|MadSci Network: Astronomy|
Hiya, Thanks for your question. This is basically just a result of the force of gravity! Gravity pulls the particles and components that form the planets together to be as close as possible; the most efficient way for this to happen is to form a sphere, which gets the most "stuff" as close as possible to the centre. Those planets, like Jupiter, which are noticably flattened obtain their shape from their rotation, which pushes outward the equitorial reigons (which are moving faster than the poles) Chris Lintott
Try the links in the MadSci Library for more information on Astronomy.