Irregular moon

In astronomy, an irregular moon, irregular satellite or 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 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 as Triton is a round moon, but is considered irregular due to its orbit.

Irregular satellites of Jupiter (red), Saturn (yellow), Uranus (green) and Neptune (blue) (excluding Triton). The horizontal axis shows their distance from the planet (semi-major axis) expressed as a fraction of the planet's Hill sphere's radius. The vertical axis shows their orbital inclination. Points or circles represent their relative sizes. Data as of August 2006.

As of November 2021, 147 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. 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.


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