section_path: History and etymology/Defining the term planet/Criticisms and alternatives to IAU definition
section_name: Criticisms and alternatives to IAU definition
paragraph_text: Criticisms and alternatives to IAU definition
  
  The IAU definition has not been universally used or accepted. In planetary geology, celestial objects are defined as planets by geophysical characteristics. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state of hydrostatic equilibrium where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:
  
  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.
  
  In the Solar System, this mass is generally less than the mass required for a body to clear its orbit; thus, some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto. (In practice, the requirement for hydrostatic equilibrium is universally relaxed to a requirement for rounding and compaction under self-gravity; Mercury is not actually in hydrostatic equilibrium, but is universally included as a planet regardless.) Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object. Dwarf planets had been proposed as a category of small planet (as opposed to planetoids as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.
  
  The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900–1000 km diameter. There is general consensus on Ceres in the asteroid belt and on the eight trans-Neptunians that probably cross this threshold— Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.
  
  Planetary geologists may include the nineteen known planetary-mass moons as "satellite planets", including Earth's Moon and Pluto's Charon, like the early modern astronomers. Some go even further and include as planets relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta; rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea; or even everything at least the diameter of Saturn's moon Mimas, the smallest planetary-mass moon. (This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moon Proteus.)
  
  Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star. The formula produces a value called π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement – though this may not be the case around very low-mass stars. In 2024, Margot and collaborators proposed a revised version of the criterion with a uniform clearing timescale of 10 billion years (the approximate main-sequence lifetime of the Sun) or 13.8 billion years (the age of the Universe) to accommodate planets orbiting brown dwarfs.