Planetary-mass moon

The distinction between a satellite and a classical planet was not recognized until after the heliocentric model of the Solar System was established. When in 1610 Galileo discovered the first satellites of another planet (the four Galilean moons of Jupiter), he referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."[2] Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed the terms "planeta" (planet), "stella" (star), "luna" (moon), and the more modern "satellite" (attendant) to describe it.[3] Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn").[4] However, when the Journal de Scavans reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites", though sometimes to Saturn as the "primary planet".[5] When William Herschel announced his discovery of two objects in orbit around Uranus in 1787, he referred to them as "satellites" and "secondary planets".[6] All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,[7] though the 1868 book Smith's Illustrated Astronomy referred to satellites as "secondary planets".[8]

In the modern era, Alan Stern considers satellite planets to be one of three categories of planet, along with dwarf planets and classical planets.[9] The term planemo ("planetary-mass object") covers all three populations.[10] Both Stern's and the IAU's definition of 'planet' depends on hydrostatic equilibrium – on the mass of the body being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity. The IAU definition specifies that the mass be great enough to overcome 'rigid-body forces', and it does not address objects that may be in hydrostatic equilibrium due to a subsurface ocean or (in the case of Io) due to magma caused by tidal heating. It is possible that all the larger icy moons have subsurface oceans.[11]

The two moons larger than Mercury have less than half its mass, and it is mass, along with composition and internal temperature, that determine whether a body is plastic enough to be in hydrostatic equilibrium. However, there are seven large moons that are more massive than the dwarf planets Eris and Pluto, which are universally believed (though not yet actually demonstrated) to be in equilibrium. These seven are Earth's Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), and the largest moons of Saturn (Titan) and of Neptune (Triton). All of these moons are ellipsoidal in shape. Another dozen moons are ellipsoidal as well, indicating that they achieved equilibrium at some point in their histories. However, it has been shown that some of these moons are no longer in equilibrium, due to them becoming increasingly rigid as they cooled over time.

Determining whether a moon is currently in hydrostatic equilibrium requires close observation, and is easier to disprove than to prove.

Earth's moon, which is entirely rocky, solidified out of equilibrium billions of years ago,[12] but most of the other six moons larger than Pluto, five of which are icy, are assumed to still be in equilibrium. (Ice has less tensile strength than rock, and is deformed at lower pressures and temperatures than rock.) The evidence is perhaps strongest for Ganymede, which has a magnetic field that indicates fluid movement of electrically conducting material in its interior, though whether that fluid is a metallic core or a subsurface ocean is unknown.[13] One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium,[14][11] as may a couple moons of Uranus (Titania and Oberon).[11] However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium.[14] The situation for Uranus's three smaller ellipsoidal moons (Umbriel, Ariel and Miranda) is unclear, as is that of Pluto's moon Charon.[12] The shapes of Eris's moon Dysnomia, Orcus's moon Vanth and Varda's moon Ilmarë are unknown, but Dysnomia is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Miranda and Mimas), Vanth is larger than Mimas, and Ilmarë is approximately the size of Mimas (within current uncertainties), so they are quite possibly ellipsoidal as well. (Or not. Vanth, Mimas and Ilmarë may all be smaller than Proteus, which is not ellipsoidal.)

List

The moons are evaluated for hydrostatic equilibrium in the general sense, not according to the IAU's narrower use of the term.

– believed to be in equilibrium

– confirmed to not be in equilibrium

– uncertain evidence

List of ellipsoidal moons, along with trans-Neptunian moons as large as Mimas[15]

Moon		Image	Radius		Mass		Density	Year of discovery	Hydrostatic equilibrium?
Name	Designation		(km)	(R☾)	(1021 kg)	(M☾)	(g/cm3)
Ganymede	Jupiter III		2634.1±0.3	156.4%	148.2	201.8%	1.942±0.005	1610
Titan	Saturn VI		2574.7±0.1	148.2%	134.5	183.2%	1.882±0.001	1655
Callisto	Jupiter IV		2410.3±1.5	138.8%	107.6	146.6%	1.834±0.003	1610	[16]
Io	Jupiter I		1821.6±0.5	104.9%	89.3	121.7%	3.528±0.006	1610
Luna	Earth I		1737.05	100%	73.4	100%	3.344±0.005	—
Europa	Jupiter II		1560.8±0.5	89.9%	48.0	65.4%	3.013±0.005	1610
Triton	Neptune I		1353.4±0.9	79.9%	21.4	29.1%	2.059±0.005	1846
Titania	Uranus III		788.9±1.8	45.4%	3.40±0.06	4.6%	1.66±0.04	1787	[11]
Rhea	Saturn V		764.3±1.0	44.0%	2.31	3.1%	1.233±0.005	1672	[14]
Oberon	Uranus IV		761.4±2.6	43.8%	3.08±0.09	4.2%	1.56±0.06	1787	[11]
Iapetus	Saturn VIII		735.6±1.5	42.3%	1.81	2.5%	1.083±0.007	1671	[14]
Charon	Pluto I		603.6±1.4	34.7%	1.53	2.1%	1.664±0.012	1978	[12]
Umbriel	Uranus II		584.7±2.8	33.7%	1.28±0.03	1.7%	1.46±0.09	1851
Ariel	Uranus I		578.9±0.6	33.3%	1.25±0.02	1.7%	1.59±0.09	1851
Dione	Saturn IV		561.4±0.4	32.3%	1.10	1.5%	1.476±0.004	1684	[14]
Tethys	Saturn III		533.0±0.7	30.7%	0.617	0.84%	0.973±0.004	1684	[14]
Dysnomia	Eris I		350±58	20.1% ± 3.3%	< 0.44[17]	< 0.6%		2005
Enceladus	Saturn II		252.1±0.2	14.5%	0.108	0.15%	1.608±0.003	1789	[14]
Miranda	Uranus V		235.8±0.7	13.6%	0.064±0.003	0.09%	1.21±0.11	1948
Vanth	Orcus I		221±5	12.7% ± 0.3%	0.02 ~ 0.06	0.03% ~ 0.08%	≈0.8	2005?
Mimas	Saturn I		198.2±0.4	11.4%	0.038	0.05%	1.150±0.004	1789	[14]
Ilmarë	Varda I		163+19 −17[18]	10.4% ± 1.2%	ca. 0.02?[19]	ca. 0.03%	1.24+0.50 −0.35 (for system)	2009

• List of gravitationally rounded objects of the Solar System