Natural satellite

A natural satellite is a celestial body that orbits another celestial body of greater mass (e.g., a planet, star, or dwarf planet), which is called its primary. For example, the Moon is a natural satellite of Earth, and Earth is a natural satellite of the Sun.

In the Solar System there are 173 known natural satellites which orbit within 6 planetary satellite systems. In addition, several other objects are known to have satellites, including three IAU-listed dwarf planets: Pluto, Haumea, and Eris.As of January 2012, over 200 minor-planet moons have been discovered. There are 76 known objects in the asteroid belt with satellites (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.