DescriptionIrregular Moon. In astronomy, an irregular moon, irregular satellite, or irregular natural satellite is a natural satellite following an orbit that is irregular in some of the following ways: Distant; inclined; highly elliptical; retrograde. They have often been captured by their parent planet, unlike regular satellites formed in orbit around them. Irregular moons have a stable orbit, unlike temporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape; Triton, for example, is a round moon but is considered irregular due to its orbit and origins. As of March 2025, 356 irregular moons are known, 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. Triton is rather unusual for an irregular moon; if it is excluded, then Nereid is the largest irregular moon around Neptune. It is currently thought that the irregular satellites were once independent objects orbiting the Sun before being captured by a nearby planet, early in the history of the Solar System. An alternative suggests that they originated further out in the Kuiper belt and were captured after the close flyby of another star. There is no widely accepted precise definition of an irregular satellite. Informally, satellites are considered irregular if they are far enough from the planet that the precession of their orbital plane is primarily controlled by the Sun, other planets, or other moons.

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