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International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology. About The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
(IUGS) is the internationally recognized body charged with fostering agreement on nomenclature and classification across geoscientific disciplines. However, they have yet to create a formal definition of the term ''
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 ...
''. As a result, there are various geophysical definitions in use among professional geophysicists, planetary scientists, and other professionals in the geosciences. As such, many professionals do not use the definition voted on 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, outreach ...
.


Definitions

Some geoscientists adhere to the formal definition of a planet that was proposed 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, outreach ...
(IAU) in August 2006. According to
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), and ...
, a planet is an
astronomical body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
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 ...
ing the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
that is massive enough to be rounded by its own
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
, and has cleared the neighbourhood around its orbit. Another widely accepted geophysical definition of a planet includes that which was put forth by planetary scientists Alan Stern and Harold Levison in 2002. The pair proposed the following rules to determine whether an object in space satisfies the definition for a planetary body. They clarified that ''the hallmark of planethood is the collective behavior of the body's mass to overpower mechanical strength and flow into an equilibrium ellipsoid whose shape is dominated by its own gravity'' and that the definition allows for an early period during which ''gravity may not yet have fully manifested itself to be the dominant force''. They subclassified planetary bodies as, * ''planets'', which orbit their stars directly * ''planetary-scale satellites'', the largest being
Luna Luna commonly refers to: * Earth's Moon, named "Luna" in Latin * Luna (goddess), the ancient Roman personification of the Moon Luna may also refer to: Places Philippines * Luna, Apayao * Luna, Isabela * Luna, La Union * Luna, San Jose Romania ...
, the Galilean satellites, Titan and Triton, with the last apparently being 'formerly a planet in its own right' * ''unbound planets'',
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 ...
s between the stars * ''double planets'', in which a planet and a massive satellite orbit a point between the two bodies (the single known example in the Solar System is Pluto–Charon) Furthermore, there are important dynamical categories: * ''überplanets'' orbit stars and are dynamically dominant enough to clear neighboring
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 in a Hubble time * ''unterplanets'', which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body. They set the boundary at Λ = 1. A 2018 encapsulation of the above definition defined all planetary bodies as planets. It was worded for a more general audience, and was intended as an alternative to the IAU definition of a planet. It noted that planetary scientists find a different definition of 'planet' to be more useful for their field, just as different fields define 'metal' differently. For them, a planet is: Some variation can be found in how planetary scientists classify borderline objects, such as the asteroids
Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pal ...
and Vesta. These two are probably surviving
protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...
s, and are larger than some clearly ellipsoidal objects, but currently are not very round (although Vesta likely was round in the past). Some definitions include them,Emily Lakdawalla et al.
What Is A Planet?
The Planetary Society, 21 April 2020
while others do not.


Other names for geophysical planets

In 2009,
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 ...
(who proposed a mathematical criterion for clearing the neighborhood) and Levison suggested that "roundness" should refer to bodies whose gravitational forces exceed their material strength, and that round bodies could be called "worlds". They noted that such a geophysical classification was sound and was not necessarily in conflict with the dynamical conception of a planet: for them, "planet" is defined dynamically, and is a subset of "world" (which also includes dwarf planets, round moons, and free floaters). However, they pointed out that a taxonomy based on roundness is highly problematic because roundness is very rarely directly observable, is a continuum, and proxying it based on size or mass leads to inconsistencies because planetary material strength depends on temperature, composition, and mixing ratios. For example, icy Mimas is round at diameter, but rocky Vesta is not at diameter. (And at much lower temperatures, icy
Salacia In ancient Roman mythology, Salacia ( , ) was the female divinity of the sea, worshipped as the goddess of salt water who presided over the depths of the ocean. Neptune was her consort. That Salacia was the consort of Neptune is implied by Varr ...
in the Kuiper belt might not have fully gravitationally collapsed even at diameter.) Thus they stated that some uncertainty could be tolerated in classifying an object as a world, while its dynamical classification could be simply determined from mass and orbital period.


Geophysical planets in the Solar System

The number of geophysical planets in the Solar System cannot be objectively listed, as it depends on the precise definition as well as detailed knowledge of a number of poorly-observed bodies, and there are some borderline cases. At the time of the IAU definition in 2006, it was thought that the limit at which icy astronomical bodies were likely to be in hydrostatic equilibrium was around in diameter, suggesting that there were a large number of dwarf planets 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 ti ...
and
scattered disk The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obje ...
. However, by 2010 it was known that icy moons up to in diameter (e.g.
Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in oth ...
) are not in equilibrium. Iapetus is round, but is too oblate for its current spin: it has an equilibrium shape for a rotation period of 16 hours, not its actual spin of 79 days. This might be because the shape of Iapetus was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation, until it became
tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
.Cowen, R. (2007). Idiosyncratic Iapetus, ''Science News'' vol. 172, pp. 104–106
references
Most geophysical definitions list such bodies anyway. (In fact, this is already the case with the IAU definition; Mercury is now known to not be in hydrostatic equilibrium, but it is universally considered to be a planet regardless.)Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) ''Mercury: The View after MESSENGER''. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73. In 2019, Grundy et al. argued that trans-Neptunian objects up to in diameter (e.g. and ) have never compressed out their internal porosity, and are thus not planetary bodies. Such a high threshold suggests that at most nine known trans-Neptunian objects could possibly be geophysical planets: Pluto, , , , , , , , and probably pass the threshold. (about in diameter) and (about in diameter) might be borderline cases: they are dark, but their densities may or may not be high enough to be geophysical planets. Consistent with this, Grundy et al. have characterised Salacia as only "dwarf planet-sized", while at the same time calling Orcus a "dwarf planet". The bodies generally agreed to be geophysical planets include the eight major planets: # Mercury #
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
# 🜨
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...
#
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosp ...
#
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousandth th ...
#
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 ...
#
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of Cro ...
#
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 ...
nine objects that scientists generally agree are dwarf planets: # ' # ' # ' # ' # ' # ' # ' # ' # ' and twenty
planetary-mass moon 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 satellite of Earth – ''the
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 ...
'' * Four satellites of Jupiter – '' Io'', '' Europa'', '' Ganymede'', and ''
Callisto Callisto most commonly refers to: * Callisto (mythology), a nymph *Callisto (moon), a moon of Jupiter Callisto may also refer to: Art and entertainment *''Callisto series'', a sequence of novels by Lin Carter *''Callisto'', a novel by Torsten Kr ...
'' * Seven satellites of Saturn – '' Mimas'', ''
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refl ...
'', '' Tethys'', '' Dione'', '' Rhea'', '' Titan'', and ''
Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in oth ...
'' * Five satellites of Uranus – '' Miranda'', '' Ariel'', '' Umbriel'', ''
Titania Titania may refer to: Astronomy * Titania (moon), the largest moon of the planet Uranus * 593 Titania, an asteroid Chemistry and mineralogy * Titania, an alternate name for titanium dioxide Fiction * Titania (A Midsummer Night's Dream), the Quee ...
'', and ''
Oberon Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fairi ...
'' * One satellite of Neptune – '' Triton'' * One satellite of 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 wor ...
'' * One satellite of Eris – '' Dysnomia'' Some other objects are sometimes included at the borderlines, such as the asteroids Pallas, Vesta, and Hygiea (larger than Mimas, but Pallas and Vesta are noticeably not round); Neptune's second-largest moon
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)'' ...
(larger than Mimas, but still not round); or some other trans-Neptunian objects like Salacia that might or might not be dwarf planets. An examination of spacecraft imagery suggests that the threshold at which an object is large enough to be rounded by self-gravity (whether due to purely gravitational forces, as with
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
and Titan, or augmented by tidal heating, as with Io and Europa) is approximately the threshold of geological activity. However, there are exceptions such as
Callisto Callisto most commonly refers to: * Callisto (mythology), a nymph *Callisto (moon), a moon of Jupiter Callisto may also refer to: Art and entertainment *''Callisto series'', a sequence of novels by Lin Carter *''Callisto'', a novel by Torsten Kr ...
and Mimas, which have equilibrium shapes (historical in the case of Mimas) but show no signs of past or present endogenous geological activity, and
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refl ...
, which is geologically active due to tidal heating but is apparently not currently in equilibrium.


Comparison to IAU definition of a planet

Some geophysical definitions are the same as the IAU definition, while other geophysical definitions tend to be more or less equivalent to the second clause of the IAU definition of planet. Stern's 2018 definition, but not his 2002 definition, excludes the first clause of the IAU definition (that a planet be in orbit around a star) and the third clause (that a planet has cleared the neighborhood around its orbit). It thus counts
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 and
planetary-mass moon 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 as planets. Five bodies are currently recognized as or named as dwarf planets by the IAU:
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 ...
,
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
(the dwarf planet with the largest known radius), Eris (the dwarf planet with the largest known mass),
Haumea , discoverer = , discovered = , earliest_precovery_date = March 22, 1955 , mpc_name = (136108) Haumea , pronounced = , adjectives = Haumean , note = yes , alt_names = , named_after = Haumea , mp_category = , orbit_ref = , epoc ...
, and
Makemake Makemake (minor-planet designation 136472 Makemake) is a dwarf planet and – depending on how they are defined – the second-largest Kuiper belt object in the classical population, with a diameter approximately 60% that of Pluto. It h ...
, though the last three have not actually been demonstrated to be dwarf planets. Astronomers normally include these five, as well as four more: , , , and .


Reaction to IAU definition

Many critics of the IAU decision were focused specifically on retaining Pluto as a planet and were not interested in debating or discussing how the term "planet" should be defined in geoscience. An early petition rejecting the IAU definition attracted more than 300 signatures, though not all of these critics supported an alternative definition. Other critics took issue with the definition itself and wished to create alternative definitions that could be used in different disciplines. The geophysical definition of a planet put forth by Stern and Levinson is an alternative to the IAU's definition of what is and is not a planet and is meant to stand as ''the'' geophysical definition, while the IAU definition, they argue, is intended more for astronomers. Nonetheless, some geologists favor the IAU's definition. Proponents of Stern and Levinson's geophysical definition have shown that such conceptions of what a planet is have been used by planetary scientists for decades, and continued after the IAU definition was established, and that asteroids have routinely been regarded as "minor" planets, though usage varies considerably.


Applicability to exoplanets

Geophysical definitions have been used to define exoplanets. The 2006 IAU definition purposefully does not address the complication of exoplanets, though in 2003 the IAU declared that "the minimum mass required for an extrasolar object to be considered a planet should be the same as that used in the Solar System". While some geophysical definitions that differ from the IAU definition apply, in theory, to
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s and
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 ...
s, they have not been used in practice, due to ignorance of the geophysical properties of most exoplanets. Geophysical definitions typically exclude objects that have ever undergone nuclear fusion, and so may exclude the higher-mass objects included in exoplanet catalogs as well as the lower-mass objects. The
Extrasolar Planets Encyclopaedia The Extrasolar Planets Encyclopaedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with ind ...
,
Exoplanet Data Explorer The Exoplanet Data Explorer / Exoplanet Orbit Database lists extrasolar planets up to 24 Jupiter masses.The Exoplane ...
and
NASA Exoplanet Archive The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of ...
all include objects significantly more massive than the theoretical 13-
Jupiter mass Jupiter mass, also called Jovian mass, is the unit of mass equal to the total mass of the planet Jupiter. This value may refer to the mass of the planet alone, or the mass of the entire Jovian system to include the moons of Jupiter. Jupiter is b ...
threshold at which deuterium fusion is believed to be supported, for reasons including: uncertainties in how this limit would apply to a body with a rocky core, uncertainties in the masses of exoplanets, and debate over whether deuterium-fusion or the mechanism of formation is the most appropriate criterion to distinguish a planet from a star. These uncertainties apply equally to the IAU conception of a planet. Both the IAU definition and the geophysical definitions that differ from it consider the shape of the object, with consideration given to
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 planeta ...
. Determining the roundness of a body requires measurements across multiple chords (and even that is not enough to determine whether it is actually in equilibrium), but exoplanet detection techniques provide only the planet's mass, the ratio of its cross-sectional area to that of the host star, or its relative brightness. One small exoplanet, Kepler-1520b, has a mass of less than 0.02 times that of the Earth, and analogy to objects within the Solar System suggests that this may not be enough for a rocky body to be a planet. Another,
WD 1145+017 b WD 1145+017 b (also known by its EPIC designation ''EPIC 201563164.01''), is a confirmed exoplanetary object, likely rocky, orbiting around and being vaporized by the white dwarf star WD 1145+017. It was discovered by NASA's ''Kepler'' spacecraf ...
, is only 0.0007 Earth masses, while SDSS J1228+1040 b may be only 0.01 Earth radii in size, well below the upper equilibrium limit for icy bodies in the Solar System. (See List of smallest exoplanets.)


See also

*
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, ...
– a list of objects generally accepted by astronomers to meet geophysical planet definitions, with many properties presented


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Publications of the Astronomical Society of the Pacific ''Publications of the Astronomical Society of the Pacific'' (often abbreviated as ''PASP'' in references and literature) is a monthly peer-reviewed scientific journal managed by the Astronomical Society of the Pacific. It publishes research and ...
, volume = 123 , number = 902 , pages = 412–422 , doi = 10.1086/659427 , arxiv = 1012.5676 , s2cid = 51769219 , issn = 1538-3873
{{cite web , url = http://www.astronomy.com/magazine/2018/05/an-organically-grown-planet-definition , title = An organically grown planet definition — Should we really define a word by voting? , last1 = Runyon , first1 = Kirby D. , last2 = Stern , first2 = S. Alan , date = 17 May 2018 , website =
Astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galax ...
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{{cite web , url = https://www.iau.org/public/themes/naming/#dwarfplanets , title = Naming of Astronomical Objects , website = International Astronomical Union , access-date = 12 October 2019 {{cite web , url = https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html , title = Exoplanet Criteria for Inclusion in the Archive , date = 26 March 2019 , website = NASA Exoplanet Archive , access-date = 12 October 2019 {{cite news , last = Chang , first = Kenneth , date = 1 September 2006 , title = Debate Lingers Over Definition for a Planet , url = https://www.nytimes.com/2006/09/01/science/space/01planet.html , website =
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 ...
, access-date = 12 October 2019
{{cite web , url = http://ww.npr.org/templates/transcript/transcript.php?storyId=89188493 , title = What Defines a Planet? (transcript) , last1 = Flatow , first1 = Ira , last2 = Sykes , first2 = Mark , date = 28 March 2008 , website =
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 n ...
, access-date = 12 October 2019
A Planet Definition Debate Alan Stern & Ron Ekers
/ref> Definition of planet Planets