system
Given the following post, respond as if you're commenting on Reddit:
human
Why is an asteroid ring a ring and not a sphere?

Given that there's no "up or down" in space? I'm guessing the answer has something to do with the gravity of the asteroids themselves, but I just can't visualize how that would work. Can somebody help me out? I know nothing about this sort of thing.
gpt
The answer is a process called accretion. Basically if you take a cloud of dust, for example, and allow all the particles inside that cloud to have some sort of motion and interaction with each other, they will form inelastic collisions such that, each time two particles collide, their trajectories become slightly closer to each other than they were to begin with. Over a great deal of time, all of these continuing averaging-out collisions means that all of the particles start moving the same direction (if you were to add up the directional vector and energy of all the particles in the dust cloud and then subtract them from each other, there would be some net rotational spin in some direction- this is the direction of the elliptical disk that forms over time). 

Particles that clump together due to gravity once they're all rotating the same direction become stars and planets and asteroids. This is the reason that most of nature forms rings or rotational planes... The solar system is (mostly) flat, the rings of Jupiter and Saturn are flat, the moons of Jupiter rotate on the same plane around it just like the planets and asteroids do around the sun, and even galaxies sometimes form disc shapes (like the Milky Way).