Uranus photographed in 2005. Clouds on its surface, along with its ring system, are visible Uranus is the seventh planet from the Sun. Grouped with Neptune as one of the two "ice giants", it is the third largest planet in the Solar System. Its orbit lies at the same distance again from the Sun to Saturn. When William Herschel, an amateur German astronomer living in England, discovered Uranus in 1781 using a home-built telescope, his discovery doubled the span of the known Solar System. Seen through a telescope, Uranus looks like a pale blue-green disc. Seen comparatively close to by Voyager 2 during its fly-by in 1986, this most featureless of planets scarcely looks any different. Only a few wispy white clouds can be made out. They are, in fact, moving round the planet at around 1100 km/h (680 mph). Apart from its rotation—unlike all the planets except Venus is spins anticlockwise—the most unusual thing about Uranus is the angle at which it orbits the Sun.
Tilted at 98° from the vertical, Uranus circles the Sun more or less on its side. Over the 84 years it takes to make a complete orbit, first one pole, then the other, faces the Sun head-on. Each pole has therefore 42 years of continuous sunlight followed by 42 years of continuous night. Astronomers think that, after Uranus formed, an Earth-sized object in the early Solar System may have collided with it, tipping it on its side. Possibly, the remnants of this object became the moons of Uranus.
Uranus has very low internal heat, and radiates virtually no excess heat. By contrast, Neptune, almost the same size and composition as Uranus, radiates more than 2.5 times as much heat into space as it receives from the Sun.
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