Chapter 41 - Planetary Rings

Abstract

Planetary rings are swarms of objects orbiting a central planet with vertical motions that are small compared to their motions within a common plane. This characteristic arises because their planets rotate fast enough that they bulge at their equators, thus defining a preferred orbital plane. This is a major contrast between rings and other astrophysical disks, such as spiral galaxies and young solar systems, which do not derive their shapes from an external gravity field but from the average angular momentum of the disk itself (in both cases, once a preferred plane is established, collisions among particles damp out the motions perpendicular to it). Planetary rings are also distinguished by a large planet/ring mass ratio, which greatly enhances the flatness of rings (their aspect ratios are as small as 10-7). However, rings do have a number of similarities with other astrophysical disks, which add to the motivation for studying them. Unlike other known disk systems that are either many light-years away or (like the early stages of our solar system) far back in time, planetary rings can be studied up close and in real time.