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Mark Thompson
Aug 24, 2011, 7:00 UTC

Sen—Until 13th March 1781, our Solar System was thought to be made up of six planets including the Earth but on this day, the Solar System almost doubled in size overnight. The discovery of Uranus by Sir William Herschel marked a turning point in our understanding of the Universe and the discovery of this new planet, well beyond the orbit of Saturn, helped evolve the understanding of our Solar System too.

Uranus is the seventh planet from the Sun and one of the gas giants. Uranus has a diameter of 50,700 kilometres (31,500 miles) - about four times that of Earth. Uranus is similar in size to Neptune.

Orbiting the Sun at an average distance of almost 2.9 billion kilometres (1.8 billion miles) means that Uranus is faint from the Earth, although it is still just visible to the naked eye under dark skies. Like all major planets at this distance from the Sun, its a large giant planet made up of significant quantities of gas although unlike Jupiter and Saturn it has considerable quantities of ice too. Not the usual kind of ice though, a form of hot dense liquid which has properties similar to conventional ice. From estimates of its mass and density its believed to have a rocky silicates core surrounded by an ice mantle and an atmosphere of hydrogen, helium and methane. Its the methane that gives Uranus its distinctive blue colour.

There are further similarities with the other giant planets, such as a ring system. Saturn of course has by far the most impressive system but there is a ring system around Uranus too. It was the second set of rings to be discovered after Saturn and their discovery was made by observation from an airborne observatory. There are thought to be 13 known rings made up of dark particles which may have come from an ancient moon of the planet which was destroyed by impact events.

One of the great mysteries of Uranus is the way it orbits the Sun. Imagine for a moment that the Sun sits at the centre of a vast sheet of paper. All of the planets orbit the Sun broadly along the plain (we call this the ecliptic) of the paper. They also rotate at varying speeds and all, with the exception of Uranus, spin on their polar axis perpendicular to the ecliptic almost like spinning tops spinning around the Solar System. The Earth's axis is actually tilted by 23.5 degrees with respect to the ecliptic whilst Uranus is tilted over by 97.8 degrees so it’s effectively lying on its back as it rolls around the Sun. Its not known what has caused this although the popular theory is that a comet or protoplanet smashed into Uranus in the early stages of its formation and knocked it over.

This strange orbital characteristic means that for about 42 years, Uranus' north pole gets continuous sunlight while the south pole gets 42 years of darkness and it gradually changes so the south pole gets the light. Because of this, the polar regions receive much more solar energy than the equatorial regions yet for some unknown reason the equator is warmer!

Like all of the outer giant planets, Uranus is accompanied by a family of natural satellites or moons. At last count their were 27 moons in orbit with its largest, Titania less than half the size of our Moon.