Geophysical definition of planet

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The International Union of Geological Sciences (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". [1] 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 dominant organization for setting planetary nomenclature. [2]

Contents

Definitions

Some geoscientists adhere to the formal definition of a planet that was proposed by the International Astronomical Union (IAU) in August 2006. [3] According to IAU definition of planet, a planet is an astronomical body orbiting the Sun that is massive enough to be rounded by its own gravity, and has cleared the neighbourhood around its orbit. [4]

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. [5]

A planetary body is defined as any body in space that satisfies the following testable upper and lower bound criteria on its mass: If isolated from external perturbations (e.g., dynamical and thermal), the body must:

  1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
  2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time (roughly the current age of the universe), so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

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,

Furthermore, there are important dynamical categories:

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: [6]

a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.

Some variation can be found in how planetary scientists classify borderline objects, such as the asteroids Pallas and Vesta. These two are probably surviving protoplanets, 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, [7] while others do not. [8]

Other names for geophysical planets

In 2009, Jean-Luc Margot (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 396-kilometre (246 mi) diameter, but rocky Vesta is not at 525-kilometre (326 mi) diameter. [9] 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. [9]

Geophysical planets in the Solar System

Under geophysical definitions of a planet, there are more satellite and dwarf planets in the Solar System than classical planets. Types of planets under the Geophysical Planet Definition (GPD).jpg
Under geophysical definitions of a planet, there are more satellite and dwarf planets in the Solar System than classical planets.

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 400 kilometres (250 mi) in diameter, suggesting that there were a large number of dwarf planets in the Kuiper belt and scattered disk. [10] However, by 2010 it was known that icy moons up to 1,500 kilometres (930 mi) in diameter (e.g. Iapetus) 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. [11] 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. [12] Most geophysical definitions list such bodies anyway. [5] [6] [7] (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.) [13]

In 2019, Grundy et al. argued that trans-Neptunian objects up to 900 to 1,000 kilometres (560 to 620 mi) in diameter (e.g. (55637) 2002 UX25 and Gǃkúnǁʼhòmdímà) have never compressed out their internal porosity, [14] [15] and are thus not planetary bodies. In 2023, Emery et al. argued for a similar threshold for chemical evolution in the trans-Neptunian region. [16] Such a high threshold suggests that at most nine known trans-Neptunian objects could possibly be geophysical planets: Pluto, Eris, Haumea, Makemake, Gonggong, Charon, Quaoar, Orcus, and Sedna pass the 900-kilometre (560 mi) threshold. [16] [17]

The bodies generally agreed to be geophysical planets include the eight major planets:

  1. ☿ Mercury
  2. ♀ Venus
  3. 🜨 Earth
  4. ♂ Mars
  5. ♃ Jupiter
  6. ♄ Saturn
  7. ⛢ Uranus
  8. ♆ Neptune

nine dwarf planets that geophysicists generally agree are planets: [17] [18]

  1. Ceres symbol (fixed width).svg Ceres
  2. Orcus symbol (fixed width).svg Orcus
  3. Pluto monogram (fixed width).svg Pluto
  4. Haumea symbol (fixed width).svg Haumea
  5. Quaoar symbol (fixed width).svg Quaoar
  6. Makemake symbol (fixed width).svg Makemake
  7. Gonggong symbol (fixed width).svg Gonggong
  8. Eris symbol (fixed width).svg Eris
  9. Sedna symbol (fixed width).svg Sedna

and nineteen planetary-mass moons:

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 (larger than Mimas, but still not round); or some other trans-Neptunian objects that might or might not be dwarf planets. [7]

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 and Titan, or augmented by tidal heating, as with Io and Europa) is approximately the threshold of geological activity. [19] However, there are exceptions such as Callisto and Mimas, which have equilibrium shapes (historical in the case of Mimas) but show no signs of past or present endogenous geological activity, [20] [21] and Enceladus, which is geologically active due to tidal heating but is apparently not currently in equilibrium. [11]

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 planets and planetary-mass moons as planets.

Five bodies are currently recognized as or named as dwarf planets by the IAU: Ceres, Pluto (the dwarf planet with the largest known radius), [22] Eris (the dwarf planet with the largest known mass), [23] Haumea, and Makemake, though the last three have not actually been demonstrated to be dwarf planets. [24] Astronomers normally include these five, as well as four more: Quaoar, Sedna, Orcus, and Gonggong.

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. [25] [26] An early petition rejecting the IAU definition attracted more than 300 signatures, though not all of these critics supported an alternative definition. [27] [28] [29]

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. [3] [30] [31] [6] 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. [32] [33]

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". [34] While some geophysical definitions that differ from the IAU definition apply, in theory, to exoplanets and rogue planets, [31] 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, Exoplanet Data Explorer and NASA Exoplanet Archive all include objects significantly more massive than the theoretical 13-Jupiter mass threshold at which deuterium fusion is believed to be supported, [35] 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. [36] [37] [38]

Both the IAU definition and the geophysical definitions that differ from it consider the shape of the object, with consideration given to hydrostatic equilibrium. 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, 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

Further reading

Related Research Articles

<span class="mw-page-title-main">Double planet</span> A binary system where two planetary-mass objects share an orbital axis external to both

In astronomy, a double planet is a binary satellite system where both objects are planets, or planetary-mass objects, and whose joint barycenter is external to both planetary bodies.

<span class="mw-page-title-main">Planet</span> Large, round non-stellar astronomical object

A planet is a large, rounded 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 by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. 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 young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.

<span class="mw-page-title-main">Pluto</span> Dwarf planet

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 known trans-Neptunian object by volume, by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has roughly one-sixth the mass of the Moon, and one-third its volume.

<span class="mw-page-title-main">Terrestrial planet</span> Planet that is composed primarily of silicate rocks or metals

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, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.

<span class="mw-page-title-main">Charon (moon)</span> Largest natural satellite of Pluto

Charon, or (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

<span class="mw-page-title-main">Natural satellite</span> Astronomical body that orbits a planet

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

<span class="mw-page-title-main">Sedna (dwarf planet)</span> Dwarf planet

Sedna is a dwarf planet in the outermost reaches of the Solar System, orbiting the Sun beyond the orbit of Neptune. Discovered in 2003, the planetoid's surface is one of the reddest known among Solar System bodies. Spectroscopy has revealed Sedna's surface to be mostly a mixture of the solid ices of water, methane, and nitrogen, along with widespread deposits of reddish-colored tholins, a chemical makeup similar to those of some other trans-Neptunian objects. Within the range of uncertainties, it is tied with the dwarf planet Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. Its diameter is roughly 1,000 km. Owing to its lack of known moons, the Keplerian laws of planetary motion cannot be employed for determining its mass, and the precise figure remains as yet unknown.

The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται, 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.

<span class="mw-page-title-main">Exomoon</span> Moon beyond the Solar System

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

<span class="mw-page-title-main">Dwarf planet</span> Small planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.

IAU definition of <i>planet</i> 2006 International Astronomical Union definition

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium, and
  3. has "cleared the neighbourhood" around its orbit.

"Clearing the neighbourhood" around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence.

In astronomy, planetary mass is a measure of the mass of a planet-like astronomical object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass (M☉), the mass of the Sun. In the study of extrasolar planets, the unit of measure is typically the mass of Jupiter (MJ) for large gas giant planets, and the mass of Earth (ME) for smaller rocky terrestrial planets.

<span class="mw-page-title-main">Planetary-mass object</span> Size-based definition of celestial objects

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, but not enough to sustain core fusion like a star.

<span class="mw-page-title-main">Planetary-mass moon</span> Planetary-mass bodies that are also natural satellites

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton – are larger and more massive than the dwarf planets Pluto and Eris.

<span class="mw-page-title-main">Ice planet</span> Planet with an icy surface

An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere.

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