The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are, at least, 140 moons within Earth's solar system, and presumably many others orbiting the planets of other stars. Typically the larger gas giants have extensive systems of moons. Mercury and Venus have no moons at all, Earth has one large moon, Mars has two tiny moons, and Pluto a large companion called Charon (sometimes considered to be a double planet).

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 foreign objects, fragments of larger moons shattered by large 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 distant observations through probes or telescopes, most theories about them are still uncertain.

Physical characteristics

Most moons in the solar system are tidally locked to their primaries; an exception is Saturn's moon Hyperion, which rotates chaotically due to a variety of external influences. No moons have moons of their own; the tidal effects of their primaries make orbits around them unstable. However, several moons have companions in their Lagrangian points (eg, Saturn's moons Tethys and Dione).

The recent discovery of Ida's moon Dactyl confirms that some asteroids also have moons. Some, like 90 Antiope, are double asteroids with two equal-sized components.

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 moons Io, Europa, Ganymede, and Callisto, Saturn's moon Titan, and Neptune's captured moon Triton. For smaller moons see the appropriate planets.

A comparative table classifying the moons of the solar system by diameter, also including a column for some notable asteroids, planets, and Kuiper belt objects.

� It is debatable whether Cruithne counts as a real moon; it is mainly placed here for comparison's sake.

In addition to the moons of the various planets there are also over 30 known asteroid moons, asteroids that orbit other asteroids.

See also

• Jupiter's natural satellites
• Saturn's natural satellites
• Uranus' natural satellites
• Neptune's natural satellites

• Timeline of natural satellites
• Naming of natural satellites

• Quasi-satellite

External links

Jupiter's moons

• Data on Jupiter's satellites (http://www.ifa.hawaii.edu/~sheppard/satellites/jupsatdata.html)
• Jupiter's new moons (discovered in 2000) (http://www.ifa.hawaii.edu/faculty/jewitt/jmoons/jmoons.html)
• Jupiter's new moons (discovered in 2002) (http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html)
• Jupiter's new moons (discovered in 2003) (http://www.ifa.hawaii.edu/~sheppard/satellites/jup2003.html)

Saturn's moons

• Saturn's new moons (discovered in 2000) (http://www.news.cornell.edu/releases/Oct00/Saturn.moons.deb.html)
• Saturn's new moon (discovered in 2003) (http://www.ifa.hawaii.edu/~sheppard/satellites/sat2003.html)

Neptune's moons

• Neptune's new moons (discovered in 2003) (http://sse.jpl.nasa.gov/whatsnew/pr/030113A.html)

All moons

• Scott Sheppard's page (http://www.ifa.hawaii.edu/~sheppard/satellites)
• JPL's Solar System Dynamics page (http://ssd.jpl.nasa.gov)