Irregular Moons

This Figure shows the Semi-major axis vs Inclination of the irregular outer moons of the giant planets (The Hill Radius is the distance that satellites are expected to be dynamically stable around a planet). Juipter's dynamical groupings are clearly visible while other planets may have dynamical groupings as well.

Irregular outer moons have large orbits, inclinations and eccentricities. Because of their orbit characteristics and small size they are believed to be objects captured during the early formation of the giant planets.

Irregular moons are thought to have been captured from heliocentric orbit when the planets were young. The best evidence for capture is that the orbits of some of the irregular moons are actually retrograde (meaning they orbit the planet in a direction opposite to Jupiter's rotation). Essentially the only plausible way to produce irregular moons is by capture. However, it is not easy for Jupiter (or any other planet) to directly capture passing asteroids from heliocentric orbit. In general, some of the initial energy of the heliocentric objects must be dissipated so that the planet can hold on to them. The origin of the dissipation that lead to the capture of irregular moons is unknown. In fact, at the present time there is no plausible source of dissipation so that capturing moons is presently almost impossible. It is theorized, however, that the youthful giant planets sustained bloated atmospheres that extended far above the cloud tops of the present planet. Friction with this atmosphere could have helped captured the irregular moons.