Question: Can you explain to me why some planets (such as venus) do not have any moons? Does it have anything to do with gravity?

Sara Imari Walker answered on 15 Jun 2011:

Hi emmajane! Great question!! Most rocky planets are expected not to have moons. It does have to do with gravity and the amount of stuff thats around. Most gaseous giants form further out and get moons, rocky planets (made of denser material) are closer in and usually don’t have moons. Individual objects in the planetary disk show differences in their growth and composition that depend on their distances from the hot central mass (the Sun). Stuff further in is rocky, no gas can condense there. Most stuff goes further out and makes big planets, with extra stuff to make moons.

So, Mercury and Venus don’t have and Mars only has two tiny moons that are probably captured asteroids. Our moon is quite unusual. Not only is it one of the biggest in the solar system, but it is around a tiny rocky planet!! The rest of the moons in our solar system of comparable size (like Europa and Titan) are circling big planets like Jupiter and Saturn. So the question is not so much why other rocky planets don’t have moons by why DO we? The answer is that we had a very unusual event happen to Earth early in its formation. In the early solar system there was lots of junk flying around, big rocks coming together to make even bigger planets. Well Earth got struck by a really big one! We collided with a protoplanet named Theia (about the size of Mars). That collision knocked a chunk of the Earth off which then formed the moon. The chunk that came off formed its own spherical object due to gravity! So ours is pretty special!