Uranus

The ice giant that’s shrouded in mystery has fascinated explorers for decades

The seventh planet from the Sun, Uranus became the first planet to be found with the aid of a telescope on 13 March 1781 by British astronomer William Herschel. On that fateful night, he described observing a “nebulous star or perhaps a comet”. Little did he know that he had just discovered Uranus, which was named after the Greek god of the sky –a name proposed by Johann Elert Bode in 1783.

240 years later, Uranus remains a puzzle. What is known is that Uranus is located about 2.9 billion kilometres (1.8 billion miles) from the Sun, about 19 times the distance from Earth to the star, meaning that one orbit of the Sun takes 84 Earth years. The planet is enormous, with a diameter of 50,724 kilometres (31,518 miles) – four times wider than Earth. Uranus has a compositional mass that is 80 per cent a fluid mixture of water, ammonia (NH3 ) and methane (CH4 )ices. It’s the methane in the outer atmosphere that gives it its blue-green colour, but the thick cloud coverage does not allow our instruments to peer down any further, and is one reason why Uranus remains an enigma.

Astronomers strongly suggest that below the planet’s cloud tops is a main atmosphere which contains mostly hydrogen and helium by composition and has traces of methane and other volatiles. Below that is the fluid icy mantle, which makes up most of its composition by mass, but it’s also theorised that the pressure and temperatures here are enough to make it ‘rain’ diamonds at that depth. Finally, at the centre is the silicate iron-nickel core, thought to be between half to just over three times the mass of Earth.