwarf planets
There is no clear definition of what constitutes a dwarf planet, and whether to classify an object as one is up to individual astronomers. Thus, the number of dwarf planets in the Solar System is unknown.
The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet – Ceres, Pluto and Eris – are generally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium. Ceres is close to equilibrium, but some gravitational anomalies remain unexplained. Eris is generally assumed to be a dwarf planet because it is more massive than Pluto.
In order of discovery, these three bodies are:
# – discovered January 1, 1801 and announced January 24, 45 years before
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 ...
. Considered a planet for half a century before reclassification as an asteroid. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006. Confirmation is pending.
# – discovered February 18, 1930 and announced March 13. Considered a planet for 76 years. Explicitly reclassified as a dwarf planet by the IAU with Resolution 6A on August 24, 2006. Five known moons.
# () – discovered January 5, 2005 and announced July 29. Called the "
tenth planet
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 ...
" in media reports. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006, and named by the IAU dwarf-planet naming committee on September 13 of that year. One known moon.
The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
brighter than +1 (and hence a minimum diameter of 838 km at the maximum
geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle) to that of an ''idealized'' flat, fully reflecting, diffusively scattering ( Lambertian) disk w ...
of 1) was to be named by a joint committee consisting of the
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
...
and the planetary working group of the IAU. At the time (and still as of 2021), the only bodies to meet this threshold were and . These bodies are generally assumed to be dwarf planets, although they have not yet been demonstrated to be in hydrostatic equilibrium, and there is some disagreement for Haumea:
These five bodies – the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) – are commonly presented as the dwarf planets of the Solar System, though the limiting factor (albedo) is not what defines an object as a dwarf planet.
The astronomical community commonly refers to other larger TNOs as dwarf planets as well. At least four additional bodies meet the preliminary criteria of Brown, of Tancredi et al., and of Grundy et al. for identifying dwarf planets, and are generally called dwarf planets by astronomers as well:
For instance, JPL/NASA called
Gonggong
Gonggong () is a Chinese water god who is depicted in Chinese mythology and folktales as having a copper human head with an iron forehead, red hair, and the body of a serpent, or sometimes the head and torso are human, with the tail of a serpe ...
a dwarf planet after observations in 2016, and Simon Porter of the Southwest Research Institute spoke of "the big eight NOdwarf planets" in 2018, referring to Pluto, Eris, Haumea, Makemake, Gonggong, , and .
More bodies have been proposed, such as and by Brown, and by Tancredi et al., and by Sheppard et al. Most of the larger bodies have moons, which enables a determination of their mass and thus their density, which inform estimates of whether they could be dwarf planets. The largest TNOs that are not known to have moons are Sedna, , and Ixion. In particular, Salacia has a known mass and diameter putting it as a borderline case by the IAU's 2006 Q&A.
At the time Makemake and Haumea were named, it was thought that
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 icy cores would require a diameter of only about 400 km (250 mi), or 3% the size of Earththe size of the moons Mimas, the smallest moon that is round, and
Proteus
In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" ''(hálios gérôn)''. ...
, the largest that is notto relax into gravitational equilibrium. Researchers thought that the number of such bodies could prove to be around 200 in the
Kuiper belt
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 ...
, with thousands more beyond.Mike Brown 'How many dwarf planets are there in the outer solar system?' Accessed November 15, 2013Stern, Alan (August 24, 2012). The PI's Perspective. , August 24, 2012. Retrieved from http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012.
This was one of the reasons (keeping the roster of 'planets' to a reasonable number) that Pluto was reclassified in the first place.
However, research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under the typical conditions of the Kuiper belt and beyond.
Individual astronomers have recognized a number of objects as dwarf planets or as likely to prove to be dwarf planets. In 2008,
Tancredi
''Tancredi'' is a ''melodramma eroico'' ('' opera seria'' or heroic opera) in two acts by composer Gioachino Rossini and librettist Gaetano Rossi (who was also to write ''Semiramide'' ten years later), based on Voltaire's play ''Tancrède'' (176 ...
et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets (Gonggong was not yet known), though the IAU did not address the issue then and has not since. Tancredi also considered the five TNOs , , , , and to most likely be dwarf planets as well.
Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size.
As of 13 September 2019, Brown's list identifies ten
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 with diameters then thought to be greater than 900 km (the four named by the IAU plus , , , , and ) as "near certain" to be dwarf planets, and another 16, with diameter greater than 600 km, as "highly likely".Mike Brown 'How many dwarf planets are there in the outer solar system?' Accessed 20 December, 2019. Notably, Gonggong may have a larger diameter () than Pluto's round moon Charon (1212 km).
But in 2019 Grundy et al. proposed, based on their studies of Gǃkúnǁʼhòmdímà, that dark, low-density bodies smaller than about 900–1000 km in diameter, such as Salacia and , never fully collapsed into solid planetary bodies and retain internal porosity from their formation (in which case they could not be dwarf planets). They accept that brighter (albedo > ≈0.2) or denser (> ≈1.4 g/cc) Orcus and Quaoar probably were fully solid:
Salacia was later found to have a somewhat higher density, comparable within uncertainties to that of Orcus, though still with a very dark surface. Despite this determination, Grundy et al. call it "dwarf-planet sized", while calling Orcus a dwarf planet. Later studies on Varda suggest that its density may also be high, though a low density could not be excluded.
Most likely dwarf planets
The trans-Neptunian objects in the following tables, except Salacia, are agreed by Brown, Tancredi et al. and Grundy et al. to be probable dwarf planets, or close to it. Salacia has been included as the largest TNO not generally agreed to be a dwarf planet and a borderline body by many criteria. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitude greater than +1, and so meet the threshold of the joint planet–minor planet naming committee of the IAU, are highlighted, as is Ceres, which the IAU has assumed is a dwarf planet since they first debated the concept.
Symbols
Ceres and Pluto received symbols like the major planets, as they were thought to be major planets when they were discovered. By the time the others were discovered,
planetary symbol
A planet symbol (or ''planetary symbol'') is a graphical symbol used in astrology and astronomy to represent a classical planet (including the Sun and the Moon) or one of the modern planets. The symbols were also used in alchemy to represent the me ...
s had mostly fallen out of use among astronomers.
Unicode
Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
includes symbols for the dwarf planets Quaoar , Sedna , Orcus , Haumea , Eris , Makemake , and Gonggong that are mostly used among astrologers: they were devised by Denis Moskowitz, a software engineer in Massachusetts. NASA has used his Haumea, Eris, and Makemake symbols, as well as the traditional astrological symbol for Pluto to refer to it as a dwarf planet. A Unicode proposal for most of Moskowitz's symbols mentions symbols that have been suggested for some smaller candidates – Salacia , Varda , Ixion or , Gǃkúnǁʼhòmdímà , Varuna , and
Chaos
Chaos or CHAOS may refer to:
Arts, entertainment and media Fictional elements
* Chaos (''Kinnikuman'')
* Chaos (''Sailor Moon'')
* Chaos (''Sesame Park'')
* Chaos (''Warhammer'')
* Chaos, in ''Fabula Nova Crystallis Final Fantasy''
* Cha ...
– but these do not have consistent, independent usage among astrologers.
Exploration
On March 6, 2015, the ''
Dawn
Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's horizo ...
'' spacecraft entered orbit around
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, becoming the first spacecraft to visit a dwarf planet. On July 14, 2015, the ''
New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' space probe flew by
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 ...
and its five moons.
Ceres displays such evidence of an active geology as salt deposits and
cryovolcano
A cryovolcano (sometimes informally called an ice volcano) is a type of volcano that erupts volatiles such as water, ammonia or methane into an extremely cold environment that is at or below their freezing point. The process of formation is known ...
s, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere.
Ceres evidently has brine percolating through its subsurface, while there is evidence that Pluto has an actual subsurface ocean.
''Dawn'' had previously orbited the asteroid Vesta. Saturn's moon Phoebe has been imaged by Cassini and before that by Voyager 2, which also encountered Neptune's moon
Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus' ...
. All three bodies show evidence of once being dwarf planets, and their exploration helps clarify the evolution of dwarf planets.
''New Horizons'' has captured distant images of Triton, Quaoar, Haumea, Eris, and Makemake, as well as the smaller candidates Ixion, , and .
Similar objects
A number of bodies physically resemble dwarf planets. These include former dwarf planets, which may still have equilibrium shape or evidence of active geology; planetary-mass moons, which meet the physical but not the orbital definition for dwarf planet; and Charon in the Pluto–Charon system, which is arguably a binary dwarf planet. The categories may overlap: Triton, for example, is both a former dwarf planet and a planetary-mass moon.
Rheasilvia
Rheasilvia is the most prominent surface feature on the asteroid Vesta and is thought to be an impact crater. It is in diameter, which is 90% the diameter of Vesta itself, and is 95% the mean diameter of Vesta, . However, the mean is affect ...
and
Veneneia
Veneneia is the second-largest crater on asteroid 4 Vesta, at 52°S latitude.
Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus' ...
is more massive than Eris or Pluto, has an equilibrium shape, and is thought to be a captured dwarf planet (likely a member of a binary system), but no longer directly orbits the sun.Phoebe is a captured
centaur
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 ...
that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history due to
radiogenic heating
A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive (a radionuclide) or stable (a stable nuclide).
Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some of ...
.
Evidence from 2019 suggests that
Theia
In Greek mythology, Theia (; grc, Θεία, Theía, divine, also rendered Thea or Thia), also called Euryphaessa ( grc, Εὐρυφάεσσα) "wide-shining", is one of the twelve Titans, the children of the earth goddess Gaia and the sky god ...
, the former planet that collided with Earth in the
giant-impact hypothesis
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago, in the Hadean ...
, may have originated in the outer Solar System rather than the inner Solar System and that Earth's water originated on Theia, thus implying that Theia may have been a former dwarf planet from the Kuiper Belt.
Planetary-mass moons
At least nineteen
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
have equilibrium shape from having relaxed under self-gravity at some point, though some have since frozen solid and are no longer in equilibrium. Seven are more massive than either Eris or Pluto. These moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun. (Indeed, Neptune's moon
Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus' ...
is a captured dwarf planet, and Ceres formed in the same region of the Solar System as the moons of Jupiter and Saturn.) Alan Stern calls planetary-mass moons "
satellite planet
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede ...
s", one of three categories of planet, together with dwarf planets and classical planets. The term ''
planemo
A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...
'' ("planetary-mass object") also covers all three populations.
Charon
There has been some debate as to whether the Pluto–
Charon
In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
system should be considered a double dwarf planet.
In a draft resolution for the
IAU definition of planet
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a ''planet'' is a celestial body that:
# is in orbit around the Sun,
# has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), an ...
, both Pluto and Charon were considered planets in a binary system.The footnote in the original text reads: "For two or more objects comprising a multiple object system.... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet." The IAU currently says Charon is not considered a dwarf planet but rather a satellite of Pluto, though the idea that Charon might qualify as a dwarf planet may be considered at a later date. However, it is no longer clear that Charon is in hydrostatic equilibrium. Also, the location of the
barycenter
In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important conc ...
depends not only on the relative masses of the bodies, but also on the distance between them; the barycenter of the Sun–Jupiter orbit, for example, lies outside the Sun, but they are not considered a binary object. Thus, a formal definition of what constitutes a binary (dwarf) planet must be established before Pluto and Charon are formally defined as binary dwarf planets.
See also
*
Centaur
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 ...
*
Lists of astronomical objects
This is a list of lists, grouped by type of astronomical object.
Solar System
* List of Solar System objects
* List of gravitationally rounded objects of the Solar System
* List of Solar System objects most distant from the Sun
* List of So ...
*
List of former planets
This is a list of astronomical objects formerly widely considered ''planets'' under any of the various definitions of this word in the history of astronomy. As the definition of ''planet'' has evolved, the de facto and de jure definitions of plane ...
*
List of gravitationally rounded objects of the Solar System
This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (but are not necessarily in hydrostatic equilibrium). Apart from the Sun itself, ...
*
List of planetary bodies
The following are lists of planets.
__NOTOC__
In the Solar System
*For a list of geophysical planets in the Solar System, see: List of gravitationally rounded objects of the Solar System This also includes a list of the eight planets according t ...
*
List of possible dwarf planets
The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond.
However, consideration of the surprisingly low densities of many large trans-Neptunian object ...
*
Lists of small Solar System bodies
This is a list of the lists of small Solar System bodies and dwarf planets.
* Lists of comets
* List of minor planets
* List of named minor planets (alphabetical)
* List of named minor planets (numerical)
* List of exceptional asteroids
* List of ...
Notes
References
External links
*
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 ...
Morning Edition
''Morning Edition'' is an American radio news program produced and distributed by NPR. It airs weekday mornings (Monday through Friday) and runs for two hours, and many stations repeat one or both hours. The show feeds live from 5:00 to 9:00 AM ...
'')
*
BBC News
BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broadca ...
Ottawa Citizen
The ''Ottawa Citizen'' is an English-language daily newspaper owned by Postmedia Network in Ottawa, Ontario, Canada.
History
Established as ''The Bytown Packet'' in 1845 by William Harris (journalist), William Harris, it was renamed the '' ...
James L. Hilton
James Lindsay Hilton (born February 21, 1957) has been an astronomer at the United States Naval Observatory since 1986. In 1999 he published a new set of ephemerides for 15 of the largest asteroids for use in the Astronomical Almanac.
Education
Hi ...