page_title: Irregular moon
section_title: Irregular satellites of Uranus and Neptune
hierarchical_section_title: Irregular moon / Observed groupings / Irregular satellites of Uranus and Neptune
context_page_description: 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.
  As of October 2019, 145 irregular moons are known, orbiting all four of the outer planets. 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.
context_section_description: According to current knowledge, the number of irregular satellites orbiting Uranus and Neptune is smaller than that of Jupiter and Saturn. However, it is thought that this is simply a result of observational difficulties due to the greater distance of Uranus and Neptune. The table at right shows the minimum radius (rₘᵢₙ) of satellites that can be detected with current technology, assuming an albedo of 0.04; thus, there are almost certainly small Uranian and Neptunian moons that cannot yet be seen.
  Due to the smaller numbers, statistically significant conclusions about the groupings are difficult. A single origin for the retrograde irregulars of Uranus seems unlikely given a dispersion of the orbital parameters that would require high impulse (Δv ≈ 300 km), implying a large diameter of the impactor (395 km), which is incompatible in turn with the size distribution of the fragments. Instead, the existence of two groupings has been speculated:
  Caliban group
  Sycorax group
  These two groups are distinct (with 3σ confidence) in their distance from Uranus and in their eccentricity.
  However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey.
  For Neptune, a possible common origin of Psamathe and Neso has been noted. Given the similar (grey) colours, it was also suggested that  Halimede could be a fragment of  Nereid. The two satellites have had a very high probability (41%) of collision over the age of the solar system.
caption: Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). For explanation, see Jupiter diagram