Irregular moon

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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. TheIrregulars.svg
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.

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.


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.


PlanetrH, 106 km [1] rmin, km [1] Number known
Neptune116167 (including Triton)

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.

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), . Irregular satellites have semi-major axes greater than 0.05 with apoapses extending as far as to 0.65 . [1] The radius of the Hill sphere is given in the adjacent table.

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[ citation needed ] 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 is usually listed as irregular despite being within 0.05 of the radius of Neptune's Hill sphere. 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.


Current distribution

The orbits of the known irregular satellites are extremely diverse, but there are certain patterns. Retrograde orbits are far more common (83%) than prograde orbits. No satellites are known with orbital inclinations higher than 55° (or smaller than 130° for retrograde satellites). In addition, some groupings can be identified, in which one large satellite shares a similar orbit with a few smaller ones.

Given their distance from the planet, the orbits of the outer satellites are highly perturbed by the Sun and their orbital elements change widely over short intervals. The semi-major axis of Pasiphae, for example, changes as much as 1.5 Gm in two years (single orbit), the inclination around 10°, and the eccentricity as much as 0.4 in 24 years (twice Jupiter's orbit period). [2] Consequently, mean orbital elements (averaged over time) are used to identify the groupings rather than osculating elements at the given date. (Similarly, the proper orbital elements are used to determine the families of asteroids.)


Irregular satellites have been captured from heliocentric orbits. (Indeed, it appears that the irregular moons of the giant planets, the Jovian and Neptunian trojans, and grey Kuiper belt objects have a similar origin. [3] ) For this to occur, at least one of three things needs to have happened:

After the capture, some of the satellites could break up leading to groupings of smaller moons following similar orbits. Resonances could further modify the orbits making these groupings less recognizable.

Long-term stability

Phoebe, Saturn's largest irregular satellite Phoebe cassini.jpg
Phoebe, Saturn's largest irregular satellite

The current orbits of the irregular moons are stable, in spite of substantial perturbations near the apocenter. [5] The cause of this stability in a number of irregulars is the fact that they orbit with a secular or Kozai resonance. [6]

In addition, simulations indicate the following conclusions:

Increasing eccentricity results in smaller pericenters and large apocenters. The satellites enter the zone of the regular (larger) moons and are lost or ejected via collision and close encounters. Alternatively, the increasing perturbations by the Sun at the growing apocenters push them beyond the Hill sphere.

Retrograde satellites can be found further from the planet than prograde ones. Detailed numerical integrations have shown this asymmetry. The limits are a complicated function of the inclination and eccentricity, but in general, prograde orbits with semi-major axes up to 0.47 rH (Hill sphere radius) can be stable, whereas for retrograde orbits stability can extend out to 0.67 rH.

The boundary for the semimajor axis is surprisingly sharp for the prograde satellites. A satellite on a prograde, circular orbit (inclination=0°) placed at 0.5 rH would leave Jupiter in as little as forty years. The effect can be explained by so-called evection resonance. The apocenter of the satellite, where the planet's grip on the moon is at its weakest, gets locked in resonance with the position of the Sun. The effects of the perturbation accumulate at each passage pushing the satellite even further outwards. [5]

The asymmetry between the prograde and retrograde satellites can be explained very intuitively by the Coriolis acceleration in the frame rotating with the planet. For the prograde satellites the acceleration points outward and for the retrograde it points inward, stabilising the satellite. [7]

Temporary captures

The capture of an asteroid from a heliocentric orbit isn't always permanent. According to simulations, temporary satellites should be a common phenomenon. [8] [9] The only observed examples are 2006 RH120 and 2020 CD3 , which were temporary satellites of Earth discovered in 2006 and 2020, respectively. [10] [11] [12]

Physical characteristics


The power law for the size distribution of objects in the Kuiper belt, where q [?] 4 and thus N ~ D . 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. TheKuiperBelt PowerLaw2.svg
The power law for the size distribution of objects in the Kuiper belt, where q ≈ 4 and thus N ~ D . 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.

Because objects of a given size are more difficult to see the greater their distance from Earth, the known irregular satellites of Uranus and Neptune are larger than those of Jupiter and Saturn; smaller ones probably exist but have not yet been observed. Bearing this observational bias in mind, the size distribution of irregular satellites appears to be similar for all four giant planets.

The size distribution of asteroids and many similar populations can be expressed as a power law: there are many more small objects than large ones, and the smaller the size, the more numerous the object. The mathematical relation expressing the number of objects, , with a diameter smaller than a particular size, , is approximated as:

with q defining the slope.

The value of q is determined through observation.

For irregular moons, a shallow power law (q ≃ 2) is observed for sizes of 10 to 100 km, but a steeper law (q ≃ 3.5) is observed for objects smaller than 10 km. An analysis of images taken by the Canada-France-Hawaii Telescope in 2010 shows that the power law for Jupiter's population of small retrograde satellites, down to a detection limit of ≈ 400 m, is relatively shallow, at q ≃ 2.5. Thus it can be extrapolated that Jupiter should have 600+600
moons 400 m in diameter or greater. [13]

For comparison, the distribution of large Kuiper belt objects is much steeper (q ≈ 4). That is, for every object of 1000 km there are a thousand objects with a diameter of 100 km, though it's unknown how far this distribution extends. The size distribution of a population may provide insights into its origin, whether through capture, collision and break-up, or accretion.

For every object of 100 km, ten objects of 10 km can be found.


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. TheIrregulars Colours.svg
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 colours of irregular satellites can be studied via colour indices: simple measures of differences of the apparent magnitude of an object through blue (B), visible i.e. green-yellow (V), and red (R) filters. The observed colours of the irregular satellites vary from neutral (greyish) to reddish (but not as red as the colours of some Kuiper belt objects).

albedo [14] neutralreddishred
low C 3–8% P 2–6% D 2–5%
medium M 10–18% A 13–35%
high E 25–60%

Each planet's system displays slightly different characteristics. Jupiter's irregulars are grey to slightly red, consistent with C, P and D-type asteroids. [15] Some groups of satellites are observed to display similar colours (see later sections). Saturn's irregulars are slightly redder than those of Jupiter.

The large Uranian irregular satellites (Sycorax and Caliban) are light red, whereas the smaller Prospero and Setebos are grey, as are the Neptunian satellites Nereid and Halimede. [16]


With the current resolution, the visible and near-infrared spectra of most satellites appear featureless. So far, water ice has been inferred on Phoebe and Nereid and features attributed to aqueous alteration were found on Himalia.


Regular satellites are usually tidally locked (that is, their orbit is synchronous with their rotation so that they only show one face toward their parent planet). In contrast, tidal forces on the irregular satellites are negligible given their distance from the planet, and rotation periods in the range of only ten hours have been measured for the biggest moons Himalia, Phoebe, Sycorax, and Nereid (to compare with their orbital periods of hundreds of days). Such rotation rates are in the same range that is typical for asteroids.

Families with a common origin

Some irregular satellites appear to orbit in 'groups', in which several satellites share similar orbits. The leading theory is that these objects constitute collisional families, parts of a larger body that broke up.

Dynamic groupings

Simple collision models can be used to estimate the possible dispersion of the orbital parameters given a velocity impulse Δv. Applying these models to the known orbital parameters makes it possible to estimate the Δv necessary to create the observed dispersion. A Δv of tens of meters per seconds (5–50 m/s) could result from a break-up. Dynamical groupings of irregular satellites can be identified using these criteria and the likelihood of the common origin from a break-up evaluated. [17]

When the dispersion of the orbits is too wide (i.e. it would require Δv in the order of hundreds of m/s)

Colour groupings

When the colours and spectra of the satellites are known, the homogeneity of these data for all the members of a given grouping is a substantial argument for a common origin. However, lack of precision in the available data often makes it difficult to draw statistically significant conclusions. In addition, the observed colours are not necessarily representative of the bulk composition of the satellite.

Observed groupings

Irregular satellites of Jupiter

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). TheIrregulars JUPITER.svg
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).

Typically, the following groupings are listed (dynamically tight groups displaying homogenous colours are listed in bold)

Animation of Himalia's orbit.

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Jupiter *
Himalia *
Callisto Animation of Himalia orbit around Jupiter.gif
Animation of Himalia's orbit.
   Jupiter ·   Himalia ·  Callisto

Sinope, sometimes included into the Pasiphae group, is red and given the difference in inclination, it could be captured independently. [15] [19] Pasiphae and Sinope are also trapped in secular resonances with Jupiter. [5] [17]

Irregular satellites of Saturn

Irregular satellites of Saturn, showing how they cluster into groups. For explanation, see Jupiter diagram TheIrregulars SATURN.svg
Irregular satellites of Saturn, showing how they cluster into groups. For explanation, see Jupiter diagram

The following groupings are commonly listed for Saturn's satellites:

Irregular satellites of Uranus and Neptune

Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). For explanation, see Jupiter diagram TheIrregulars NEPTUNE URANUS.svg
Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). For explanation, see Jupiter diagram
Planetrmin [1]
Jupiter1.5 km
Saturn3 km
Uranus7 km
Neptune16 km

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: [15]

These two groups are distinct (with 3σ confidence) in their distance from Uranus and in their eccentricity. [20] However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey. [16]

For Neptune, a possible common origin of Psamathe and Neso has been noted. [21] Given the similar (grey) colours, it was also suggested that Halimede could be a fragment of Nereid. [16] The two satellites have had a very high probability (41%) of collision over the age of the solar system. [22]


Distant Cassini image of Himalia Himalia.png
Distant Cassini image of Himalia

To date, the only irregular satellites to have been visited by a spacecraft are Triton and Phoebe, the largest of Neptune's and Saturn's irregulars respectively. Triton was imaged by Voyager 2 in 1989 and Phoebe by the Cassini probe in 2004. Voyager 2 also captured a distant image of Neptune's Nereid in 1989, and Cassini captured a distant, low-resolution image of Jupiter's Himalia in 2000. New Horizons captured low-resolution images of Jupiter's Himalia and Elara in 2007. There are no spacecraft planned to visit any irregular satellites in the future.

Related Research Articles

Nereid (moon) Large moon of Neptune

Nereid, or Neptune II, is the third-largest moon of Neptune. Of all known moons in the Solar System, it has the most eccentric orbit. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.

Natural satellite Astronomical body that orbits a planet

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body. While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

Caliban (moon) moon of Uranus

Caliban is the second-largest retrograde irregular satellite of Uranus. It was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars using the 200-inch Hale telescope together with Sycorax and given the temporary designation S/1997 U 1.

Elara (moon) Moon Of Jupiter

Elara is a prograde irregular satellite of Jupiter. It was discovered by Charles Dillon Perrine at Lick Observatory in 1905 in photographs taken with the 36" Crossley reflecting telescope which he had recently rebuilt. It is the eighth-largest moon of Jupiter and is named after Elara, one of Zeus's lovers and the mother of the giant Tityos.

Pasiphae (moon) Moon of Jupiter

Pasiphae, formerly spelled Pasiphaë, is a retrograde irregular satellite of Jupiter. It was discovered in 1908 by Philibert Jacques Melotte and later named after the mythological Pasiphaë, wife of Minos and mother of the Minotaur from Greek legend.

Moons of Jupiter Natural satellites of the planet Jupiter

There are 80 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: Io; Europa; Ganymede; and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 76 known moons and the rings together composing just 0.003% of the total orbiting mass.

Skathi (moon) Moon of Saturn

Skathi, also named Saturn XXVII and originally spelled Skadi, is a natural satellite of the planet Saturn. Skathi is one of Saturn's irregular moons, in its Norse group of satellites. It was discovered on September 23, 2000 by a team of astronomers led by Brett Gladman. The team announced their discovery on December 7, 2000, along with seven other satellites of Saturn. The moon was named after Skaði, a figure in Norse mythology, as part of an effort to diversify the largely Greek and Roman names of astronomical objects.

Moons of Uranus Natural satellites of the planet Uranus

Uranus, the seventh planet of the Solar System, has 27 known moons, most of which are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner and major moons all have prograde orbits, while orbits of the irregulars are mostly retrograde. The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the Earth's Moon. The orbits of the regular moons are nearly coplanar with Uranus's equator, which is tilted 97.77° to its orbit. Uranus's irregular moons have elliptical and strongly inclined orbits at large distances from the planet.

Moons of Neptune Natural satellites of the planet Neptune

The planet Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid. Neptune's outermost moon Neso, which has an orbital period of about 26 Julian years, orbits farther from its planet than any other moon in the Solar System.

The naming of moons has been the responsibility of the International Astronomical Union's committee for Planetary System Nomenclature since 1973. That committee is known today as the Working Group for Planetary System Nomenclature (WGPSN).

Himalia group

The Himalia group is a group of prograde irregular satellites of Jupiter that follow similar orbits to Himalia and are thought to have a common origin.

In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton. Inner satellites are distinguished from other regular satellites by their proximity to the parent planet, their short orbital periods, their low mass, small size, and irregular shapes.

Nice model

The Nicemodel is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur — where it was initially developed in 2005 — in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.

In astronomy, a regular moon is a natural satellite following a relatively close and prograde orbit with little orbital inclination or eccentricity. They are believed to have formed in orbit about their primary, as opposed to irregular moons, which were captured.

Retrograde and prograde motion Relative directions of orbit or rotation

Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object. It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.

The five-planet Nice model is a recent variation of the Nice model that begins with five giant planets, the four plus an additional ice giant in a chain of mean-motion resonances.

The jumping-Jupiter scenario specifies an evolution of giant-planet migration described by the Nice model, in which an ice giant is scattered inward by Saturn and outward by Jupiter, causing their semi-major axes to jump, quickly separating their orbits. The jumping-Jupiter scenario was proposed by Ramon Brasser, Alessandro Morbidelli, Rodney Gomes, Kleomenis Tsiganis, and Harold Levison after their studies revealed that the smooth divergent migration of Jupiter and Saturn resulted in an inner Solar System significantly different from the current Solar System. During this migration secular resonances swept through the inner Solar System exciting the orbits of the terrestrial planets and the asteroids, leaving the planets' orbits too eccentric, and the asteroid belt with too many high-inclination objects. The jumps in the semi-major axes of Jupiter and Saturn described in the jumping-Jupiter scenario can allow these resonances to quickly cross the inner Solar System without altering orbits excessively, although the terrestrial planets remain sensitive to its passage.

Satellite system (astronomy) Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.


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