Natural satellite
|
Moons of solar system scaled to Earth's Moon |
A
natural satellite is a non-
man-made object that orbits a
planet or other body larger than itself. It is commonly referred to as a
moon (not capitalized). The term
natural satellite may also refer to a
planet orbiting a
star, as is the case with the eight or nine planets orbiting the
Sun. There are 240 known moons within the
Solar system, including 80 orbiting
minor planets (not the bodies in the
asteroid belt) and over 150 orbiting the planets. Other stars and their planets also have natural satellites.
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.
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 major 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. In addition to the moons of the various planets there are also over 80 known moons of the
asteroids and other
minor planets.
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
Trans-Neptunian Objects for comparison.
*
Mars' natural satellites*
Asteroid moon*
Jupiter's natural satellites*
Saturn's natural satellites*
Uranus' natural satellites*
Neptune's natural satellites*
Pluto's natural satellites*
Irregular satellites
*
Timeline of discovery of Solar System planets and their natural satellites*
Naming of natural satellites*
Quasi-satellite*
Extrasolar moonJupiter's moons
*
Data on Jupiter's satellites*
Jupiter's new moons (discovered in 2000)*
Jupiter's new moons (discovered in 2002)*
Jupiter's new moons (discovered in 2003)Saturn's moons
*
Saturn's new moons (discovered in 2000)*
Saturn's new moon (discovered in 2003)Neptune's moons
*
Neptune's new moons (discovered in 2003)All moons
*
Natural Satellite Physical Parameters (JPL-NASA, with refs)*
Moons of the Solar System (The Planetary Society)*
Scott Sheppard's page*
Major moons in order from the Sun*
JPL's Solar System Dynamics page*
Moon of an Object? First Photo of Satellite Beyond the Solar System*
USGS list of named moons*
Upper size limit for moons explained
