So only our own solar system has been strictly defined thus far.
A moon is a natural satellite that orbits a planet.
A planet has three criteria:
- It must orbit the Sun
- It must have enough gravity to have molded itself into a spherical shape
- It must have cleared its orbit (this is why Pluto got ousted a decade or so ago)
So many planets have moons (all but Venus and Mercury). In my opinion being a moon doesn’t mean you’re any less awesome than a planet. Some of the most bizarre and incredible places in the solar system are moons:
- Titan, which has a thick atmosphere and lakes of liquid methane
- Europa and Enceladus which are giant snowballs that have entire liquid oceans underneath their surface… and potentially life
- Phobos is likely an asteroid that was captured by Mars and is doomed
- Triton a moon of Neptune is possibly captured as well. It could’ve been a planet once (some suggest it may be a captured exoplanet); also doomed, however it will first form an incredibly beautiful, bright ring around Neptune before final total destruction
- Io, moon of Jupiter, is the most volcanic place in the solar system. The gravitational stress of nearby Jupiter stretches and crushes this moon so much that the interior is superheated
- Our moon, the Moon, was created out of the remains of Earth and an alien planet that crashed into a young Earth, long ago.