Planetary ring is a disk or ring of dust, moonlets, or other small objects orbiting a planet or similar body. The most notable planetary rings in the Solar System are those around Saturn, but the other three gas giants (Jupiter, Uranus and Neptune) also possess ring systems. On 26 March 2014 was announced the discovery of rings around the minor planet Chariklo during the observation of a stellar occultation on 3 June 2013. 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 possess 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. Pluto is not known to have any ring systems, though the New Horizons probe might find a ring system when it visits in 2015. All of the jovian planets have a system of rings. Jupiter has four faint rings: a flattened main ring, a puffier inner ring, and two wispy outer rings that are inside the orbit of Io. The rings are made of very small, dark particles the size of smoke particles. They are produced by dust kicked up from the tiny innermost moons of Jupiter by impacts on the moons. Upper left-center is the Galileo spacecraft's view of Jupiter's faint rings when looking back toward the Sun---the best position for viewing very faint rings made of tiny particles. The lower right graphic is a description of the rings and where they are with respect to some small moons very close to Jupiter. Jupiter's ring system is composed of three parts: an outermost gossamer ring, a flat main ring, and an innermost donut-shaped halo. Saturn's Rings The planet with the spectacular ring system is Saturn. Icy particles spread out into large, flat rings make up Saturn's ring system that can be seen with even low-power telescopes on the Earth's surface. The rings of the other jovian planets are dark and faint, so they were discovered only relatively recently with either powerful telescopes or by spacecraft flybys. Saturn's rings were discovered by Christian Huygens in 1659. Galileo's telescope was too small to make them look like more than just a couple of bumps on either side of the planet. In 1675 Giovanni Cassini discovered a gap between the two large (A & B) rings, now called the Cassini division in his honor. With improved telescopes, astronomers were able to see that one of the large rings was in fact, two rings (B & C) and there is a gap in the A ring (the Encke division). There is also a hint of another ring closer to the planet than the C ring (the D ring). When the Pioneer and Voyager spacecraft flew by, astronomers found more rings and complex structure in the rings. The rings that are visible in even low-power telescopes on the Earth (A, B, and C) extend from about 74,000 kilometers to about 137,000 kilometers from Saturn's center (or 1.23 to 2.28 Saturn radii). The rings are very thin, less than a hundred meters thick. A scale model of the rings with the width equal to a single piece of regular paper would be about 100 meters across! Collisions between the ring particles keeps the ring system very flat and all of the particle orbits circular. In 1859 James C. Maxwell (of electromagnetism fame), showed that the rings could not be solid, but, rather a swarm of particles. A solid ring the width of Saturn's ring system would become unstable and break up. James Keeler proved Maxwell correct in 1895 when he measured the doppler shifts of different parts of the rings and found that the outer parts of the ring system orbited at a slower speed than the inner parts. The rings obeyed Kepler's third law and, therefore, must be made of millions of tiny bodies each orbiting Saturn as a tiny mini-moon. Spectroscopy of the rings shows that the particles are made of frozen water.

• published: 12 Apr 2015
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