In astronomy, seventh major planet from the Sun. It was discovered in 1781 by the English astronomer William Herschel during a telescopic survey of the sky.
Uranus is almost 15 times as massive as the Earth, and its volume exceeds that of the latter by more than 50 times. Stellar occultation measurements conducted by James Elliot and other American investigators in 1977 revealed the existence of nine sharply defined, narrow, dark rings lying from 1.6 to 1.95 RU (where RU is Uranus' equatorial radius of 26,000 km [16,120 miles]). A tenth narrow ring and broad dust bands between the narrow rings were discovered in 1986 by the U.S. Voyager 2 space probe during its flyby of the planet. Three of the rings are circular, while the others have a large range in eccentricity and are variable (by up to a factor of five) in width. All of the rings consist primarily of boulder-sized chunks of dark matter about 1 m (3 feet) in diameter. The composition of the ring particles has not yet been determined, but investigators speculate that they are made up of rock fragments and water ice intermixed with a black, amorphous polymer, which might account for the coal-black colour of the particles. The gaps between the rings are filled with numerous bright dust particles.
In addition to its system of rings, Uranus has at least 15 satellites. The five major moons--Miranda, Umbriel, Ariel, Oberon, and Titania--have diameters ranging from 472 to 1,578 km (293 to 980 miles). The other moons are substantially smaller and lie inside the orbit of Miranda, the innermost of the major Uranian satellites. Two of the moonlets seem to act as "shepherds" of the ring dubbed Epsilon, gravitationally confining its constituent particles.
Data transmitted by Voyager 2 revealed that Uranus is surrounded by a magnetic field comparable in strength to those around the Earth and Saturn. Trapped within this field is a radiation belt more intense than the Earth's. The axis of the Uranian field is tilted 58.6 from the planet's axis of rotation. This is an exceedingly large inclination when compared with that of the magnetic fields of other planets (e.g., the Earth's field is tilted a mere 11). The magnetic field of Uranus is swept back into a long "tail" by the solar wind (outflow of charged particles from the Sun's corona). Because the field is inclined so sharply, the tail is twisted much like a corkscrew.
An analysis of radio emissions from Uranus' magnetic field has enabled investigators to estimate the rotation period of the planet to be about 17.24 hours. Uranus' axis of rotation is unusual in that it lies in the plane of its orbit, and so Uranus appears to spin on its side. The axes of all other planets in the solar system are roughly perpendicular to the plane of their orbits.
The interior of Uranus is thought to consist of a mixture of ice and gas. A rocky core appears to be small or nonexistent.
The atmosphere of Uranus appears to extend to an altitude of thousands of kilometres in successive layers. The innermost layer most likely consists of clouds of methane ice, while the upper reaches are composed principally of hydrogen and helium. The atmospheric hydrogen and helium occur in proportions consistent with solar values. The average heat radiated by Uranus is equivalent to that of a blackbody--i.e., an object that emits all radiation incident upon it--of the same cross-section radiating at 59.1 K. This is the so-called effective temperature of the planet and is equal to the actual temperature of the atmospheric gases at a pressure of about 0.4 bar (one bar is roughly equal to the Earth's sea-level pressure). Temperature decreases with altitude from this point, reaching a minimum of 52 K at the 70-millibar level, and then increases with height up to a maximum of 750 K at the very top of the atmosphere.
Like Jupiter and Saturn, Uranus has bands of clouds oriented parallel to circles of constant latitude. Zonal winds travel in a retrograde direction at low latitudes (near the equator) and in a prograde direction at higher latitudes; the wind velocities are several times greater than those of the Earth and appear to increase with latitude.
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