Uranus was discovered in 1781 by William Herschel, who first mistook the planet for a comet. Uranus takes 84 years to circle the Sun. The planet is 4.0 times bigger than the Earth and 14.5 as massive.
Uranus has a characteristic green colour and a spectrum dominated by absorption bands of methane and hydrogen. While the precise composition of the atmosphere is unknown, a common idea is that it is similar to that of Jupiter but depleted in hydrogen and helium relative to other gases such as methane. From the Earth, Uranus shows a tiny featureless disk, making determinations of its spin orbit difficult. Indeed, values quoted range from 10.7 to 23 hours. Assuming that the satellites orbit close to the planet's equatorial plane, the spin axis must be tilted at 98 degrees to the ecliptic, giving rise to unusual seasons on the planet. For a quarter of each orbital period a particular pole points in the general direction of the Sun, while the other is in darkness.
The planet has a deep gaseous atmosphere in which clouds must exist that are not evident from the outside. Temperatures in the upper atmosphere are near -110 degrees. Very little is known about the planet's internal structure or whether Uranus possesses a magnetic field. Models suggest a three-layer interior structure consisting of a hydrogen-rich atmosphere, an icy mantle, and a rocky core.
The year 1976 saw the discovery of the planet's unique ring system, made up of more than nine individual, thin rings. Unlike the rings of Saturn, those of Uranus consist of dark particles. It has been proposed that each of the rings is associated with a tiny satellite, or perhaps a pair of satellites.
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