The seventh planet from the Sun, orbiting every 84 years at a mean distance of 2871 million km (19.2 au). It is blue-green in colour due to the absorption of red light by methane in the upper atmosphere. Its mean opposition magnitude is +5.5, making it just visible to the naked eye under favourable conditions, but it was unknown until 1781 when discovered telescopically by F. W. Herschel. Uranus is distinctly ellipsoidal in shape (equatorial diameter 51 118 km, polar diameter 49 946 km). Its density, 1.3 g/cm³, is the lowest of the planets except Saturn. Its rotation axis is tilted at over 90° to its orbital plane, so that its rotation is retrograde, and it presents its poles and its equator alternately towards the Sun as it orbits. The rotation period of the visible surface ranges from about 16 hours at 70° south to about 18 hours near the equator, but radio observations indicate that the core rotates in 17 h 14 m.

Uranus has a thick atmosphere composed of 83% hydrogen, 15% helium, and 2% methane (molecular percentages). Thicker clouds of methane at the 1 bar pressure level overlie deeper opaque clouds that probably consist of hydrogen sulphide. The temperature near the top of the atmosphere is around −220°C. Internally, Uranus is thought to have a small rocky core at a high temperature, probably surrounded by a layer of icy materials, topped by a layer of hydrogen and helium. Unlike the other gas giants, Uranus does not emit more heat than it receives. The interior is probably as hot as that of Neptune, but the heat may be prevented from escaping as effectively as a result of some unknown process. Its magnetic field has a strength of about 2.5 × 10⁻⁵ tesla at the equator, similar to the Earth’s. Uranus’s magnetic axis is not centred in the core, but one-third of the way to the surface, and is tilted at nearly 60° to its spin axis.

The atmosphere of Uranus shows few visible features. Computer-enhanced images have revealed extremely faint dark belts or bands with bright zones between them. Dark and bright spots occur, but these are also very faint. Each latitude has its own characteristic rotation period, with the shortest rotation periods at around 70° south. This may be a result of the high axial inclination of Uranus; since each pole of Uranus spends long periods facing the Sun, convection cells originating at the pole could produce winds that blow against the rotation of the planet near the equator, due to the Coriolis force.

Uranus has thirteen known rings, all far fainter and less extensive than those of Saturn. The innermost ring, known as Zeta, is 35–40 000 km from the centre of Uranus; the brightest ring, called Epsilon, is 51 100 km from the centre and 20–100 km wide. The rings differ from those of Jupiter and Saturn in that they are slightly eccentric, and do not perfectly lie in the equatorial plane of the planet. They are comparatively dust-free and composed of much bigger particles, generally over a metre in size. The Epsilon Ring is composed mainly of large icy boulders, but is dark grey in colour with an albedo of a few per cent. Uranus has 27 known satellites.

http://hubblesite.org/image/2175/news_release/2007-32

http://hubblesite.org/news_release/news/1998-35