Planetary-mass moon

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A planetary-mass moon is a planetary-mass object that is a natural satellite of another non-stellar celestial object. Because of their mass, these moons are large and ellipsoidal (sometimes spherical) in shape due to hydrostatic equilibrium caused by internal partial melting and differentiation and/or from tidal or radiogenic heating, in some cases forming a subsurface ocean. Planetary-mass moons are sometimes called satellite planets by some planetary scientists such as Alan Stern, who are more concerned with whether a celestial body has planetary geology (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics). Thus they consider planetary-mass moons to be a subset of the planets. This conceptualization of planets as three classes of objects (classical planets, dwarf planets and satellite planets) has not been accepted by the International Astronomical Union (the IAU). Two moons in the Solar System, Ganymede and Titan, are larger than the terrestrial planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive than Mercury. Additionally, seven moons — Ganymede, Titan, Callisto, Io, Luna (Earth's Moon), Europa, and Triton — are larger and more massive than the largest and most massive dwarf planets Pluto and Eris.