Geophysical definition of planet
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.
Definitions[]
Some geoscientists adhere to the formal definition of a planet that was proposed by the International Astronomical Union (IAU) in August of 2006.[2] 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.[3]
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.[4]
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:
- 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,
- 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, the largest being 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 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 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 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:[5]
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,[6] while others do not.[7]
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 km diameter, but rocky Vesta is not at 525 km diameter.[8] (And at much lower temperatures, icy Salacia in the Kuiper belt might not have fully gravitationally collapsed even at 850 km 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.[8]
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 400 km 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 1500 km 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.[4][5][6] (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 1000 km in diameter (e.g. (55637) 2002 UX25 and Gǃkúnǁʼhòmdímà) have never compressed out their internal porosity,[9] 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, Eris, Haumea, Makemake, Gonggong, Charon, Quaoar, Sedna, and probably Orcus pass the 900 km threshold. Salacia (about 850 km in diameter) and Varda (about 740 km 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".[14]
The bodies generally agreed to be geophysical planets include the eight major planets: