Prof. Dale E. Gary
Minor Bodies of the Solar System: Pluto, Moons, and Rings
Pluto was discovered in 1930 by Clyde Tombaugh, after a decades long search in vain by Percival Lowell. It has been known as the 9th planet of the solar system since that time, but perhaps we were a bit too hasty. Since then a large number of other objects that are very similar to Pluto have been found, and it is now recognized that these are part of the Kuiper Belt group of objects. In fact, there is a group of objects called Plutinos that have the same orbital characteristics as Pluto: they cross Neptune's orbit, and have a 3:2 resonance with Neptune.
All four of the giant planets have rings, although none are easily visible except the rings of Saturn. The rings probably have several different origins. Jupiter's faint rings are due to dust from meteroid strikes on small moons nearby. Saturn's rings are due to the breakup of a fairly large body (about 250 km in diameter) due to tidal forces. The faint rings of Uranus and Neptune may also be due to tidal disruption, but if so the bodies are much smaller. The rings of Uranus and Neptune are kept narrow by the action of shepherd moons, which "herd" the ring particles and keep them in a narrow range of orbits.
Note that the rings are not long-lived phenomena. Saturn's rings may not last more than a few hundred million years before the material eventually falls onto the planet.
outer edges of the Gossamer Ring are due to similar particles from Almathea
and Thebe, but because these moons have larger orbital inclinations their
particles are spread out over a larger thickness that exactly match their
Uranus' outer epsilon ring showing the two
shepard moons, 1986U7 and 1986U8, which
act in concert to keep the ring narrow.
Uranus' Rings as seen in forward (top) and backward (bottom)
scattered light. Note that the outer (epsilon) ring doesn't match up
in the two figures because it is rather eccentric (elliptical).
The Galilean moons are Io, Europa, Ganymede, and Callisto. They are said to from a miniature solar system, since when Jupiter first formed it would have had its own "solar nebula" in a disk orbiting its equator. From the inner disk, Io formed. Io is entirely rocky, without significant icy compounds. As we go out from Jupiter, the moons become less dense, and hence must have a larger percentage of icy material, just as the planets do in going out from the Sun. Jupiter has many many additional moons, with more yet to be discovered. Most of the small ones are likely captured asteroids.
Neptune's largest satellite is Triton and it is an oddball in the sense that it orbits in the opposite direction (retrograde) from all of the other large moons, which obey the right-hand rule. There is no way that the moon could have formed in that configuration. In fact, Triton is thought to have been captured from the Kuiper Belt some time in the past, and it is very much like what Pluto must be like--largely made of ices. Because it orbits retrograde, it is slowly spiralling toward Neptune due to tidal interactions. At some point it will come too close to Neptune (within about 2.5 planet radii), and will be tidally disrupted. The timescale for this is about 100 million years, so if we were to go into the future by 100 million years we might find that Saturn has no rings any longer, but now Neptune does! And since Triton is 2700 km in diameter, it is more than 1000 times the mass of Saturn's rings, so it will make quite a ring!