Irregular moon

In Astronomy, an irregular satellite is a natural satellite following an inclined, sometimes retrograde orbit and believed to be captured as opposed to a regular satellite, formed in situ.

Eighty-six irregular satellites have been discovered since 1997 orbiting all four giant planets. Only Phoebe the largest irregular satellite of Saturn and Himalia, the largest irregular satellite of Jupiter and a dozen others have beendiscovered earlier.

There's no widely accepted precise definition of the irregular satellite. Typically, the semi-major axis is compared with the Hill sphere (the gravitational influence sphere) $r_{H}$. Irregular satellites have semi-major axes greater than 0.05 $r_{H}$ and apoapsis up to 0.65 $r_{H}$.