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planetary ringsFlat rings of material – consisting of numerous small bodies and particles of rock and ice – known to surround the four giant planets. Saturn's rings were discovered in 1610 by Galileo but not recognized for what they were until Christiaan Huygens correctly described them in 1659. NASA's Pioneer and Voyager spacecraft confirmed that the rings were indeed made up of innumerable smaller bodies ranging in size from a fraction of a millimeter to 30 meters across. The two Voyager spacecraft confirmed the existence of Jupiter's rings, Uranus' rings, and Neptune's rings, which are all much fainter than those of Saturn, All these ring systems probably came into being either because a satellite was shattered to fragments through collision with an asteroid or comet or because a satellite or captured interplanetary body came too close to the planet concerned and was torn apart by gravitational tidal forces. See also Roche limit.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006