Icy Uranus – first planet to be discovered in the Solar System

Uranus, the first new planet to be added to the Solar System, in 1781, is another giant world, after Jupiter and Saturn. But it is classed as an ice giant rather than a gas giant, a label it shares with the next and outermost planet, Neptune. The difference is that they are composed of more ice and rock, and heavier elements than found in Jupiter and Saturn.

Uranus from Voyager 2
Uranus imaged by Voyager 2, the only spacecraft to visit it. Its cloud tops are bland, revealing no detail. Image credit: NASA

Uranus’ atmosphere is mainly hydrogen and helium plus a little methane which gives the planet its blue-green colour. There are also traces of water and ammonia. Uranus has a dense liquid core of water, methane and ammonia. Its mass is 14.5 times that of Earth.

Puzzlingly, Uranus is the coldest of the planets, with temperatures around -224 degrees C in the cloud tops. It emits little energy of its own.

The planet’s discoverer, Sir William Herschel, also found two of its moons in 1787, later named Oberon and Miranda. Brewing tycoon William Lassell found two more, Ariel and Umbriel, in 1851, and the next, Miranda, was not spotted until 1948 by U.S. astronomer Gerard Kuiper.

Today 27 are known, thanks to NASA’s Voyager 2 probe, the Hubble Space Telescope and powerful ground-based telescopes. Most are named after characters in Shakespeare.

The discovery of the moons quickly revealed that Uranus was lying on its side and effectively rolling around in its orbit. This axial tilt of nearly 98 degrees was probably caused by a cosmic collision in its distant past.

A pair of images of Uranus from the Keck II telescope, with the north pole to their right, reveal new details about the planet's enigmatic atmosphere
The wind patterns of Uranus are revealed in a pair of images taken with the Keck II telescope on Hawaii. Credit: Lawrence Sromovsky, Pat Fry, Heidi Hammel, Imke de Pater

It means the planet experiences extreme seasons, because for around 21 years, one hemisphere is almost completely pointed towards the Sun and the other pointed away, receiving no sunlight. For another 21 years, the hemispheres are reversed. Between these two extremes, Uranus is sideways on to the Sun, with both hemispheres experiencing similar amounts of daylight and darkness during their 17h 14m long day.

Uranus’s extreme seasons have a strong effect on its atmosphere. Some of the fastest winds in the Solar System have been detected, blowing at 800 km (500 miles) per hour.

Another surprise for astronomers came when they were watching Uranus pass in front of a star in 1977 in what is termed an occultation. The star repeatedly blinked on and off, revealing the presence of rings around the planet. The spaceprobe Voyager 2 confirmed their existence in 1986 when it became the only spacecraft to visit the planet. As with Jupiter’s rings, they are totally inconspicuous, unlike the splendid rings around Saturn.