Why isn the moon considered a planet?
While we don’t yet have a good definition of a planet, a planet is considered to be something that orbits the Sun on it’s own, while a moon is something that orbits something that orbits the Sun. Asteroids can (and do) have moons, to my knowledge there are no moons with their own moons. Actually, some space scientists think that Earth and the Moon should be considered a double planet system, since they are so similar in size (compared to, say, Jupiter and any of its moons). But since they didn’t form at the same time (the Moon formed due to a huge collision with Earth) this idea probably will never catch on.
