Geophysical definition of 'planet'

In 2002, planetary scientists Alan Stern and Harold Levison proposed the following algorithm to determine whether an object in space satisfies the definition for a planetary body. It was designed with the aim of retaining Pluto as a planet.[1]

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, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

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, which in the Solar System are seven (Luna, the Galilean satellites, Titan and Triton, with the last apparently being 'formerly a planet in its own right')
• unbound planets, rogue planets between the stars
• double planets, in which a planet and a massive satellite orbit a point between the two bodies (the single 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 planetesimals 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 revision of the algorithm 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:[2]

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.

Under geophysical definitions of a planet, there are at least as many satellite and dwarf planets in the Solar system as classical planets, and very likely more.

The number of geophysical planets in the Solar system is unknown. 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 km in diameter, suggesting that there were a large number of dwarf planets in the Kuiper belt and Scattered Disk, making dwarf planets the most common type of planet in the Solar System.[3] However, it's since been shown that icy moons up to 1500 km in diameter are not in equilibrium, and that at least some trans-Neptunian objects up to 900 to 1000 km in diameter are not even solid bodies, suggesting that there may be only a few dwarf planets in the Solar system. 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.[4] 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 geological activity, and Enceladus, which is geologically active due to tidal heating but is apparently not currently in equilibrium.[5]

Geophysical definitions are 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 (that a planet be in orbit around the sun) 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),[6] Eris (the dwarf planet with the largest known mass),[7] Haumea and Makemake, though the last three have not actually been demonstrated to be dwarf planets.[8]

Geophysical definitions of a planet are alternative definitions of what is and is not a planet.[9][10][2] An early petition rejecting the IAU definition attracted more than 300 signatures, though not all critics supported a geophysical definition.[11][12][13] Proponents of a 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.[14][15] Many critics of the IAU decision were focused specifically on retaining Pluto as a planet, without specifying what a planet should be.[16][17]

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,"[18] which is equivalent to the geophysical limit. While geophysical definitions apply in theory to exoplanets and rogue planets,[10] 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,[19] 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.[20][21][22]

Both geophysical definitions and the IAU definition 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.)