system
Given the following post, respond as if you're commenting on Reddit:
human
Why are amalgomations of objects around a gravity source roughly disc shaped/flat? (ie Saturn's Rings, the Solar System, or the Milky Way)


I would not necessarily expect gravity to align randomly distributed objects into the disc shape that seems so prevalent. Will objects distributed in a random spherical shape around a large gravity source always eventually collapse to a disc shape in a way that is calculable based on the initial distribution of the objects? Or is there something else at work?
gpt
You're really talking about three different things.

Consider a galaxy. It can be nearly any shape you can think of: spherical, elliptical, lens-shaped, or just blobby, like a cloud. Some galaxies in particular have a spiral appearance. These are the ones you were most likely thinking of when you asked the question.

As to why a spiral galaxy has a disc shape instead of any other shape, it has to do with angular momentum and self-interaction. A collapsing cloud of uniformly distributed dust will inevitably have some net angular momentum. As the cloud continues to collapse under its own gravitation, the dust interacts with itself such that particles with orbits outside the plane of the cloud's overall angular momentum tend to get perturbed into orbits that lie within the plane of that angular momentum.

The solar system is thought to have formed in basically the same way; not from a truly uniform cloud of dust, but the overall principle is the same. The planets in our solar system lie in a plane — mostly — because that's where the *stuff* was from which the planets congealed.

As for Saturn's rings, there are two conflicting theories about where they might've come from. One theory says the matter that makes up the rings used to be a moon — a moon with a name, believe it or not; it's called Veritas — that dipped too close to the planet and was pulled apart by tidal forces. In that case, the rings occupy a plane because the moon was orbiting in a plane when it came apart.

The other theory is that the matter that makes up the rings is stuff left over from when the planet and its moons first formed, but because of its proximity to Saturn it was unable to congeal together into a moon. In that case, it occupies a plane because the same principle that pulls spiral galaxies into a planar shape also pulled the protoplanetary cloud that became Saturn into a disc.

So basically it all boils down to conservation of angular momentum. Once you set something in rotation, that rotation doesn't go away, and due to self-interaction of clouds of stuff, it in fact tends to become more and more well defined as time goes on.