Uranus

Uranus is the sixth planet on our journey through the Solar System. It is a featureless planet and it is known as an ice giant. The reason it is called an ice giant as opposed to a gas giant like Jupiter and Saturn, is that while the planet is composed of mostly hydrogen and helium, ices such as water, methane and ammonia are more common. The planet also has a small rocky core.

Uranus taken by Voyager 2 in 1986. Credit: NASA

Although Uranus is largely featureless in most images, it is still an interesting place. The atmosphere is the coldest in the Solar System and the planet actually rotates on its side. This means that the seasons are nothing like those on Earth. Each side of the planet experiences 21 years of darkness and then 21 years of daylight! Scientists think this may have been as a result of a collision with an Earth sized protoplanet (baby planet) during the formation of the Solar System. A year on Uranus lasts 86 Earth years and a day on Uranus is equivalent to 17 hours on Earth.

Up close pictures taken by the Voyager spacecraft showed that Uranus has some cloud formations. Observations show that the clouds on Uranus become more dynamic as the planet approaches equinox. Wind speeds on the planet can be up to 900 km/h. Uranus has rings but the rings are dark and hard to see. There are 10 rings in total around Uranus. The planet has 27 moons and something that is unique about them is that they are named after characters from the works of William Shakespeare and Alexander Pope. The five main moons are Miranda, Ariel, Umbriel, Titania, and Oberon.

The first five planets were all known since ancient times. The ancient people noticed that these stars didn’t appear to move the same as the others. This was not the case with Uranus. Uranus is 2.9 billion kilometres from the Sun. To put this into perspective, at this distance it takes 2 hours 40 minutes for light to travel from the Sun to Uranus. In comparison it takes 8 minutes for light to travel from the Sun to the Earth. Uranus is visible as a very faint star, just visible to the naked eye from a very dark sky.

Throughout history there are records of various people adding Uranus to their journals and sky charts but they all mistook it for a star. The reason for this is that given its huge distance from the Sun and the length of time it takes to orbit the Sun, it moves very slowly against the background stars. This changed on the 17th March 1781 when an English astronomer, named William Herschel observed Uranus through his telescope. He noticed the movement against the background stars and reported to the Royal Society that he had discovered a comet. Over the next few months and years, other astronomers observed the planet and calculated its orbit. They began to suspect it was a planet but it took some time to convince Herschel it was in fact a planet.

Having been the first planet to be discovered in modern times the next problem was to find a name for it. Initially Herschel wanted to call it Georgium Sidus, after King George III. This did not go down well with other astronomers and eventually another astronomer named Johann Bode, suggested Uranus. He argued that Uranus who was the Greek god of the sky was a more appropriate name. Even though the planet was officially named Uranus, the argument continued and some people continued to refer to it by other names for up to 70 years after the discovery.

Uranus is one of the least explored planets. It takes a long time to get there and power supply is difficult as sunlight is a lot weaker so far out and this rules out solar panels. The only spacecraft to have visited Uranus is Voyager 2 in 1986, and even then it was just a quick flyby. For the moment there are no planned missions to Uranus but there are a number of missions that have been proposed.

The crescent view of Uranus, captured by Voyager 2 as it passed in 1986. Credit: NASA

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