Irregular Moon

In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which formed in situ.

Ninety-three irregular satellites have been discovered since 1997, orbiting all four of the giant planets (Jupiter, Saturn, Uranus and Neptune). Before 1997, only ten were known, including Phoebe, the largest irregular satellite of Saturn, and Himalia, the largest irregular satellite of Jupiter. Sycorax, the largest irregular moon of Uranus, was discovered in 1997. It is currently thought that the irregular satellites were captured from heliocentric orbits near their current locations, shortly after the formation of their parent planet. An alternative theory, that they originated further out in the Kuiper belt, is not supported by current observations.