The seventh planet from the Sun and the third largest of the gas giants (narrowly beating out Neptune) with a diameter about four times that of Earth. 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. 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.

(For further details see Uranus »)

(Source: The Internet Encyclopedia of Science »)