An irregular satellite, also known as an irregular moon or an irregular natural satellite, is a natural satellite following a distant, inclined and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites, which have formed in orbit around them.
As of July 2018, about 125 irregular moons have been discovered, orbiting all four of the outer planets (Jupiter, Saturn, Uranus, and Neptune). The largest of each planet are Himalia of Jupiter, Phoebe of Saturn, Sycorax of Uranus, and Triton of Neptune. 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.