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.
William Lassell, the discoverer of Triton
Phoebe, Saturn's largest irregular satellite
The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.
The power law for the size distribution of objects in the Kuiper belt, where q ≈ 4 and thus N ~ D−3. That is, for every Kuiper beld object of a particular size, there are approximately 8 times as many objects half that size and a thousands times as many objects one-tenth that size.
Animation of Triton
This diagram illustrates the differences of colour in the irregular satellites of Jupiter (red labels), Saturn (yellow) and Uranus (green). Only irregulars with known colour indices are shown. For reference, the centaur Pholus and three classical Kuiper belt objects are also plotted (grey labels, size not to scale).
For comparison, see also colours of centaurs and KBOs.
The Kuiper belt (green), in the Solar System's outskirts, is where Triton is thought to have originated.
The orbits of Jupiter's irregular satellites, showing how they cluster into groups. Satellites are represented by circles that indicate their relative sizes. An object's position on the horizontal axis shows its distance from Jupiter. Its position on the vertical axis indicates its orbital inclination. The yellow lines indicate its orbital eccentricity (i.e. the extent to which its distance from Jupiter varies during its orbit).
Artist's impression of Triton, showing its tenuous atmosphere just over the limb.
Animation of Himalia's orbit. ··
Clouds observed above Triton's limb by Voyager 2.
Irregular satellites of Saturn, showing how they cluster into groups. For explanation, see Jupiter diagram
Interpretative geomorphological map of Triton
Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). For explanation, see Jupiter diagram
Triton's bright south polar cap above a region of cantaloupe terrain
Distant Cassini image of Himalia
Cantaloupe terrain viewed from 130,000 km by Voyager 2, with crosscutting Europa-like double ridges. Slidr Sulci (vertical) and Tano Sulci form the prominent "X".
Animation of Saturn's Inuit group of satellites {{legend2|RoyalBlue| Kiviuq}}{{·}}{{legend2|Lime| Ijiraq}}{{·}}{{legend2|Gold| Paaliaq}}{{·}}{{legend2|OrangeRed| Siarnaq}}{{·}}{{legend2|Cyan|Tarqeq}}
Tuonela Planitia (left) and Ruach Planitia (center) are two of Triton's cryovolcanic "walled plains". The paucity of craters is evidence of extensive, relatively recent, geologic activity.
Animation of Phoebe's orbit. {{legend2|RoyalBlue| Saturn}}{{·}}{{legend2| Magenta | Phoebe}}{{·}}{{legend2|Cyan|Titan}}
NASA illustration detailing the studies of the proposed Trident mission
71 irregular moons of Jupiter (with Callisto for comparison)
Neptune (top) and Triton (bottom) three days after flyby of Voyager 2
58 irregular moons of Saturn (with Iapetus for comparison)
thumb|Close up of the volcanic province of Leviathan Patera, the caldera in the center of the image. Several pit chains extend radially from the caldera to the right of the image, while the smaller of the two cryolava lakes is seen to the upper left. Just off-screen to the lower left is a fault zone aligned radially with the caldera, indicating a close connection between the tectonics and volcanology of this geologic unit.
9 irregular moons of Uranus
thumb|Dark streaks across Triton's south polar cap surface, thought to be dust deposits left by eruptions of nitrogen geysers
6 irregular moons of Neptune (excluding Triton)
thumb|Two large cryolava lakes on Triton, seen west of Leviathan Patera. Combined, they are nearly the size of Kraken Mare on Titan. These features are unusually crater free, indicating they are young and were recently molten.

The term does not refer to shape as Triton is a round moon, but is considered irregular due to its orbit.

- Irregular moon

Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons.

- Triton (moon)

4 related topics


Nereid imaged by Voyager 2 in 1989

Nereid (moon)

Third-largest moon of Neptune.

Third-largest moon of Neptune.

Nereid imaged by Voyager 2 in 1989
Nereid's highly eccentric orbit around Neptune.

Nereid is third-largest of Neptune's satellites, and has a mean radius of about 180 km. It is rather large for an irregular satellite.

The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.

Cassini image of Phoebe

Phoebe (moon)

Cassini image of Phoebe
Animation of Phoebe's orbit.
Artist's impression of the Phoebe ring, which dwarfs the main rings
Cassini closeup of Phoebe from 13 June 2004; the crater Euphemus is at top center
Named craters on Phoebe
Phoebe (with NGC 4179 in the lower right corner) as imaged with a 24" telescope
Map of Phoebe's middle latitudes. The higher latitudes have been clipped from the main map, but can be seen in the polar projections.
Map of Phoebe's south polar region
Map of Phoebe's north polar region
3D map showing Phoebe's once spherical shape

Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was discovered by William Henry Pickering on March 18, 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart.

Phoebe is the second largest retrograde satellite in the Solar System after Triton.

Elliptic orbit by eccentricity ····

Orbital eccentricity

Kepler orbits.svg:

Kepler orbits.svg:

Elliptic orbit by eccentricity ····
Plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next years. The arrows indicate the different scales used, as the eccentricities of Mercury and Mars are much greater than those of Venus and Earth. The 0 point on this plot is the year 2007.

Neptune's largest moon Triton has an eccentricity of 0 (0), the smallest eccentricity of any known moon in the Solar System; its orbit is as close to a perfect circle as can be currently measured.

However, smaller moons, particularly irregular moons, can have significant eccentricity, such as Neptune's third largest moon Nereid (0.75).

Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)

Retrograde and prograde motion

Object in the direction opposite the rotation of its primary, that is, the central object .

Object in the direction opposite the rotation of its primary, that is, the central object .

Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)
The orange moon is in a retrograde orbit.

Retrograde satellites are generally small and distant from their planets, except Neptune's satellite Triton, which is large and close.