A ring system is a collection of rings, usually surrounding a planet, that are made up of small particles in orbit around the planet. Ring systems are found around many different types of planets, including gas giants and ice giants. The best-known example of a ring system is the one around Saturn, which consists of eight main rings and several smaller ones.
Ring systems are believed to form in one of two ways. The first is through the accretion of material onto a planet; this can happen, for example, when a protoplanetary disk breaks up near the planet’s Roche limit. The second way is through the gravitational interactions between a planet and its moons; for example, Saturn’s rings are thought to have been formed when one or more of its moons were destroyed by impacts.
The composition of ring systems varies depending on their origin. For example, Saturn’s rings are mostly water ice with some rocky material mixed in, while the rings around Jupiter are made mostly of dust particles. Ring systems can also be very dense; for example, Jupiter’s innermost ring has about 10 times as much material as Saturn’s Rings do. This article will focus on the former type of ring system—those that form through accretion—and specifically on Saturn’s rings.
Saturn’s rings were first observed by Galileo Galilei in 1610 using a primitive telescope. At that time they appeared as two bright bands on either side of Saturn itself; however, later observers with better telescopes could resolve them into individual rings. It was not until 1789 that William Herschel discovered that there were actually multiple rings (he initially identified six). In 1850 John Dreyer finally compiled all known observations into a single map showing eight distinctrings: three major ones (C-, B-, and A-rings) separated by gaps (the Cassini Division), and five fainter ones outside them (D-, E-, F-, G-, and H-rings). These latter five were later shown to be composed of countless tiny individualringlets themselves separated by even narrower gaps (the Encke Division being one notableexample).
Each major ring is divided into numerous narrow concentric bands or “subrings”, each justa few kilometers wide; these subrings appear to be caused by differential orbital speeds within agiven region causing shearing forces which act to break up an originally continuous structureinto discrete clumps or “streamers”. There may also be minor vertical height variations withdepths typically less than 10 meters within any given subring due mainly to self-gravity effects(such as “waviness” or “undulations”). Finally, there can be localized regions where particlemotion becomes chaotic (“disordered”) instead following regular orbits; such areas generallyappear empty except possibly for extremely fine filamentary structures (“strands”).