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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.
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"), Mars has two tiny moons, and Pluto has three, including a large companion called Charon (Pluto and Charon are sometimes considered a double planet).
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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.
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.
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 |
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| 3000-4000 | Luna | Io |
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| 2000-3000 | Triton | 2003 UB313 Pluto |
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| 1000-2000 | Rhea |
Charon |
90377 Sedna 90482 Orcus |
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| 100-1000 | Himalia |
Enceladus | Miranda |
Proteus |
S/2005 P 12 S/2005 P 22 |
1 Ceres 2 Pallas |
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| 50-100 | Thebe |
Pandora | Caliban |
Thalassa |
(Too many to list) | |||
| 10-50 | Phobos |
Carme |
Siarnaq
Atlas |
Ophelia
Cordelia |
S/2002 N 1 |
(Too many to list) | ||
| less than 10 | Cruithne1 | At least 47, see Jupiter's natural satellites |
Erriapo Narvi |
(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 64 and 200 km.
In addition to the moons of the various planets there are also over 30 known asteroid moons, asteroids that orbit other asteroids.
| The Solar System |
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| Sun - Mercury - Venus - Earth (Moon) - Mars - Asteroid belt - Jupiter |
| Saturn - Uranus - Neptune - Pluto - Kuiper belt - Scattered disc - Oort cloud |
| See also astronomical objects and the solar system's list of objects, sorted by radius or mass. |