A natural satellite

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A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, dwarf planets, and minor planets.

As of July 2009[update], 336 bodies are formally classified as moons. They include 168 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroid moons, and 58 satellites of Trans-Neptunian objects, some of which will likely turn out to be dwarf planets. Some 150 additional small bodies were observed within Saturn's ring system, but they were not tracked long enough to establish orbits. Planets around other stars are likely to have natural satellites as well, although none have been observed.
Nineteen moons are large enough to be round, and one, Titan, has a substantial atmosphere.

Of the inner planets, Mercury and Venus have no moons; Earth has one large moon, known as the Moon; and Mars has two tiny moons, Phobos and Deimos. 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. It has been suggested that a few moons, notably Europa, one of Jupiter's Galilean moons, may harbour life, though there is currently no direct evidence to support this claim.

Among the dwarf planets, Ceres has no moons. Pluto has three known satellites, the relatively large Charon and the smaller Nix and Hydra. Haumea has two moons, and Eris has one. 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.