Asked by Mark West
As with many common terms, the astronomical community has never really got around to making a formal definition of what makes a moon (they only came up with a formal classification for planets in 2006!). So as it stands, a moon is simply any natural body in orbit around a planet or other non-stellar object (so asteroids can have moons, and it’s even possible for a moon to have its own moon, although none have been discovered so far).
Known moons in our solar system range in size from Jupiter’s giant satellite Ganymede (larger than the planet Mercury at 5,268 km or 3,273 miles across) down to small bodies less than 10km (6 miles) across. Jupiter has at least 50 of these tiny satellites, Saturn at least 36, and many more certainly await discovery. These smaller moons are mostly a mix of captured asteroids and comets in long, eccentric orbits around their planets, and ‘shepherd moons’ whose gravity helps regulate the structure of planetary rings.
Of course, each one of the tiny orbiting fragments that makes up the rings themselves is technically a satellite in its own right, but so far astronomers have not decided to start naming them individually, and true ‘moons’ are restricted to those orbiting in relatively empty surroundings. The term ‘moonlet’, meanwhile, usually refers to larger, perhaps temporary, structures found inside the body of the rings, but is also, confusingly, sometimes used to refer to natural satellites of asteroids.
Answered by astronomy and space author Giles Sparrow