Uranus is a gas giant with 27 known moons and a system of rings. It was discovered by William Herschel in 1781. It is the only planet apart from Venus that rotates in the opposite direction to the other planets, called retrograde rotation. The axis of rotation is inclined at 97.86º, almost on its side.

Uranus is composed mainly of rock and various ices without a rocky core, the material appears to be evenly distributed.

The atmosphere consists of 83% Hydrogen, 15% Helium and just less than 2% Methane with traces of Acetylene and other hydrocarbons. The blue-green colour of Uranus is caused by Methane in the upper atmosphere absorbing red light.

The magnetic field is different from previously studied planets in that it is considerably off centre and is tilted at 60º to the axis of rotation. This is probably due to the internal structure of the planet. Unlike Jupiter and Saturn which have large deep electrically conducting layers of metallic Hydrogen which supported large circulating electric currents resulting in powerful magnetic fields, Uranus is believed to have a thin shallow layer of conducting material resulting in a weaker off centre magnetic field.

Uranus has a faint ring system first discovered in 1977 and confirmed in 1986 when Voyager 2 flew by the planet. The rings vary in their composition, the prominent Epsilon ring consisting of rocks up to 10 metres in diameter while the fainter rings are made of particles the size of fine dust.

The innermost ring is 1986U2R at 38,000 km from the centre of the planet. Next is 6 then 5 then 4 followed by Alpha, Beta, Eta, Gamma, Delta, Lambda and Epsilon at 51,000 km from the planets centre.

The rings vary in thickness from 15 km for the Epsilon ring to 100 metres for 1986U2R.

Uranus has 27 known moons Titania the largest, along with Oberon were discovered by William Herschel in 1787. William Lassell discovered Ariel and Umbriel and in 1948 Gerard Kuiper (of Kuiper Belt fame) found Miranda. No further moons were discovered until the visit of Voyager 2 1986.

Miranda is a most unusual object. Only 470 kilometres in diameter its surface features are jumbled and chaotic. It has canyons 20 kilometres deep and mixes of old and young features. Some scientists believe that Miranda experienced one or more collisions in the past that shattered the moon into a number of pieces which then slowly reassembled under gravitational force.

Cordelia and Ophelia are shepherd moons for the epsilon ring. Cordelia is the innermost known moon and is 15 kilometres in diameter with Ophelia having a diameter of 16 kilometres.

More information about the moons can be found at:

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus&Display=Moons