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). While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.
In the Solar System, there are six planetary satellite systems containing 205 known natural satellites. Four of the five IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018[update], there are 334 other minor planets known to have natural satellites.
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 the Neptune–Triton system at 0.055, the Saturn–Titan system at 0.044, the Jupiter–Ganymede system at 0.038, and the Uranus–Titania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter of Pluto.