Irregular moon

In Astronomy, an irregular satellite is a natural satellite following an inclined, sometimes retrograde orbit and believed to be captured as opposed to a regular satellite, formed in situ.

Eighty-six irregular satellites have been discovered since 1997 orbiting all four giant planets. Only fourteen others were discovered earlier. This includes Phoebe, the largest irregular satellite of Saturn, and Himalia, the largest irregular satellite of Jupiter. It is thought that the irregular satellites have been captured from heliocentric orbits, near their current locations, early after the planet formation. Their origin in Kuiper Belt space is not supported by the current observations.

Definition

There's no widely accepted precise definition of the irregular satellite. Typically, the semi-major axis is compared with the Hill sphere (the gravitational influence sphere) $r_{H}$. Irregular satellites have semi-major axes greater than 0.05 $r_{H}$ and apoapsis up to 0.65 $r_{H}$.

Orbits

Irregular satellites of Jupiter (red), Saturn (yellow), Uranus (green) and Neptune (blue)

Current distribution

The diagram illustrates the orbits of the irregular satellites of the giants planets discovered so far. The semi-major axes are expressed as a fraction of the planet’s Hill sphere’s radius and the inclination is represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde.

Origin

The capture from a heliocentric orbit requires one of the following

• energy dissipation (e.g. in the primordial gas cloud)
• a substantial (40%) extension og the planet's Hill sphere in a brief period of time (thousands of years)
• transfer of energy in a three-body interaction.

The last mechanisms, involving a collision (or close encounter) of two satellites could result in one of them losing energy and be captured.

Long-term stability

Remarkably, the current orbits prove stable in numerical simulations, in spite of substantial perturbations near apocentre . A number of irregulars are proven to be in secular or Kozai resonance belived to be the cause of this stability. In addition, the simulations indicate that

• orbits with inclinations higher than 50° (130° for retrograde orbits) are not stable, their eccentricity growing quickly resulting in the satellite being lost
• retrograde orbits are more stable than prograde (stable retrograde orbits can be found further from the planet)

Physical characteristics

Colours

Observed colours vary from neutral to reddish but not as red as Kuiper Belt objects. Each planet's system displays slightly different characteristics; Jupiter's irregulars are consistent with C and D-type asteroids, Saturn's irregulars are redder, similar to Iapetus (but different from Phoebe). Only the irregulars of Uranus show the very red colours typical for classical KBOs.