Rings and a large family of satellites are features common to all four giant planets. There are variations in emphasis and scale, but the similarities between each ring-satellite system outweigh their differences.
Ring-satellite systems
Most outer satellites of each giant planet travel in eccentric orbits,usually in the opposite direction to the spin on their planet.Furthermore, many of these orbits are inclined at 〉30° relative to their planet’s equator. The typically eccentric, retrograde, and inclined nature of their orbits earns these bodies the name‘irregular satellites’, quite apart from the fact that (being at most about 100 kilometres, and more often only a few kilometres,across) they have far too little gravity to pull themselves into spherical shapes. The irregular satellites are the most numerous class: at the last count, Jupiter had 55, with orbital semi-major axes ranging from 105 to 400 Jupiter radii; Saturn had 38, with orbits from 184 to 417 Saturn radii; Uranus had 9, 167–818Uranus radii; and Neptune had 6, 223–1,954 Neptune radii.The ‘regular satellites’ are the large ones in near-circular prograde orbits, much closer to their planets, and with very low inclinations. Jupiter has 4 (the ones discovered by Galileo) whose orbital semi-major axes range from 5.9 to 26.3 Jupiter radii.These are substantial worlds, and geologically have much in common with the terrestrial planets, though of course they do not satisfy the IAU defi nition of a planet. Saturn has 8 (all but one considerably smaller than Jupiter’s, and orbiting at 3–59 Saturn radii), and Uranus has 5 (at 5–23 Uranus radii). Neptune has one large satellite, Triton, orbiting at 14 Neptune radii, that would be regarded as ‘regular’, except for its retrograde orbit. An important characteristic, shared by all regular satellites (including Triton), is that tidal forces have such a grip on them that they are in synchronous rotation, rotating once per orbit, so that (like the Earth’s Moon) they keep the same face towards their planet.