system
Given the following post, respond as if you're commenting on Reddit:
human
If the gravitational pull of a planet is the same in all directions, why does Saturn, for example, have rings in only one plane? Shouldn't it be inside of a "shell" of debris instead of just having rings?
gpt
A shell or ball is what you'd expect if the particles don't collide with each other. This is why you can get elliptical galaxies - stars almost never collide and don't even have close encounters very often, so once you get a ball of stars, the stars will just keep on buzzing around in a ball for a very long time. This is also supposed to be the case for dark matter. The dark matter particles don't really collide with each other, so they just stay in a big puffy "halo" around the galaxy.

However, the dust and rocks and moonlets in a planetary ring *can* collide with each other. So if you some particles in a "polar" orbit, going up over the north pole and back around the south pole, and other particles in an "equatorial orbit", going in circles around the equator, then these particles will smash into each other. Unless all the particles are orbiting in the same plane, their orbits will cross and they'll collide. These collisions transfer momentum between the particles, and also get rid of kinetic energy. Eventually, through enough collisions, everything will settle down until you get a disc or a ring. Then all the particles can have nice circular orbits without bumping into each other. (Another way to think of it is this: you can get rid of energy, but you can't get rid of momentum. A ring or disc is the lowest energy system you can get while still conserving angular momentum).

This is true for more than just planetary rings. Gas and plasma particles in space will bump into each other too. So when you get a lot of gas coming together to form a galaxy - or, on a smaller scale, a chunk of gas coming together to form a star - it will also collapse into a disc. For a galactic gas disc, this will collapse to form stars, so you get a disc of stars. For a stellar gas disc, this will collapse to form planets, so you get all the planets within the same plane. It's not that the stars or planets need to be in a disc - neither really is good at collisions - it's that the gas they formed from was in a disc.