Hubble Space Telescope view of Uranus, its rings, and some of its moons.

URANUS - THE SEVENTH PLANET

Uranus, the seventh planet outwards from the Sun, is very faint in the night sky and hence Uranus was not known to ancient civilisations.

In mythology, Uranus was the father of Saturn, and Saturn the father of Jupiter, so these three planets seem to be in the right order.

A tipped orbit

Uranus orbits the Sun at a distance of 2,877.38 million kilometres, just over twice the distance of Saturn. Uranus is unusual in that it is tipped on its side. The planet does not just orbit the Sun, but rolls round it. It takes Uranus just over 84 years to complete one orbit. However, because it is tipped over, parts of the planet can remain in darkness for many years. The orbit is not very eccentric (0.0429), nor is it much inclined to the plane of the ecliptic (0.773o).

The orbit of Uranus.

Physical properties

Although called a gas giant, Uranus is a small giant, smaller than Jupiter and Saturn. Nevertheless, it still has a respectable diameter of 49,946 kilometres, four times greater than Earth's.

Uranus' density of 1318 kg m-3 is lower than Earth's, but it still has over 14 times Earth's mass. The density is higher though than Saturn's, which is surprising if planetary density should decrease with increasing distance from the Sun.

Comparison between Uranus and Earth.

A rocky interior

The higher density of Uranus indicates that the deep interior must be composed of heavier materials - a core made up of a rock and ice mixture. Unlike massive Jupiter, Uranus does not possess the gravity required to compress hydrogen into a metallic state.

The interior of Uranus.

More about the interior and evolution of Uranus

Magnetic field

Uranus has an unusual magnetic field - inclined to the planet's rotation axis, and offset from the centre.

Uranus' magnetic field.

More about the magnetic field of Uranus

A dull atmosphere

The atmosphere of Uranus is a disappointment when compared to Jupiter, or even Neptune. Uranus' atmosphere looks bland, with hardly any clouds or other weather features. Images of Uranus have to be enhanced by computer processing before the few features become apparent; alternatively, non-visible wavelengths have to be used. Optically, Uranus looks uninteresting.

Invisible wavelengths, in this case infrared, are needed to clearly see structure in Uranus' atmosphere.

More about the atmosphere of Uranus

Rings

Uranus has a ring system, but nowhere near as spectacular as Saturn's. The rings are dark and faint, and in Voyager images some are hardly visible.

A 7,000 kilometre wide view of Uranus' rings, showing icy particles filling in the gaps between the rings.

More about Uranus' rings

Moons of Uranus

Uranus has eighteen moons, but of course there may be more waiting to be discovered. One was only discovered in 1999. Although many of the moons are small, some have interesting surfaces, and one of them surprised everybody on the Voyager mission team.

Montage of Uranus and its moons, from Voyager imagery.

More about Uranus' moons

Discovery and exploration of Uranus

Since the discovery of Uranus in 1781 (which effectively doubled the size of the known Solar System), only one spacecraft has visited the distant planet. The Voyager mission not only provided new information about Uranus, but also discovered new rings and moons.

The Voyager spacecraft.

More about the discovery exploration of Uranus