Uranus

Uranus is the seventh closest planet to the Sun and the third largest and fourth heaviest of the planets. Its diameter (50,000km) is four times that of Earth with a mass over 14 times that of Earth.

Spins on it's Side

Uranus orbits the Sun once very 84 years (at about 2900 million km) but is unusual in that it spins on its side (with an axial tilt of 97 degrees). This means that its moons and also its faint ring system also orbit in plane perpendicular to the plane of the ecliptic.

It is believed to be comprised of a small rocky core surrounded by a deep mantle of water, ammonia and methane. This is in turn surrounded by an atmosphere of hydrogen, helium and methane with an upper cloud layer.

Colder than Expected

Another oddity in Uranus is the fact that it is very cold. All the other gas giant planets emit more heat radiation than they receive due to very hot cores, but Uranus does not. A temperature of -224 degrees C has been measured in Uranus' atmosphere - the coldest in the solar system.

Uranus' Rings

Uranus has the second most extensive ring system of the solar system after Saturn. The rings, which are very difficult to see from ground based observations, were first discovered in 1977 by measuring the intensity of a star as Uranus and its rings passed in front of it. There are 13 known rings with radii of 38,000km to 98,000km. They are comprised of ice and some darker material which results in them being much darker than the rings of Saturn.

Moons

Uranus has 27 known moons with sizes ranging from over 1500 km diameter down to under 20km. The moons consist of ice, rock and other trace elements. Some of the inner moons undergo gravitation interactions with each other which may in many millions of years lead to instabilities and collisions.

Uranus and Man

Uranus, under clear dark skies, is actually visible to the naked eye. However it is very dim and its 84 year periodicity means that it moves slowly across the sky. However it is interesting that it was not noticed by the ancients and was only observed for the first time by Sir William Herschel in 1781 using a telescope. It was initially named Georgium Sidus (George's Star) by Herschel after King George III. However this unpopular name was eventually discarded and it was renamed Uranus after the Greek god of the sky. Uranus is the only planet to be named after a Greek god, rather than a Roman deity.

To date Uranus has been visited only once - by the Voyager 2 spacecraft. The fly-by occurred in 1986 and resulted in the discovery of 10 new moons and 2 rings. It also measured the chemical composition of the atmosphere and photographed the planet and its moons. This data is still being studied and In 2016 researchers claimed to have discovered evidence for two new moons which may be causing disturbances in its inner most rings.

A "Uranus Orbiter and Probe" mission is in the study stages.

Voyager at Uranus

Voyager Images of Uranus