I'd like to understand better the assertion that planets are spheres because of gravity.

The thing which I believe is true, but don't completely get, is that any arrangement of a sufficient number of molecules, all with zero velocity, in isolation from anything else, will over time form a sphere due to gravitational attraction between the molecules.

If this is true, what is the argument for why this will happen? (i.e. what is the proof that I can't construct a weird initial arrangement of molecules for which it won't happen?) Can we do some math and see it pop out? If this is false, what additional stipulations do I need?