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. , 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 Planet Hill radiusrH (106 km) rH (°) Number known Farthest knownsatellite (106 km) Jupiter 51 4.7 103 24.2 (0.47rH) Saturn 69 3.0 261 28.0 (0.41rH) Uranus 73 1.5 10 20.4 (0.28rH) Neptune 116 1.5 9 (including Triton) 50.7 (0.44rH)
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), . Irregular satellites have semi-major axes greater than 0.05 with apoapses extending as far as to 0.65 . 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 .
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.
Capture A planet cannot capture an object directly from heliocentric orbit without some external mechanism; an object can be captured temporarily but has the energy to eventually escape. For it to enter permanent orbit around the planet, the object must be slowed down in some way while it is close enough. In modern times, there is no mechanism available for a planet to capture a moon, so their captures must have occurred much earlier in the Solar System's history.
The prevailing theory today is n-body capture, gravitational interactions between three or more bodies that slows an object down enough such that it becomes captured. The Nice model predicts that many small bodies were scattered inward from the Kuiper belt region. When one made an approach to two giant planets in close proximity to each other, the small body could be captured in a permanent orbit around one of the planets. When the planets later moved away and their orbits stabilized, the object would remain bound.
There are variants of this theory. A close encounter between an incoming binary object and the planet (or possibly an existing moon) could result in one component of the binary being captured. Such a route has been suggested as most likely for Triton. Another theory floats the idea of trans-Neptunian objects being injected due to a close passing star and a fraction of these injected TNOs captured by the giant planets.
Historically there were other capture models proposed. In the gas drag scenario, moons that initially are in temporary capture have their speed slowed down by friction and drag with the primordial gas cloud around planets during planet formation. This would slow them down enough such that their orbits become permanent. Another proposal was "pull-down", where a planet undergoes a sudden increase of mass, pulling in nearby material and undergoing runaway growth. This would increase the planet's gravitational influence, such that unbound objects passing nearby would find themselves permanently captured. Another idea is that a collision (or close encounter) happens between an incoming body and a satellite, resulting in the incoming body losing energy and being captured.
However, the gas drag and pull-down theories cannot explain the irregular satellites of the ice giants, as they were expected not to have the required significant gas envelopes and did not undergo runaway growth phases. Another problem is that after planetary formation, the planetary migration that later occurred would have destabilized any previously captured objects. There may have been previous generations of irregular moons captured by different ways, but the irregular moons existing today were likely captured relatively late, in a gas-free environment.
Collisional history The irregular moons were captured into a relatively small amount of orbital space, making collisions unavoidable over the history of the Solar System. The total irregular moon population may have been much larger when initially captured, then ground down through many collisions, losing up to 99% of their starting mass. This may produce large quantities of dust that may spread onto the surfaces of large regular moons.
Many of the irregular moons that exist today are expected to be collisional fragments of an originally larger body. When one suffers a collision, it may fragment into many more irregular moons, forming a collisional family. The fragments usually end up following similar orbits to that of the original, allowing for the identification of these families.
Orbits 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 between 60° and 130°; moreover, apart from Nereid, no irregular moon has inclination less than 26°, and inclinations greater than 170° are only found in Saturn's system. 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 million km 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). 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.)
Long-term stability The current orbits of the irregular moons are stable, in spite of substantial perturbations near the apocenter. The cause of this stability in a number of irregulars is the fact that they orbit with a secular or Kozai resonance.
In addition, simulations indicate the following conclusions: Orbits with inclinations between 50° and 130° are very unstable: their eccentricity increases quickly resulting in the satellite being lost Retrograde orbits are more stable than prograde (stable retrograde orbits can be found further from the planet) 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.
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.
Physical characteristics
Size distributions
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.