In astronomy, an irregular moon, irregular satellite, or irregular natural satellite is a natural satellite following an orbit that is irregular in some or all of the following ways: distant; inclined; highly elliptical; retrograde. They have often been captured from elsewhere by their parent planet, unlike regular satellites that 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. Given their great distances from their parent planet, the term outer moon may also be used interchangeably. However, the outermost large moons of a planet may also be called "outer moons", such as Iapetus, Oberon, or Callisto, which might cause confusion. Only the giant planets have irregular moons. As of April 2026, 383 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.
Definition
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. In practice, the satellite's semi-major axis is compared with the radius of the planet's Hill sphere (that is, the sphere of its gravitational influence), r H {\displaystyle r_{H}} . Irregular satellites have semi-major axes greater than 0.05 r H {\displaystyle r_{H}} with apoapses extending as far as to 0.65 r H {\displaystyle r_{H}} . The radius of the Hill sphere is given in the adjacent table: Uranus and Neptune have larger Hill sphere radii than Jupiter and Saturn, despite being less massive, because they are farther from the Sun. However, no known irregular satellite has a semi-major axis exceeding 0.47 r H {\displaystyle r_{H}} . Earth's Moon seems to be an exception: it is not usually listed as an irregular satellite even though its precession is primarily controlled by the Sun and its semi-major axis is greater than 0.05 of the radius of Earth's Hill sphere. On the other hand, Neptune's Triton, which is probably a captured object, is usually listed as irregular despite being within 0.05 of the radius of Neptune's Hill sphere, so that Triton's precession is primarily controlled by Neptune's oblateness instead of by the Sun. Neptune's Nereid and Saturn's Iapetus have semi-major axes close to 0.05 of the radius of their parent planets' Hill spheres: Nereid (with a very eccentric orbit) is usually listed as irregular, but not Iapetus.
Origin and evolution
Source population Unlike regular satellites, the irregular satellites have orbits that are too distant, eccentric, and inclined to have formed in the circumplanetary disk around their planets, so it is generally accepted that irregular satellites were captured from heliocentric orbits. Before they were captured in orbit around a planet, the irregular satellites were initially part of some group of objects directly orbiting the Sun. The Nice model suggests that this original population was the protoplanetary disk at distances beyond Neptune. This trans-Neptunian planetesimal disk was originally much larger and more massive, stretching between 24–50 AU, with a mass of several tens of times that of Earth. 99.9% of it was lost due to the gravitational effects of Neptune when it entered the region due to planetary migration. A portion of these objects were dispersed inward and found themselves trapped under the gravitational influence of the giant planets, becoming the irregular satellites and the Jupiter and Neptune trojans. Other objects migrated into the main asteroid belt and the Hilda group, becoming the P and D-type asteroids. Still others were ejected into the Oort cloud, while the surviving population remains as the hot population of the Kuiper belt. Because these groups of small bodies are purported to come from the same source population, they should all share similar physical characteristics.
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