Saturn
The Ringed planet.
Saturn is 6th planet in the solar system, the second most massive (about 95 earth masses), and the second highest volume, though it's also the least dense (less dense than on average than liquid water). It is made mainly of hydrogen and helium gas, with a possibly rocky core in the center, and additional trace elements. Saturn's atmosphere, though looking bland in most far away pictures, has intense cyclones and storms, and the second fastest winds in the solar system on average (After Neptune).
Saturn's orbit isn't perfectly circular; at perihelion, it's barely 9 A.U.[1] from the sun, but then it swings out until it's over 10 A.U. away from the sun at aphelion. One complete orbit takes a hair under 29-and-a-half years.
Saturn's magnetic field is the second largest in the solar system, and is probably generated by metallic hydrogen in the core of the planet. The plasma within the field primarily comes from the moon Enceledas, which shoots water through geysers in the south pole that is than ionized, while some other material comes from Titan, and a bit from other moons. Energy comes from both the Solar Wind and from Saturn's rotation, somewhat of a mixture of Jupiter's magnetosphere (Where rotation supplies most of the energy), and other planetary magnetospheres (Where the solar wind supplies almost all the energy). As with other solar system bodies with magnetic fields, Saturn has its own Auroras.
The Moons of Saturn are mostly crater covered ice and rock balls, but the largest, Titan, has a thick atmosphere (about 1.5x the pressure of Earth's), and has a number of hydrocarbon lakes, erosion channels, rivers, and other surface features. Titan may also have ice volcanoes, or other geologic processes. Another moon, Enceledas, has the previously mentioned water geysers, and possible an underground liquid water layer. The moons of Saturn also help maintain some of the Rings, by providing material through collisions, and gravitationally interacting with the rings to keep them stable.
The Rings
The rings of Saturn are composed of large numbers of ice and rock fragments, possibly formed from a collision of a moon that than never came back together, or possibly formed from material that was too close to Saturn to form a moon in the first place. These clumps often combine and then split, maintaining and redistributing material throughout the rings. Normally, when particles clump together, their tiny gravity is enough to make them keep accreting more and more material -- this is how the planets formed in the first place. But the rings are so close to Saturn that the big planet's own gravity prevents this from happening. This magical distance from a large object, within which no gravitationally-held-together satellites can exist, is called the Roche Limit.
Scientists still debate the age of the rings, which may be relatively recent (in solar system terms), or may be almost as old as the solar system.
The ring system actually consists of thousands of individual rings, each orbiting the planet at a slightly different distance. There are gaps where few or no rings exist, created by the gravitational interaction between the rings and the moons. The rings are extremely thin, no more than a kilometer thick from one side to the other and perhaps as thin as ten meters. While a few of the fragments making up the rings are made of rock, almost all of the ring fragments are made of plain old water ice, ranging in size from 10 meters across all the way down to a centimeter across.
Larger rock/ice balls around 10 km in diameter, called "shepherd moons", orbit along with the rings inside the ring plane. The tiny gravity of these moonlets is thought to keep the rings in line, hence their name.
- ↑ A.U. stands for Astronomical Unit, the average distance between the Earth and the sun