Planets can revolve around the sun. Moons can revolve around the planets. What keeps a moon from having their own moon?
Interesting. Is this true for all moons, or just ours? Is it shape dependent, for example are the gravity fields of Mars' potato moons even lumpier? Dala was telling me about binary planets again last night, where the center of gravity is between two planets and not in a planet, like Pluto and its moon. That exo planet and its possible moon are so massive, I wonder if they might actually be binary planets too . . .the lunar gravity field is 'lumpy' and not smooth; satellites in orbit at low altitude will become unstable quickly.
Knowing whether that is true or not may have to wait for discoveries of moons orbiting distant exoplanets. Moons themselves are much harder to detect and only one promising candidate has been found so far – a possible exomoon orbiting the Jupiter-sized exoplanet Kepler-1625b. That possible moon – about the size of Neptune – is large enough and far enough from its planet that submoons should be possible as well. Astronomers will need to verify that primary moon first – if it does exist – before looking for any submoons.
Maybe moons are too small to have a big enough gravitational force? I know some big moons can have rings : https://en.wikipedia.org/wiki/Rings_of_Rhea How big does a rock that gravitates around a moon has to be, before it's called a "moon" ?