Uranus is the seventh major planet in our solar system, and the third of four gas giant planets. It is 31,800 miles (51,200 kilometers) in diameter, just under four times the diameter of Earth. Like the other gas giant planets, Uranus consists mostly of gas. Its pale blue-green, cloudy atmosphere is made of 83 percent hydrogen, 15 percent helium, and small amounts of methane and other gases. Uranus gets its color because the methane in the atmosphere absorbs reddish light and reflects bluish-greenish light. Deep down below its atmosphere, a slushy mixture of ice, ammonia, and methane is thought to surround a rocky core.

Although it orbits the Sun in a perfectly ordinary, near-circular ellipse every eighty-four Earth years, Uranus has an extremely odd rotation compared to the other major planets. It rotates on its side, almost like a bowling ball rolling down its lane, and its polar axis is parallel rather than perpendicular to its orbital plane. This means that one end of Uranus faces the Sun for an entire half of its orbit, while the other end faces away during that time. So one “day” on Uranus is equal to forty-two Earth years! Most astronomers think that at some point in its history, Uranus was struck by a large (at least planet-sized) object that knocked it onto its “side,” causing this unusual motion.

Uranus is orbited by at least twenty-seven moons and thirteen thin rings. During its flyby of Uranus, the Voyager 2 space probe discovered a large and unusually shaped magnetic field around Uranus (probably unique because of the planet’s odd rotational motion) and a chilly cloud-top temperature of –350 degrees Fahrenheit (–210 degrees Celsius).