Ring structures, composed of numerous individual small bodies and dust, surrounding the four largest outer planets - Jupiter, Saturn, Uranus and Neptune.
The rings of Saturn were discovered as soon as Galileo became the first person to turn a telescope on the sky in 1610. In 1857, James Clerk Maxwell demonstrated theoretically that the rings must be made up of many unconnected particles, and this was later confirmed by spectroscopic observations showing that the inner particles orbit more quickly than the outer ones. In 1977, nine narrow rings around Uranus were detected when the planet occulted a star. In 1979,Voyager 1 discovered a faint band around Jupiter and, in the early 1980s, what seemed to be incomplete ring arcs were detected around Neptune, again during an occultation. In 1989, Voyager 2 showed that there are complete rings around Neptune and that some ╘clumpiness' gave rise to the impression of incomplete arcs.
Virtually all the planetary rings lie within their Roche limits. In a disc of debris around a newly formed planet, material beyond the Roche limit could coalesce into satellites while nearer to the planet, the tidal forces would prevent the satellites from forming.
The rings around Jupiter are faint and tenuous, and their reflection qualities show that many of the particles can be no bigger than 1 or 2 micrometres. Dust of this size must be constantly replenished, perhaps by impacts on boulder-sized objects in the ring.
Saturn's rings are far more complex and extensive than any others. The rings readily visible from Earth were labelled A, B and C, C being the faint inner Crepe Ring. The A and B rings are separated by the Cassini Division and there is also a narrow but conspicuous gap towards the outer edge of the A ring, known as the Encke Division or Encke Gap. Voyager 1 detected material inside the C ring, which was called the D ring. Beyond the A ring lie further narrow, tenuous rings, known as the E, F and G rings. The ring particles are thought to consist of a mixture of water ice and dust, ranging in size from a few micrometres up to a hundred metres. However, the composition is not uniform, as demonstrated in Voyager images showing marked colour variations. These images also show that the rings consist of thousands of narrow, closely spaced ╘ringlets'. Many of the observed structures are attributable to the gravitational action of satellites. For example, Pandora and Prometheus act as ╘shepherds', confining the F ring, and the Cassini Division lies where a satellite with an orbital period half that of Mimas would lie. (This is an example of a resonance phenomenon.).
The nine rings of Uranus found in 1977 are labelled, in order of increasing distance from the planet: 6,5,4,alpha, beta,eta,gamma,delta and epsilon. Two further rings were found by Voyager 2 in 1986 and also a pair of satellites, Ophelia and Cordelia, shepherding the epsilon ring. The nine main rings appear to be composed of metre-sized boulders. But in backlighting, Voyager 2 also saw many slender ringlets composed of dust.
Two main rings orbit Neptune, with a diffuse sheet of material extending outwards from the inner of the two. There is also a tenuous third ring nearer the planet. The outer ring has bright clumps, about 8í in extent, which seem to be dominated by dust-sized particles. The narrowness of the rings suggests that they are shepherded by unseen satellites. In 1991, the names Galle, Leverrier and Adams were given to the three most prominent rings, in order of distance from the planet.
