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. It is also predicted that Phobos, a moon of Mars, will break up and form into a planetary ring in about 50 million years. Its low orbit, with an orbital period that is shorter than a Martian day, is decaying due to tidal deceleration. Jupiter Jupiter's ring system was the third to be discovered, when it was first observed by the Voyager 1 probe in 1979, and was observed more thoroughly by the Galileo orbiter in the 1990s. Its four main parts are a faint thick torus known as the "halo"; a thin, relatively bright main ring; and two wide, faint "gossamer rings". The system consists mostly of dust. Saturn Saturn's rings are the most extensive ring system of any planet in the Solar System, and thus have been known to exist for quite some time. Galileo Galilei first observed them in 1610, but they were not accurately described as a disk around Saturn until Christiaan Huygens did so in 1655. The rings are not a series of tiny ringlets as many think, but are more of a disk with varying density. They consist mostly of water ice and trace amounts of rock, and the particles range in size from micrometers to meters. Uranus Uranus's ring system lies between the level of complexity of Saturn's vast system and the simpler systems around Jupiter and Neptune. They were discovered in 1977 by James L. Elliot, Edward W. Dunham, and Jessica Mink. In the time between then and 2005, observations by Voyager 2 and the Hubble Space Telescope led to a total of 13 distinct rings being identified, most of which are opaque and only a few kilometers wide. They are dark and likely consist of water ice and some radiation-processed organics. The relative lack of dust is due to aerodynamic drag from the extended exosphere-corona of Uranus. Neptune The system around Neptune consists of five principal rings that, at their densest, are comparable to the low-density regions of Saturn's rings. However, they are faint and dusty, much more similar in structure to those of Jupiter. The very dark material