Planetary-mass moon

Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean.

Two moons in the Solar System, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive.

Additionally, seven – Ganymede, Titan, Callisto, Io, Luna, Europa, and Triton – are larger and more massive than the dwarf planets Pluto and Eris.

The distinction between a satellite and a classical planet was not recognized until after the heliocentric model of the Solar System was established.

[4] However, when the Journal de Scavans reported Cassini's discovery of two new Saturnian moons (Tethys and Dione) 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 (Titania and Oberon) in 1787, he referred to them as "satellites" and "secondary planets".

[10] Stern's and the IAU's definition of 'planet' depends on hydrostatic equilibrium – on the body's mass being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity.

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

[11] However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium.

[16] In addition to not being in equilibrium, Mimas and Tethys have very low densities and it has been suggested that they may have non-negligible internal porosity,[17][18] in which case they would not be satellite planets.