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A natural Satellite is a '''moon''' (not capitalized), that is, any natural object that orbits a Planet . The term ''natural satellite'' may also refer to a planet orbiting a star, as is the case with the earth and the sun. There are 240 known moons within the Solar System , including 155 orbiting the traditional nine planets and 80 more orbiting Minor Planet s, and there are presumably many others orbiting the planets of other stars. The large Gas Giant s have extensive systems of moons, including half a dozen comparable in size to Earth's moon. Mercury and Venus have no moons at all, Earth has one large moon ("The Moon " or "Luna"), Mars has two tiny moons, and Pluto has at least three satellites, including a large companion called Charon . Pluto-Charon and some of the Asteroid systems are sometimes considered to be Double Planet s. ORIGIN Most moons are assumed to have been formed out of the same collapsing region of Protoplanetary Disk that gave rise to its primary. However, there are many exceptions and variations to this standard model of moon formation that are known or theorized. Several moons are thought to be captured asteroids; others may be fragments of larger moons shattered by impacts, or (in the case of Earth's Moon ) a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few observations via Probes or Telescope s, most theories about their origins are still uncertain. ORBITAL CHARACTERISTICS Most moons in the solar system are Tidally Locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions are Saturn 's moon Hyperion , which rotates chaotically due to a variety of external influences, and the outermost moons of the gas giants, which are too far away to become 'locked' (an example is Saturn's moon Phoebe ). It is not possible for a moon to have moons of its own: the tidal effects of their primaries would make such a system unstable. However, several moons have small companions in the Lagrangian Point s of their orbits (e.g., Saturn's moons Tethys and Dione ). The recent discovery of 243 Ida 's moon Dactyl confirms that some Asteroid s also have Moon s. Some, like 90 Antiope , are double asteroids with two equal-sized components. The asteroid 87 Sylvia has two moons. See Asteroid Moon for further information. MOONS OF THE SOLAR SYSTEM The largest moons in the solar system (those bigger than about 3000 km across) are Earth's Moon , Jupiter 's Galilean Moon s Io , Europa , Ganymede , and Callisto , Saturn 's moon Titan , and Neptune 's captured moon Triton . For smaller moons see the articles on the appropriate planet. The following is a comparative table classifying the moons of the solar system by diameter. The column on the right includes some notable planets, asteroids and Kuiper Belt objects for comparison. 1) Cruithne is not a real moon; it is mainly placed here for comparison's sake. 2) Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 44 and 130 km. In addition to the moons of the various planets there are also over 80 known moons of the Asteroid s and other Minor Planet s. SEE ALSO
EXTERNAL LINKS Jupiter's moons
Saturn's moons Neptune's moons All moons
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