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.
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.
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:
| Diameter(km) | Earth | Mars | Asteroids | Jupiter | Saturn | Uranus | Neptune | Pluto |
|---|---|---|---|---|---|---|---|---|
| 5000+ | Ganymede | Titan | ||||||
| 4000-5000 | Callisto | |||||||
| 3000-4000 | Luna | Europa Io |
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| 2000-3000 | Triton | |||||||
| 1000-2000 | 1 Ceres | Iapetus Rhea Dione Tethys |
Ariel Umbriel Titania Oberon |
Charon | ||||
| 500-1000 | 4 Vesta 2 Pallas |
|||||||
| 100-500 | (Too many to list) | Amalthea | Phoebe Hyperion Enceladus Mimas Janus Epimetheus |
Sycorax Miranda Puck Portia |
Proteus Nereid Larissa Galatea Despina |
|||
| 50-100 | (Too many to list) | Himalia Thebe |
Pandora Prometheus |
Setebos Prospero Stephano Caliban 1986U10 Belinda Rosalind Juliet Desdemona Cressida Bianca Cordelia Ophelia |
Thalassa Naiad |
|||
| 10-50 | Phobos Deimos |
(Too many to list) | Sinope Pasiphae Carme Ananke Elara Lysithea Leda Adrastea Metis |
Helene Calypso Telesto Atlas Pan |
Trinculo | |||
| less than 10 | Cruithne¹ | S/2000 J11 S/2000 J1 Iocaste Praxidike Harpalyke Isonoe Erinome Taygete Chaldene S/2002 J1 Kalyke Megaclite Callirrhoe Euporie Kale Orthosie Thyone Euanthe Hermippe Pasithee Eurydome Aitne Sponde Autonoe S/2001 J11 S/2003 J2 S/2003 J3 S/2003 J4 S/2003 J5 S/2003 J6 S/2003 J7 S/2003 J8 S/2003 J9 S/2003 J10 S/2003 J11 S/2003 J12 S/2003 J13 S/2003 J14 S/2003 J15 S/2003 J16 S/2003 J17 S/2003 J18 S/2003 J19 S/2003 J20 S/2003 J21 |
||||||
| unknown | Themisto |
Ymir Paaliaq Siarnaq Tarvos Kiviuq Ijiraq Thrym Skadi Mundilfari Erriapo Albiorix Suttung S/2003 S1 |
S/2002 N1 S/2002 N2 S/2002 N3 |
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
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets