A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Two moons in the Solar System are larger than the planetMercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planet. The concept of satellite planets – the idea that planetary-mass objects, including planetary-mass moons, are planets – is used by some planetary scientists, such as Alan Stern, who are more concerned with whether a celestial body has planetary geology than where it orbits. This conceptualization of planets as three classes of objects has not been accepted by the International Astronomical Union. In addition, the IAU definition of 'hydrostatic equilibrium' is quite restrictive – that the object's mass be sufficient for gravity to overcome rigid-body forces to become plastic – whereas planetary-mass moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean.
Early history
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, he referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness." Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed the terms "planeta", "stella", "luna", and the more modern "satellite" to describe it. Giovanni Cassini, in announcing his discovery of Saturn's moonsIapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne. 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". When William Herschel announced his discovery of two objects in orbit around Uranus in 1787, he referred to them as "satellites" and "secondary planets". All subsequent reports of natural satellite discoveries used the term "satellite" exclusively, though the 1868 book Smith's Illustrated Astronomy referred to satellites as "secondary planets".
Modern concept
In the modern era, Alan Stern considers satellite planets to be one of three categories of planet, along with dwarf planets and classical planets. The term planemo covers all three populations. 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 due to magma caused by tidal heating. It is possible that all the larger icy moons have subsurface oceans. 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 and Pluto, which are universally believed to be in equilibrium. These seven are Earth's Moon, the four Galilean moons of Jupiter, and the largest moons of Saturn and of Neptune. 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.
Current equilibrium moons
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, but most of the other six moons larger than Pluto, five of which are icy, are assumed to still be in equilibrium. 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. One of the mid-sized moons of Saturn may also be in equilibrium, as may a couple moons of Uranus. However, the other ellipsoidal moons of Saturn are no longer in equilibrium. The situation for Uranus's three smaller ellipsoidal moons is unclear, as is that of Pluto's moon Charon. 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, Vanth is larger than Mimas, and Ilmarë is approximately the size of Mimas, so they are quite possibly ellipsoidal as well.
List
The moons are evaluated for hydrostatic equilibrium in the general sense, not according to the IAU's narrower use of the term.
