Editing Hydrostatic equilibrium (section)

=== Planetary geology ===
{{see also|List of gravitationally rounded objects of the Solar System}}
{{further|Clairaut's theorem (gravity)}}

The concept of hydrostatic equilibrium has also become important in determining whether an astronomical object is a [[planet]], [[dwarf planet]], or [[small Solar System body]]. According to the [[definition of planet]] that was adopted by the [[International Astronomical Union]] in 2006, one defining characteristic of planets and dwarf planets is that they are objects that have sufficient gravity to overcome their own rigidity and assume hydrostatic equilibrium. Such a body often has the differentiated interior and geology of a world (a [[planemo]]), but near-hydrostatic or formerly hydrostatic bodies such as the proto-planet [[4 Vesta]] may also be differentiated and some hydrostatic bodies (notably [[Callisto (moon)|Callisto]]) have not thoroughly differentiated since their formation. Often, the equilibrium shape is an [[oblate spheroid]], as is the case with Earth. However, in the cases of moons in synchronous orbit, nearly unidirectional tidal forces create a [[scalene ellipsoid]]. Also, the purported dwarf planet {{dp|Haumea}} is scalene because of its rapid rotation though it may not currently be in equilibrium.

Icy objects were previously believed to need less mass to attain hydrostatic equilibrium than rocky objects. The smallest object that appears to have an equilibrium shape is the icy moon [[Mimas (moon)|Mimas]] at 396&nbsp;km, but the largest icy object known to have an obviously non-equilibrium shape is the icy moon [[Proteus (moon)|Proteus]] at 420&nbsp;km, and the largest rocky bodies in an obviously non-equilibrium shape are the asteroids [[2 Pallas|Pallas]] and [[4 Vesta|Vesta]] at about 520&nbsp;km. However, Mimas is not actually in hydrostatic equilibrium for its current rotation. The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet [[Ceres (dwarf planet)|Ceres]], which is icy, at 945&nbsp;km, and the largest known body to have a noticeable deviation from hydrostatic equilibrium is [[Iapetus (moon)|Iapetus]] being made of mostly permeable ice and almost no rock.<ref>{{Cite journal |last=Thomas |first=P.C. |date=July 2010 |title=Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission |url=http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf |journal=Icarus |volume=208 |issue=1 |pages=395–401 |doi=10.1016/j.icarus.2010.01.025 |bibcode=2010Icar..208..395T |archive-date=23 December 2018 |archive-url=https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf |url-status=dead }}</ref> At 1,469&nbsp;km Iapetus is neither spherical nor ellipsoid. Instead, it is rather in a strange walnut-like shape due to its unique [[Eequatorial ridge on Iapetus|equatorial ridge]].<ref name="Castillo2007">{{cite journal| last=Castillo-Rogez| first=J. C.|author2=Matson, D. L. |author3=Sotin, C. |author4=Johnson, T. V. |author5=Lunine, Jonathan I. |author6= Thomas, P. C. | title=Iapetus' geophysics: Rotation rate, shape, and equatorial ridge| journal=Icarus| date=2007| volume=190| issue=1| pages=179–202| doi=10.1016/j.icarus.2007.02.018| bibcode=2007Icar..190..179C}}</ref> Some icy bodies may be in equilibrium at least partly due to a subsurface ocean, which is not the definition of equilibrium used by the IAU (gravity overcoming internal rigid-body forces). Even larger bodies deviate from hydrostatic equilibrium, although they are ellipsoidal: examples are Earth's [[Moon]] at 3,474&nbsp;km (mostly rock),<ref>{{cite journal |last1=Garrick-Bethell |first1=I. |last2=Wisdom |first2=J |last3=Zuber |first3=MT |title=Evidence for a Past High-Eccentricity Lunar Orbit |journal=Science |date=4 August 2006 |volume=313 |issue=5787 |pages=652–655 |doi=10.1126/science.1128237 |pmid=16888135 |bibcode=2006Sci...313..652G |s2cid=317360 }}</ref> and the planet [[Mercury (planet)|Mercury]] at 4,880&nbsp;km (mostly metal).<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>

In 2024, Kiss et al. found that {{dp|Quaoar}} has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium but that its shape became "frozen in" and did not change as it spun down because of tidal forces from its moon [[Weywot]].<ref name="Kiss2024">
{{cite journal
 |display-authors = etal
 |first1 = C. |last1 = Kiss
 |first2 = T. G. |last2 = Müller
 |first3 = G. |last3 = Marton
 |first4 = R. |last4 = Szakáts
 |first5 = A. |last5 = Pál
 |first6 = L. |last6 = Molnár
 |title      = The visible and thermal light curve of the large Kuiper belt object (50000) Quaoar
 |journal    = Astronomy & Astrophysics
 |date       = March 2024
 |volume     = 684
 |issue      = 
 |pages      = A50
 |doi        = 10.1051/0004-6361/202348054
 |arxiv      = 2401.12679
 |bibcode    = 2024A&A...684A..50K
}}</ref> If so, this would resemble the situation of Iapetus, which is too oblate for its current spin.<ref name="Science News">Cowen, R. (2007). Idiosyncratic Iapetus, ''Science News'' vol. 172, pp. 104–106. [http://www.sciencenews.org/articles/20070818/bob8ref.asp references] {{Webarchive|url=https://web.archive.org/web/20071013165655/http://www.sciencenews.org/articles/20070818/bob8ref.asp |date=2007-10-13 }}</ref><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 = 2015-09-25| archive-date = 2018-12-23| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> Iapetus is generally still considered a [[planetary-mass moon]] nonetheless<ref name=planetarysociety>Emily Lakdawalla et al., [https://www.planetary.org/worlds/what-is-a-planet What Is A Planet?] {{Webarchive|url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |date=2022-01-22}} The Planetary Society, 21 April 2020</ref> though not always.<ref name=ChenKipping>{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free}}</ref>

Solid bodies have irregular surfaces, but local irregularities may be consistent with global equilibrium. For example, the massive base of the tallest<!--as opposed to highest--> mountain on Earth, [[Mauna Kea]], has deformed and depressed the level of the surrounding crust and so the overall distribution of mass approaches equilibrium.