Uranus

Uranus is the seventh planet from the Sun. It is a giant ball of gas and icy material that lies far out in the cold edge of the solar system. Uranus is about four times wider than Earth. About 63 Earths could fit inside it. The planet takes 84 Earth years to finish one trip around the Sun.

Uranus was discovered in 1781 by a British astronomer named William Herschel. It was the first planet ever found with a telescope. The five closer planets had been known since ancient times, because people could see them with just their eyes. Herschel's discovery suddenly made the solar system twice as big as anyone thought.

The planet looks pale blue-green. That color comes from a gas called methane high up in its clouds. Methane soaks up the red light from the Sun and sends blue light back out into space. Below the clouds, Uranus is made mostly of water, ammonia, and methane in slushy, icy forms. Because of all this icy material, scientists call Uranus an "ice giant," along with Neptune.

The strangest thing about Uranus is the way it spins. Most planets spin like tops, with their poles pointing up and down. Uranus is tipped on its side. Its axis points almost straight at the Sun. Scientists think a huge object, maybe the size of Earth, slammed into Uranus long ago and knocked it over. Because of this tilt, each pole on Uranus gets 42 years of sunlight, followed by 42 years of darkness.

Uranus has rings, but they are thin and dark. They are nothing like the bright, wide rings of Saturn. The planet also has 28 known moons. They are named after characters from the plays of William Shakespeare and the poems of Alexander Pope. The biggest moon, Titania, is about half the size of Earth's Moon.

Only one spacecraft has ever visited Uranus. In 1986, NASA's Voyager 2 flew past it and took close-up pictures for the first time. After that quick visit, the spacecraft flew on toward Neptune. No mission has returned since. Many scientists want to send another probe soon, because there is still a lot we do not know. We are not sure why Uranus gives off so little heat, or what its deep inside looks like. Some researchers argue it should be the next big target for planetary exploration.