Why Are Planets Rings Flat?

Why are planets rings flat? It seems like they would orbit in random paths. Do their gravities pull each other into a disc? Also are there planets with more than one ring that are at a different inclination for each other?

The explanation for this is that the orbits of the planets are coplanar. During the Solar System’s formation, the planets formed out of a disk of dust which surrounded the Sun. As the disk of dust was a disk, all in a plane, all of the planets formed in a plane as well.

Rings and disks are common in astronomy. When a cloud collapses, the conservation of angular momentum amplifies any initial tiny spin of the cloud. As the cloud spins faster and faster, it collapses into a disk. This is the maximal balance between gravitational collapse and centrifugal force created by rapid spin. The result is the coplanar planets, the thin disks of spiral galaxies, and the accretion disks around black holes.

Then why does Pluto follow an orbital path that is not co-planar?

It’s not too far from the general plane of our solar system, only 17° from the ecliptic. A better example would be Eris, at 44°. While the solar system was forming. Protoplanets were all over the place, gravitationally interacting and smashing into each other. In the inner system and with the largest of these bodies, their mutual influences mostly cancelled out variation. This leaves them roughly on the same plane. Farther out and with smaller bodies, there’s much less gravitational influence between them.Hence, they stay in these very inclined eccentric orbits. And another factor is close encounters with other bodies. These can throw planets out of line with everything else, and they may stay like that for a very long time.

There can be any number of reasons. A common mistake people make is that planets form in disks and hence are somewhat coplanar and then remain so throughout their lifetime. This is not the case due to orbital migration mechanisms. In fact, from exoplanet observations, we think there are many systems that show noncoplanar behaviour even among planets much larger than Pluto (being small its orbit can be perturbed much easier by another object).

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