• The seventh planet of the solar system, discovered in 1781 by William Herschel. Uranus was the first planet to be discovered, rather than simply being observed in the night sky, and its presence was a huge shock to contemporary science. It is now known that earlier astronomers including Flamsteed had also charted Uranus – which can just be seen as a naked-eye object – and failed to register it as a planet. Uranus takes 84 years to orbit the Sun – it is still fewer than three Uranus years since we found out about it – and has an average distance of 19AU from the Sun. Uranus is one of the giant gas planets, 15 times as massive as the Earth and 52,000km in diameter. Its day of 16 hours is the oddest in the solar system, since the axis of Uranus’s rotation is at 98¦ to the plane of its orbit. This means that Uranus is in effect rotating ‘backwards.’ Explanations for this behaviour must cope with the fact that Uranus’s large satellites Miranda, Ariel, Umbriel, Titania and Oberon orbit the planet exactly in the plane of its equator. Uranus also has a system of ten dark, narrow rings extending out to 51,200km from the planet, discovered in 1977 by observers in an airborne observatory high in the Earth’s atmosphere. Our knowledge of Uranus was expanded vastly in 1986 by a flyby of the planet by the US spacecraft Voyager 2. Its images show Uranus as a planet almost bare of the spectacular cloud features found on Jupiter and Saturn. Uranus has a magnetic field comparable in strength to the Earth’s, tilted steeply both to its angle of rotation and to its orbit. Voyager 2 also revealed ten small satellites orbiting closer to Uranus than the five satellites previously known, some of which seem to have a role in shaping the ring system. Uranus also has an extensive magnetospheric system. Uranus’s atmosphere seems to be dominated by hydrogen, helium and methane, and there are also more complex molecules like acetylene. Below the clouds, Uranus may have an icy mantle (possibly including solid ammonia) and perhaps a rocky core. The planet has a density of about 1.2gm/cc.