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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. There are 172 known natural satellites orbiting six of the eight planets, and eight orbiting three of the five IAU-listed dwarf planets. As of January 2012, over 200 minor-planet moons have been discovered. There are 76 objects in the asteroid belt with satellites, four Jupiter...  MORE

A natural satellite, or moon, is a celestial body that orbits a planet or smaller body, which is called its primary. There are 172 known natural satellites orbiting six of the eight planets, and eight orbiting three of the five IAU-listed dwarf planets. As of January 2012, over 200 minor-planet moons have been discovered. There are 76 objects in the asteroid belt with satellites, four Jupiter trojans, 39 near-Earth objects, and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, though none have yet been observed. Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The large gas giants have extensive systems of natural satellites, 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 natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life, though there is currently no direct evidence of life. LESS