Editing Definition of planet (section)

=== Hydrostatic equilibrium ===
[[File:Proteus (Voyager 2).jpg|thumb|left|[[Proteus (moon)|Proteus]], a moon of [[Neptune]], is irregular, despite being larger than the spheroidal [[Mimas (moon)|Mimas]].]]
The [[International Astronomical Union|IAU's]] definition mandates that planets be large enough for their own [[gravity]] to form them into a state of [[hydrostatic equilibrium]]; this means that they will reach a round, [[ellipsoid]]al shape. Up to a certain mass, an object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own [[centre of mass]] until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are [[spheroid]]s, and several, such as the larger moons of Saturn and the dwarf planet {{dp|Haumea}}, have been further distorted into ellipsoids by rapid rotation or [[tidal force]]s, but still in hydrostatic equilibrium.<ref>{{cite web| author= Brown, Michael E.| title=2003EL61| work= California Institute of Technology| url=http://www.gps.caltech.edu/~mbrown/2003EL61| access-date=May 25, 2006}}</ref>)

However, there is no precise point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his article, "how are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and non-round shapes, so any boundary would be an arbitrary choice."<ref name=Soter /> Furthermore, the point at which an object's mass compresses it into an ellipsoid varies depending on the chemical makeup of the object. Objects made of ices,{{Ref label|D|d|none}} such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.<ref name=browndwarf>{{cite web|title=The Dwarf Planets|author=Mike Brown|url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/|access-date=August 4, 2007}}</ref> Heat energy, from [[gravitational collapse]], [[impact event|impacts]], tidal forces such as orbital resonances, or [[radioactive decay]], also factors into whether an object will be ellipsoidal or not; Saturn's icy moon Mimas is ellipsoidal (though no longer in hydrostatic equilibrium), but Neptune's larger moon [[Proteus (moon)|Proteus]], which is similarly composed but colder because of its greater distance from the Sun, is irregular. In addition, the much larger Iapetus is ellipsoidal but does not have the dimensions expected for its current speed of rotation, indicating that it was once in hydrostatic equilibrium but no longer is,<ref name="Thomas2010">{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = September 4, 2015| archive-date = December 23, 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> and the same is true for Earth's moon.<ref>Garrick-Bethell et al. (2014) "The tidal-rotational shape of the Moon and evidence for polar wander", ''Nature'' 512, 181–184.</ref><ref name=Bursa>{{Cite journal|last=Bursa|first=M.|date=October 1, 1984|title=Secular Love Numbers and Hydrostatic Equilibrium of Planets|url=https://ui.adsabs.harvard.edu/abs/1984EM&P...31..135B|journal=Earth, Moon, and Planets|volume=31|issue=2 |pages=135–140|doi=10.1007/BF00055525|bibcode=1984EM&P...31..135B |s2cid=119815730 |issn=0167-9295}}</ref> Even Mercury, universally regarded as a planet, is not in hydrostatic equilibrium.<ref name="Mercury">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.</ref> Thus the IAU definition is not taken literally even by the IAU, as it includes Mercury as a planet; its requirement for hydrostatic equilibrium is in practice ignored in favour of a requirement for roundedness.<ref>{{Cite tweet |user=plutokiller |last=Brown |first=Mike |number=1624127764969459713 |title=The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Merucry & the moon}}</ref>