Natural satellite

A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. There are 240 known moons within the Solar system, including 155 orbiting the traditional nine planets and 80 more orbiting minor planets, and there are presumably many others orbiting the planets of other stars.

The large gas giants 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 planets.

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 telescopes, 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 points of their orbits (e.g., Saturn's moons Tethys and Dione).

The recent discovery of 243 Ida's moon Dactyl confirms that some asteroids also have moons. 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 moons 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.

Diameter (km)	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto	Other objects
5000-6000			Ganymede	Titan
4000-5000			Callisto					Mercury
3000-4000	Moon		Io Europa
2000-3000						Triton		2003 UB313 Pluto
1000-2000				Rhea Iapetus Dione Tethys	Titania Oberon Umbriel Ariel		Charon	2005 FY9 2003 EL61 90377 Sedna 90482 Orcus 50000 Quaoar
500-1000				Enceladus				1 Ceres 20000 Varuna 28978 Ixion 2 Pallas 4 Vesta (and many others)
250-500				Mimas Hyperion	Miranda	Proteus Nereid		10 Hygiea 511 Davida 704 Interamnia 3 Juno S/2005 (2003 UB313) 1 S/2005 (2003 EL61) 1 (and many others)
100-250			Amalthea Himalia	Phoebe Janus Epimetheus Prometheus	Sycorax Puck Portia	Larissa Galatea Despina		S/2005 (2003 EL61) 2 (too many to list)
50-100			Thebe Elara Pasiphaë	Pandora	Caliban Juliet Belinda Cressida Rosalind Desdemona Bianca	Thalassa Naiad S/2002 N 4	S/2005 P 12 S/2005 P 22	(too many to list)
10-50		Phobos Deimos	Carme Metis Sinope Lysithea Ananke Leda Adrastea	Siarnaq Atlas Helene Albiorix Telesto Pan Paaliaq Calypso Ymir Kiviuq Tarvos Ijiraq	Ophelia Cordelia Setebos Prospero Stephano Perdita Ferdinand Francisco Margaret Trinculo Mab Cupid	S/2002 N 1 S/2002 N 2 S/2002 N 3 Psamathe		(too many to list)
less than 10	Cruithne1		At least 47, see Jupiter's natural satellites.	At least 22, see Saturn's natural satellites.				(too many to list)

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 asteroids and other minor planets.