Irregular Satellites of the Jovian Planets
This figure shows the sizes and orbital elements of the irregular satellites of the outer planets. At present, Jupiter is known to have 31 irregular satellites (shown in red); Saturn has 13 (green); and Uranus has 5 (blue). Neptune's only known irregular satellite is Nereid. The size of each satellite is shown by a circle whose size is proportional to the diameter of the object in the ratio 1:1000 to the horizontal scale on the plot. The largest objects shown are Saturn's Nereid (340 km diameter), Saturn's Phoebe (240 km diameter) and Jupiter's Himalia (180 km diameter). The center of each circle denotes the present orbital elements of the satellites scaled to the Hill radii of the planets. (The Hill radius is the radius of a sphere around a planet, within which planetary tidal forces on a small body are larger than the tidal forces of the Sun. Numerically, the Hill radius is 0.35 AU for Jupiter, 0.44 AU for Saturn, 0.47 AU for Uranus, and 0.78 AU for Neptune.) The planetocentric semi-major axes and inclinations of the satellites are shown in polar coordinates, with the orbital inclination being the angle counted from the X-axis (prograde orbits for [0,90] deg, and retrograde orbits for [90,180] deg). The planetocentric distance of each body varies within the range shown by the line segments extending radially from the coordinate center. The length of a segment is 2ae, where a is the semimajor axis and e is the eccentricity.
The clumping of bodies at several different locations in this diagram suggests a few groups of irregular satellites with similar orbits, each of which may have originated by the disruption of one parent body. Such disruptions are hypothetized to occur early in the formation of the Solar System due to collisions, tides or gas pressure. The same mechanisms may have played an important role for the parent body's capture from heliocentric to planetocentric orbit, or in its formation at the time when the regular satellites formed and scattered a large number of bodies from their formation site to larger planetocentric distances.
The lack of irregular satellites near inclinations of 90 degrees is believed to be due to the effect of the Kozai resonance, but may also occur due to the mechanism involved in the satellites' origin. The Kozai resonance is a special feature of the orbital dynamics at large inclinations which forces large oscillations of the pericenter distance. Hypothetical satellites with inclinations near 90 degrees may have struck the planet, or may have been ejected into heliocentric orbit when they suffered close encounters with massive regular satellites orbiting at 0.002-0.05 Hill radii from Jupiter, Saturn and Uranus. The lack of irregular satellites at Neptune (other than Nereid), if observationally confirmed, may have resulted from the gravitational effect of its massive satellite Triton, which is believed to have been captured onto an initially eccentric orbit and may have ejected many pre-existing irregular satellites before tides circularized its orbit.
Read more about Irregular Satellites
For further information:
Jovian irregulars (David Jewitt)
Saturnian irregulars (Brett Gladman)
Uranian irregulars (Brett Gladman)
Minor Planet Center headlines (scroll to "Natural Satellites")
JPL Solar System Dynamics Planetary Satellite Orbits
JPL Solar System Dynamics Planetary Satellite Sizes and Discovery Data
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