Two views of Uranus, one in true color (left) and the other in false color, compiled from images returned Jan. 17, 1986, by Voyager 2. The spacecraft was 9.1 million km (5.7 million miles) from the planet, several days from closest approach.
Two sides of Uranus, imaged through colored filters, showing long-lived clouds drifting across the surface. Image: Lawrence Sromovsky, UW-Madison.
Uranus was discovered by William Herschel in 1781. Others had seen it earlier – at its brightest it is just visible to the naked eye – but had taken it to be a star because it doesn't move perceptibly from one night to the next. The earliest recorded sighting was in 1690 when John Flamsteed catalogued it as 34 Taurus. William Herschel originally called it Georgium Sidus (George's Star) in honor of King George III, while French astronomers began calling it Herschel. It was Johann Bode who proposed the name Uranus, after the Greek god of the heavens, but this didn't come into common usage until around 1850.
Uranus's most extraordinary feature is the tilt of its axis – almost 98° (or 82° if it is taken to be retrograde), so that the planet effectively spins around on its side. As a result, for part of its orbit, one pole continually faces the Sun while the other is in total darkness. Half an orbit later, the roles (and poles) are reversed. In between, the Sun rises and sets around the equator normally. For Uranus to be in such a position, it was almost certainly struck a formidable blow by another massive object. Since the plane containing its 13 rings (see Uranus, rings) and 27 known moons (see Uranus, moons) is similarly tilted, this impact presumably took place during or shortly after the accretionary phase of the planets.
The atmosphere of Uranus is composed of hydrogen (83%), helium (15%), methane (2%), and trace amounts of acetylene and other hydrocarbons. Its bluish hue stems from an upper methane haze that absorbs strongly at red wavelengths – leaving a featureless blue planet in our telescopes. During Voyager 2's flyby in 1986, Uranus's banded cloud patterns were extremely bland and faint; more recent Hubble Space Telescope observations, however, have shown a more strongly banded appearance now that the Sun is getting closer to being directly overhead at Uranus' equator, a position it will assume in 2007.
A single image of Uranus taken in 2004 shows 18 distinct cloud systems – eight more than Voyager saw during its entire months-long flyby. And one set of images taken with the Keck II telescope in Hawaii in summer 2004 shows an extremely bright cloud reaching up high above the planet's opaque methane layers in its southern hemisphere.
Until 10 years ago, the only pictures sharp enough to show any cloud features were those from Voyager. But then images from the Hubble Space Telescope began to exceed Voyager's resolution, and over the past four years images from the 10-meter Keck telescope, using its new adaptive optics system and an infrared camera, have done even better. These observations have shown that some storm systems on Uranus come and go within days, while others can persist for years. The fastest winds ever recorded, seen in late 2003, were up to 420 kilometers per hour.
Uranus's magnetic field is odd in that it isn't centered on the center of the planet and that is tilted almost 60° with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. (Neptune has a similarly displaced magnetic field, suggesting that this is not necessarily a result of Uranus's axial tilt.) The magnetosphere is twisted by the planet's rotation into a long corkscrew shape behind the planet.
Composition of Uranus
Uranus probably has a solid, rocky core, surrounded by a deep layer of ammonia and water that has condensed into an icy slush. The outer layer is made up primarily of liquid hydrogen and helium.