Moons (Natural Satellites)

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as moons, a derivation from the Moon of Earth.

In the Solar System, there are six planetary satellite systems containing 209 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, and Eris. As of November 2021, there are 442 other minor planets known to have natural satellites.

Definition of a moon

There is no established lower limit on what is considered a “moon”. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn’s rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.

The natural satellites orbiting relatively close to the planet on prograde, regular satellites are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created 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. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.

The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated from the collision of two large proto-planetary objects. The material that would have been placed in orbit around the central body is predicted to have accreted to form one or more orbiting natural satellites. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.