Planets

Saturn's Rings: As Saturn travels around the Sun, we see its majestic ring system from different angles. These images were taken with the Hubble Space Telescope from the year 1996 (bottom left) to 2000 (top right).

Notice how thin the rings are! They are 300,000 km across, and only a kilometer thick at most. They are made up of chunks of ice, most of which are about the size of softballs, though there are larger bodies, some a few kilometers across, and smaller pieces, all the way down to very fine dust. Each ring particle orbits Saturn like a tiny moon.

Ancient people noticed that while most of the stars did not move, certain bright "stars" wandered through the constellations of the zodiac. These were the planets. Mercury, Venus, Mars, Jupiter, and Saturn are visible to the unaided eye. The planets Uranus and Neptune and the dwarf planets Ceres, Pluto and Eris were discovered with telescopes.

Many space missions have been sent to the planets, but they have all been unmanned probes. No human has yet traveled beyond our Moon to another planet, but in the coming decades, the first person may set foot on Mars.

The inner planets, Mercury, Venus, Earth and Mars, are called the terrestrial or earthlike planets. They are rocky planets with metal cores which have solid surfaces. The outer planets, Jupiter, Saturn, Uranus and Neptune, are jovian or jupiterlike. They have no solid surfaces, and are made mostly of hydrogen and helium, which is why they are also known as the gas giants.

The dwarf planet Ceres is a cratered ball of rock and is the largest object in the asteroid belt. The dwarf planets Pluto and Eris are frigid, icy worlds, and are part of the Kuiper Belt.