A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. (There are no known natural satellites of moons.)
Two hundred and forty bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets,[1] 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets are very likely to have natural satellites, although none have yet been observed.
The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth‘s moon: the four Galilean moons, Saturn‘s Titan, and Neptune‘s Triton. Saturn has an additional six mid-sized moons massive enough to have achieved hydrostatic equilibrium, and Uranus has five. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon (the Moon); and Mars has two tiny moons, Phobos and Deimos. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do), Eris has one, Dysnomia, and Pluto has three known satellites, Nix, Hydra, and a large companion called Charon. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.
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