Ring system (astronomy)
A ring system is a disk or ring orbiting an astronomical object that is composed of solid material such as dust, moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as a planetary ring system.
The most prominent planetary rings in the Solar System are those around Saturn, but the other three giant planets (Jupiter, Uranus and Neptune) also have ring systems. Recent evidence suggests that ring systems may be found around other types of astronomical objects, including minor planets, moons, and brown dwarfs.
Ring systems of planets
There are three ways that thicker planetary rings (the rings around planets) have been proposed to have formed: from material of the protoplanetary disk that was within the Roche limit of the planet and thus could not coalesce to form moons; from the debris of a moon that was disrupted by a large impact; or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet's Roche limit. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it now appears that Saturn's rings might be quite old, dating to the early days of the Solar System.
The composition of ring particles varies; they may be silicate or icy dust. Larger rocks and boulders may also be present, and in 2007 tidal effects from eight 'moonlets' only a few hundred meters across were detected within Saturn's rings.
Sometimes rings will have "shepherd" moons, small moons that orbit near the inner or outer edges of rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.
Saturn's narrow F Ring has the small satellites Prometheus and Pandora orbiting just inside and outside it, and traditionally they have both been viewed as shepherds. However, recent studies indicate that only Prometheus contributes to the ring's confinement.
Several of Jupiter's small innermost moons, namely Metis and Adrastea, are within Jupiter's ring system and are also within Jupiter's Roche limit. It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiter's tidal forces, possibly facilitated by impacts of ring material on their surfaces.
Uranus's ε ring also has two shepherd satellites, Cordelia and Ophelia, acting as inner and outer shepherds respectively. Both moons are well within Uranus's synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration.
Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2's images showed them to be complete rings with bright clumps. It is thought that the gravitational influence of the shepherd moon Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.
Rings systems of minor planets and moons
Reports in March 2008 have suggested that the Saturnian moon Rhea may have its own tenuous ring system, which would make it the only moon known to have a ring system. A later study published in 2010 revealed that imaging of Rhea from the Cassini mission was inconsistent with the predicted properties of the rings, suggesting that some other mechanism is responsible for the magnetic effects that had led to the ring hypothesis.
One minor planet is known to have rings, the centaur 10199 Chariklo. It has two rings, perhaps due to a collision that caused a chain of debris to orbit it. The rings were discovered when astronomers observed Chariklo passing in front of the star UCAC4 248-108672 on June 3, 2013 from seven locations in South America. While watching, they saw two dips in the star's apparent brightness just before and after the occultation. Because this event was observed at multiple locations, the conclusion that the dip in brightness was in fact due to rings orbiting the asteroid is unanimously the leading theory. The observations revealed what is likely a 19-kilometer (12-mile)-wide ring system that is about 1,000 times closer than the Moon is to Earth. In addition, astronomers suspect there could be a moon lying amidst the asteroid's ring debris. If these rings are the leftovers of a collision as astronomers suspect, this would give fodder to the idea that moons (such as the Moon) form through collisions of smaller bits of material. Chariklo's rings have not been officially named, but the discoverers have nicknamed them Oiapoque and Chuí, after two rivers near the northern and southern ends of Brazil.
A second centaur, 2060 Chiron, is also suspected to have a pair of rings. Based on stellar-occultation data that were initially interpreted as resulting from jets associated with Chiron's comet-like activity, the rings are proposed to be 324 (± 10) km in radius. Their changing appearance at different viewing angles can explain the long-term variation in Chiron's brightness over time.
It is also predicted that Phobos, a moon of Mars, will break up and form into a planetary ring in about 50 million years, due to its low orbit with an orbital period that is shorter than a Martian day, causing tidal deceleration that continuously lowers its orbit.
Rings around exoplanets
Because all giant planets of the Solar System have rings, the existence of exoplanets with rings is plausible. Although particles of ice, the material that is predominant in the rings of Saturn, can only exist around planets beyond the frost line, within this line rings consisting of rocky material can be stable in the long term. Such ring systems can be detected for planets observed by the transit method by additional reduction of the light of the central star if their opacity is sufficient. As of January 2015, no such observations are known.
A sequence of occultations of the star 1SWASP J140747.93-394542.6 observed in 2007 over 56 days was interpreted as a transit of a ring system of a (not directly observed) substellar companion dubbed “J1407b”. This ring system is attributed to have a radius of about 90 million km (about 200 times the one of Saturn's rings). In press releases, the term super-Saturn was dubbed. However, the age of this stellar system is only about 16 million years, which suggests that this structure, if real, is a protoplanetary disk rather than a stable ring system in an evolved planetary system.
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