Natural satellite

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.

Most of the 205 known natural satellites of the planets are natural satellites. Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton are the seven largest and most massive natural satellites in the Solar System (see List of natural satellites § List). Triton is more massive than all smaller natural satellites combined.

In the Solar System, there are six planetary satellite systems containing 207 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.[1] As of September 2018, there are 334 other minor planets known to have natural satellites.[2]

A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter.[3] 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 and about 1/80th of it mass.[4] The next largest ratios are the NeptuneTriton system at 0.055 (with a mass ratio of about 1 to 5000), the SaturnTitan system at 0.044 (with the second mass ratio next to the Earth-Moon system, 1 to 4250), the JupiterGanymede system at 0.038, and the UranusTitania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter of Pluto.

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