I've been wondering for a while now why some objects in the universe (such as stars, planets, etc.) are round, while others are flat (galaxies, planetary rings and protoplanetary disks).
I know these shapes come to be because of gravity. In planets, the gravity from the core pulls everything to the centre, thus, rounding the planet (same with stars). But with Saturn, for example, why have its rings formed as rings and not as a spherical layer of ice surrounding the planet?
The same with the Kuiper belt and asteroid belt, I would presume the mechanics there are the same as with a planetary orbit, hence, why they are belts and not clouds. But then what about the oort cloud? Why is that a huge cloud in the outermost solar system and not a huge ring of ice.
As for the solar system itself, I've read that the solar system is kind of egg-shaped, but shouldn't it be just like the galaxy (or the other way round)?
I've tried looking into this and wondering about what different laws of physics may apply in each case, but it's all really confusing.