Why are planets spherical? I am a complete novice when it comes to astronomy, but someone asked me the above question casually. It seems as though planets are all roughly spherical, whereas other celestial bodies aren't necessarily. Is there are particular reason for this?
That's mainly because of the gravity, and of size. Small objects, e.g. a stone, create only a very weak gravitation. The stone is much harder than necessary to withstand this force caused by its own gravity.

Earth is much larger. The gravity at the surface causes the weight we feel. A huge mountain on Earth would be flattened under its own weight, because the matter it's made of isn't hard enough to withstand this weight.
By hills filling up valleys the planet as a whole becomes spherical.

A fast-rotating planet gets a slightly flattened ellipsoid shape.