Definition
There is no single, universally accepted definition of what constitutes an irregular (as opposed to a regular) satellite. Instead, there are several definitions which, although they differ in detail, end up giving consistent classifications for almost all the satellites in the Solar System.

The key discriminant is that the regular satellites have orbits which are small (in comparison with the total range of gravitational influence of the parent planet), nearly circular and close to the equatorial planes of their planets. The irregular satellites, in contrast, have orbits which are large, eccentric and inclined relative to the parent planet equatorial plane.

The important difference between the regular and irregular satellites, so defined, is that the regulars formed by accretion in pre-planetary disks analogous to the disk around the sun from which the planets formed. The irregular satellites cannot have formed in such disks (since they pay little heed to the planetary equators) and instead are thought to have been captured.

The two classes of satellite thus have very different formation histories, and can potentially tell us quite different things about the way planets formed.

David Jewitt.

David Jewitt Home Page

Irregular Satellites Page