Why we stopped calling moons satellites, and what counts as one
Kepler coined satellite in 1610, borrowing the Latin satelles, a guard or attendant, for the four companions Galileo had spotted around Jupiter. The word served astronomers for centuries until Sputnik launched in 1957 and it came to mean machines. So moon, lowercase, returned to scientific papers, with Earth's own Moon keeping its capital letter.
The Solar System's six planetary families hold 456 satellites with confirmed orbits, and hundreds of minor planets have companions too, including Pluto, Eris and several other dwarf planets. Mercury and Venus have none; Mars has two tiny ones, Phobos and Deimos. The giants host crowds, among them six bodies comparable to our Moon: Jupiter's four Galilean moons, Saturn's Titan and Neptune's Triton. There is no agreed smallest moon. Bodies about a kilometre wide count, while smaller objects inside Saturn's rings and around asteroids get called moonlets.
Orbits hint at origins. Regular moons, close in and circling the same way their planet spins, probably condensed from the same patch of disc that built the planet. Irregular ones, on distant, tilted or backward paths, are likely captured asteroids. Triton is the odd one: large and on a tight circular orbit, yet moving backwards, suggesting it was a captured dwarf planet. Capture can also be brief. The small object 2006 RH120 circled Earth for about nine months across 2006 and 2007 before drifting off.
Most regular moons are tidally locked, showing one face to their planet, though Saturn's Hyperion tumbles chaotically under Titan's pull. Distant irregular moons such as Phoebe spin in roughly ten hours while taking hundreds of days to orbit. Saturn also hosts Trojan moons: Telesto and Calypso lead and trail Tethys, and Helene and Polydeuces do the same for Dione. No moon of a moon has ever been confirmed; a hinted ring around Rhea was not seen when Cassini looked.
Several of the nineteen moons big enough to be rounded by gravity are still active. Io is the most volcanic body in the Solar System, heated by tidal flexing, while Enceladus and Triton vent geyser-like plumes. Titan has a thick atmosphere and hydrocarbon lakes. Europa, Ganymede, Callisto, Titan and little Enceladus are thought to hide oceans of liquid water beneath their ice.
Source: Natural satellite
