A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a galactic center, but these uses are rare. Instead, the term is normally used to identify non-artificial satellites of planets, dwarf planets, or minor planets.

There are 240 known moons within the Solar System, including 162 orbiting the planets,^[1] 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. 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. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon (the Moon); and Mars has two tiny moons, Phobos and Deimos. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do), Eris has one, Dysnomia, and Pluto has three known satellites, Nix, Hydra, and a large companion called Charon. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.

Origin

Moons orbiting relatively close to the planet on prograde orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that gave rise to its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. The Earth-Moon^[2] and possibly Pluto-Charon systems^[3] are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting moons. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process.

Orbital characteristics

Tidal locking

Most regular 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 include Saturn's moon Hyperion, which rotates chaotically because of a variety of external influences.

In contrast, the outer moons of the gas giants (irregular satellites) are too far away to become 'locked'. For example, Jupiter's moon Himalia, Saturn's moon Phoebe and Neptune's Nereid have rotation period in the range of 10 hours compared with their orbital periods of hundreds of days.

Satellites of satellites

No "moons of moons" (natural satellites that orbit the natural satellite of another body) are known. It is uncertain whether such objects can be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects (most notably the primary) would perturb the orbit of the moon's moon until it broke away or impacted its primary. In theory, a secondary satellite could exist in a primary satellite's Hill sphere, outside of which it would be lost because of the greater gravitational pull of the planet (or other object) that the primary satellite orbits. For example, the Moon orbits the Earth because the Moon is 370,000 km from Earth, well within Earth's Hill sphere, which has a radius of 1.5 million km (0.01 AU or 235 Earth radii). If a Moon-sized object were to orbit the Earth outside its Hill sphere, it would soon be captured by the Sun and become a dwarf planet in a near-Earth orbit.

Trojan satellites

Two moons are known have small companions at their L₄ and L₅ Lagrangian points, which are about sixty degrees ahead of and behind the body in its orbit. These companions are called Trojan moons, because their positions are comparable to the positions of the Trojan asteroids relative to Jupiter. Such objects are Telesto and Calypso, which are the leading and following companions respectively of Tethys; and Helene and Polydeuces, which are the leading and following companions of Dione.

Asteroid satellites

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. In addition to the moons of the various planets there are also over 80 known moons of the dwarf planets, asteroids and other small solar system bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

The following is a comparative table classifying the moons of the solar system. The column on the right includes some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparison.

Terminology

The first known natural satellite was the Moon (luna in Latin). Until the discovery of the Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.

Christiaan Huygens, the discoverer of Titan, was the first to use the term moon for such objects, calling Titan Luna Saturni or Luna Saturnia – "Saturn's moon" or "The Saturnian moon", because it stood in the same relation to Saturn as the Moon did to the Earth.

As additional moons of Saturn were discovered, however, this term was abandoned. Giovanni Domenico Cassini sometimes referred to his discoveries as planètes in French, but more often as satellites, using a term derived from the Latin satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite or artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has come to be linked primarily with artificial objects flown in space – including, sometimes, even those which are not in orbit around a planet.

As a consequence of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with the Earth's moon on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used.

The definition of a moon

There has been some debate about the precise definition of a moon. This debate has been caused by the presence of orbital systems where the difference in mass between the larger body and its satellite are not as pronounced as in more typical systems. Two examples include the Pluto-Charon system, and the Earth-Moon System. The presence of these systems has caused a debate about where to precisely draw the line between a double body system, and a main body-satellite system, the most commonly agreed upon definition is whether the barycentre is below the surface of the larger body, though this is not official, and somewhat arbitrary. At the other end of the spectrum there are many ice/rock clumps that form the ring systems on the Solar System's gas giants and there is no set point to define when one of these clumps is large enough to be classified as a moon, though the term "moonlet" may be sometimes used to refer to extremely small objects in orbit around a larger body, there is again no official definition.

Natural satellites of planets and dwarf planets

• Earth's natural satellite, see Moon
• Mars' natural satellites
• Jupiter's natural satellites
• Saturn's natural satellites
• Uranus' natural satellites
• Neptune's natural satellites
• Pluto's natural satellites
• Eris' natural satellite, see Dysnomia

Natural satellites of Small Solar System bodies

• 2003 EL61's natural satellites