Hi Sophia,

Good question. The answer has to do with a fundamental aspect of the universe. Planets are mostly spherical because the way that gravity works. Gravity pulls mass together from all directions equally. In space, this leads to a sphere of mass. It can get much more complicated than this simple answer if there isn't very much mass that is getting pulled together by gravity. But with large enough bodies, this is the reason.

Because most planets spin, they bulge at their equators so this makes them not quite spheres.

Laura