A natural satellite, or moon, is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body).
In the Solar System, there are six planetary satellite systems containing 205 known natural satellites.
IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other minor planets known to have moons.A planet usually has at least around 10000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is the unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth. The next largest ratios are Neptune at 0.055, Saturn at 0.044, Jupiter at 0.038, and Uranus at 0.031. For the category of planetoids, among the five that are known in the Solar System, Charon has the largest ratio, being 0.52 the diameter of Pluto.