The

International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world. About Fo ...

(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, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

". As a result, there are various geophysical definitions in use among professional geophysicists, planetary scientists, and other professionals in the geosciences. Many professionals opt to use one of several of these geophysical definitions instead of the definition voted on by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

, the dominant organization for setting planetary nomenclature.

Definitions

Some geoscientists adhere to the formal definition of a planet that was proposed by the

(IAU) in August 2006. According to

IAU definition of planet The International Astronomical Union (IAU) adopted in August 2006 the definition made by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi that stated, that in the Solar System, a ''planet'' is a celestial body that: # is in o ...

, a planet is an

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...

ing the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

that is massive enough to be rounded by its own

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

, and has cleared the neighbourhood around its orbit. Another widely accepted geophysical definition of a planet includes that which was put forth by

planetary scientist Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their ...

Alan Stern Sol Alan Stern (born November 22, 1957) is an American engineer, planetary scientist and space tourist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved ...

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 explain their reasoning by noting that this definition delineates the evolutionary stages and primary features of planets more clearly. Specifically, they claim 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, Spanish and other languages * Luna (goddess) In Sabine and ancient Roman religion and myth, Luna is the divine embodiment of the Moon (Latin ''Lūna'' ). She is often presented as t ...

, the

Galilean satellites The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede, Callisto, Io, and Europa. They are the most readily visible Solar System objects after Saturn, the dimmest of ...

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

, and

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' ...

, 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) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from ...

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. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

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 * Pa ...

and Vesta. These two are probably surviving

protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitatio ...

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 with expertise in planetary sciences and SETI. Career Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries ...

(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 Mimas, also designated Saturn I, is the seventh-largest natural satellite of Saturn. With a mean diameter of , Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15 ...

is round at diameter, but rocky Vesta is not 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 times ...

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 objec ...

. However, by 2010 it was known that icy moons up to in diameter (e.g.

Iapetus In Greek mythology, Iapetus (; ; ), 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 other sources. Iapetus was linked ...

) 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. In 2023, Emery et al. argued for a similar threshold for chemical evolution in the trans-Neptunian region. Such a high threshold suggests that at most nine known trans-Neptunian objects could possibly be geophysical planets: Pluto, , , , , , , , and pass the threshold. The bodies generally agreed to be geophysical planets include the eight major planets: #

☿ Planetary symbols are used in astrology and traditionally in astronomy to represent a classical planet (which includes the Sun and the Moon) or one of the modern planets. The classical symbols were also used in alchemy for the seven metals known t ...

'' Mercury'' #

♀ file:SBB Giruno by Stadler (46959878215).jpg, Gender symbols on a public toilet in Switzerland, alt=Image shows male and female symbols incised deeply A gender symbol is a pictogram or glyph used to represent sexual differentiation, sex and gender ...

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

'' # 🜨 ''

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

'' #

♂ file:SBB Giruno by Stadler (46959878215).jpg, Gender symbols on a public toilet in Switzerland, alt=Image shows male and female symbols incised deeply A gender symbol is a pictogram or glyph used to represent sexual differentiation, sex and gender ...

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

'' # ♃ ''

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 Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

'' # ♄ ''

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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

'' # ⛢ ''

Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

'' # ♆ ''

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

'' nine dwarf planets that geophysicists generally agree are planets: # ' # ' # ' # ' # ' # ' # ' # ' # ' and nineteen

planetary-mass moon A planetary-mass moon is a planetary-mass object. They are large and ellipsoidal (sometimes spherical) in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in t ...

s: * One satellite of Earth – Moon decrescent symbol

''the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

'' * Four satellites of Jupiter – '' Io'', ''

Europa Europa may refer to: Places * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliffs, Alexan ...

'', '' Ganymede'', and ''

Callisto CALLISTO (''Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations'') is a reusable VTVL Prototype, demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the CNES, French ...

'' * Seven satellites of Saturn – ''

'', ''

Enceladus Enceladus is the sixth-largest moon of Saturn and the 18th-largest in the Solar System. It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. It is covered by clean, freshly deposited snow hundreds of meters thick, ...

'', '' Tethys'', '' Dione'', '' Rhea'', ''

'', and ''

'' * Five satellites of Uranus – '' Miranda'', ''

Ariel Ariel may refer to: Film and television *Ariel Award, a Mexican Academy of Film award * ''Ariel'' (film), a 1988 Finnish film by Aki Kaurismäki *, a Russian film directed by Yevgeni Kotov * ''ARIEL Visual'' and ''ARIEL Deluxe'', a 1989 and 1991 ...

'', ''

Umbriel Umbriel () is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem '' The Rape of the Lock''. Umb ...

'', '' Titania'', and ''

Oberon Oberon () is a king of the fairy, fairies in Middle Ages, 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 ...

'' * One satellite of Neptune – ''

'' * One satellite of Pluto – ''

Charon In Greek mythology, Charon or Kharon ( ; ) is a psychopomp, the ferryman of the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the worlds of the living and ...

'' 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 ( ; ) 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''). Some who ascribe a specific domain to Prote ...

(larger than Mimas, but still not round); or some other trans-Neptunian objects 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 Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...

and

, or augmented by tidal heating, as with Io and

) is approximately the threshold of geological activity. However, there are exceptions such as

and

, which have equilibrium shapes (historical in the case of Mimas) but show no signs of past or present endogenous geological activity, and

, 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 around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

s and

s as planets. Five bodies are currently recognized as or named as dwarf planets by the IAU: Ceres,

(the dwarf planet with the largest known radius), Eris (the dwarf planet with the largest known mass),

Haumea Haumea ( minor-planet designation: 136108 Haumea) is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team heade ...

, and

Makemake Makemake ( minor-planet designation: 136472 Makemake) is a dwarf planet and the largest of what is known as the classical population of Kuiper belt objects, with a diameter approximately that of Saturn's moon Iapetus, or 60% that of Pluto. It ...

, 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 confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s and

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 (also known as Encyclopaedia of exoplanetary systems and Catalogue of Exoplanets) is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains ...

, Exoplanet Data Explorer 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 The 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 ...

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, also called hydrostatic balance and 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. I ...

. 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 may refer to: Arts and entertainment * ''The Walking Dead'' (TV series) * ''Watership Down'' * ''White Dwarf'' (magazine) Businesses and organizations Government agencies * Royal Canadian Air Force Women's Division * War Department ...

, is only 0.0007 Earth masses, while

SDSS J1228+1040 b SDSS may refer to: * Sloan Digital Sky Survey, a major multi-filter imaging and spectroscopic redshift survey * Social Democratic Party of Slovakia * Spatial Decision Support System, a GIS based decision aiding system * Independent Democratic Serb ...

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.)

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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 r ...

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Astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, access-date = 12 October 2019 , archive-date = 10 October 2019 , archive-url = https://web.archive.org/web/20191010153028/http://astronomy.com/magazine/2018/05/an-organically-grown-planet-definition , url-status = live {{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 , archive-date = 31 October 2013 , archive-url = https://web.archive.org/web/20131031154417/https://www.iau.org/public/themes/naming/#dwarfplanets , url-status = live {{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 , archive-date = 9 October 2023 , archive-url = https://web.archive.org/web/20231009113108/https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html , url-status = live {{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 =

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