Natural satellite

The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are, at least, 138 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).

Table of contents

1 Origin

2 Physical characteristics

3 Moons of the Solar system

4 See also

5 External links

5.1 Jupiter's moons
5.2 Saturn's moons
5.3 Neptune's moons
5.4 All moons

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.

Diameter(km)	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto	Other objects
5000+			Ganymede	Titan
4000-5000			Callisto					Mercury
3000-4000	Moon		Io Europa
2000-3000						Triton		Pluto
1000-2000				Rhea Iapetus Dione Tethys	Titania Oberon Umbriel Ariel		Charon	Sedna 2004 DW Quaoar Varuna Ixion
100-1000			Himalia Amalthea	Enceladus Mimas Hyperion Phoebe Janus Epimetheus Prometheus	Miranda Sycorax Puck Portia	Proteus Nereid Larissa Galatea Despina		Ceres Pallas Juno Vesta (and many others)
50-100			Thebe Elara Pasiphaë	Pandora	Caliban Juliet Belinda Cressida Rosalind Desdemona Bianca	Thalassa Naiad S/2002 N4		(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 S/1986 U10 S/2001 U2 S/2001 U3 S/2003 U3 Trinculo S/2003 U1 S/2003 U2	S/2002 N1 S/2002 N2 S/2002 N3 S/2003 N1		(Too many to list)
less than 10	Cruithne¹		Callirrhoe Themisto Praxidike Megaclite Kalyke Iocaste Taygete Harpalyke S/2000 J11 Thyone S/2003 J6 Hermippe Autonoe S/2003 J7 S/2003 J5 S/2003 J1 Chaldene Isonoe Erinome S/2003 J20 Euanthe Eurydome S/2002 J1 Aitne S/2003 J8 S/2003 J3 Euporie S/2003 J21 S/2003 J18 S/2003 J22 Orthosie S/2003 J16 S/2003 J15 S/2003 J17 S/2003 J11 S/2003 J19 Pasithee Kale S/2003 J4 Sponde S/2003 J13 S/2003 J23 S/2003 J10 S/2003 J14 S/2003 J2 S/2003 J12 S/2003 J9	Erriapo S/2003 S1 Skadi Mundilfari Suttung Thrym				(Too many to list)

¹ It is debatable whether Cruithne counts as a real moon; it is mainly placed here for comparison's sake.

External links

Jupiter's moons

Saturn's moons

Neptune's moons

Neptune's new moons (discovered in 2003)

All moons

The Solar System

(For other objects and regions, see: List of solar system objects, Astronomical objects)