The Planet Uranus - Astronomy

When the planet Uranus was discovered in 1781, it was believed to be a comet. We now know that it is a complex world, and so distant that astronomers must use math to predict its behavior.

Uranus’s Surface -- Cold and Gaseous

Named after an ancient sky god, Uranus is one of the Gas Giants (along with Jupiter, Saturn, and Neptune) and has no solid surface. Its atmosphere is primarily hydrogen and helium, with small amounts of methane, ammonia, and water. Uranus’ liquid core makes up approximately 80 percent of the planet’s mass; this core consists mainly of icy substances such as water, methane, and ammonia. The planet’s surface is covered by blue-green clouds made of methane crystals, and it is believed beneath that is a layer of thicker clouds, followed by an ocean of water and dissolved ammonia. Near the cloud tops, the temperature is approximately -355 degrees Fahrenheit, rising to about 4,200 degrees Fahrenheit at the ocean and 12,600 degrees Fahrenheit at the core.

Far From the Sun

Uranus is the seventh planet from the Sun (at a distance of about 2.4 billion miles) with an orbit lasting 84 years. It is the most distant planet that can be viewed without a telescope. Discovered in 1781 by British astronomer William Herschel, Uranus was named by German astronomer Johann E. Bode. Most of what scientists know about Uranus comes from the Voyager 2 mission, which came within 50,600 miles of the planet’s cloud tops. Because the planet is so far, astronomers have only recently begun to understand it. Its 11 rings weren’t discovered until 1977, and of its 27 known moons, several were discovered in 2003.

The Tilted Planet

Like many planets, Uranus is tilted on its axis. However, Uranus is so tilted that its axis is nearly level with its path around the Sun. It is believed this was caused by a collision with an Earth-sized object when Uranus was still young. Because of this tilt, the planet’s seasons last 20 years. However, because the planet is so far from the Sun, the temperatures on both sides of the planet are approximately the same. The planet spins on its axis at different rates at different locations. The planet’s interior rotates once every 17 hours 14 minutes, but most of the atmosphere rotates significantly faster--the area near the South Pole rotates once every 14 hours.

Mass, Density, and Pressure

Though Uranus has a mass 14 1/2 times larger than Earth’s, it has only 1/20 the mass of fellow Gas Giant Jupiter, the largest planet in the Solar System. The atmospheric pressure on Uranus 1.3 times that of Earth, and could crush anything entering the atmosphere. The planet’s average density (about 1.27 grams per cubic centimeter, or about 1 1/4 times the density of water) is about one-fourth that of the Earth.