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Uranus

Royal Greenwich Observatory
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Information Leaflet No. 43: 'Uranus'.

Uranus:

Uranus is the seventh planet of the Solar System. It has a diameter of about 52,400 km, a mass 14.6 times that of the Earth, and orbits the Sun every 84 years. Its distance from the Sun varies between 18.3 and 20.1 Astronomical Units. (One Astronomical Unit is the mean distance of the Earth from the Sun).
From the Earth Uranus can be seen with a small telescope. Even with large telescopes it can only be seen as a small greenish disk with no surface details visible.

The Discovery of Uranus:

Uranus was the first planet to be discovered that was not known to the ancients. William Herschel announced its discovery in 1781 although initially he thought that it was a comet. After he had realised that it was a planet, he wanted to call it Georgium Sidus, after his patron King George III. The name Uranus was originally suggested by Bode, but it was not until 1850 when John Couch Adams (see the pamphlet 'Neptune') suggested the change of name that it was universally adopted.

Herschel was not the first astronomer to record Uranus but he was the first to recognise that it was not a star. The earliest records of Uranus are by Flamsteed in 1690 (he called it a star, 34 Tauri), 1712, and four times in 1715.
There are at least 15 other known sightings by three other astronomers before Herschel's discovery.

Results from Voyager 2:

Our knowledge of the outer planets in the Solar System has been revolutionised by the amazing results from the close flybys of each by Voyager 2 which was at its closest to Uranus on January 24, 1986.

The most striking observation was that the planet Uranus shows almost no features! It appears as a large green-blue ball from Earth and from the Voyager pictures. Astronomers already knew that Uranus rotated about an axis that is tilted at 98° to its orbital plane. This means that Uranus is 'lying on its side' and rotating in the opposite sense to that of the other planets. The effect on the surface is that the seasons on Uranus are linked to its orbital period about the Sun, so near the poles winter will last 42 earth-years!

The Voyager data favour a model for the internal structure of the planet with a fairly small, rocky core surrounded by a deep superdense atmosphere of gases and ices of water, ammonia and methane. Above this, there is an atmosphere of hydrogen, helium with clouds of methane and ammonia and water ice. The temperature of the 'surface' is -214°C.

Uranus has a magnetic field from which the rotation rate of the unseen 'body' of the planet can be deduced as just over 17 hours. This is smaller than the rate determined from the few cloud structures seen, indicating that there are strong jet streams present on Uranus rather like those on the Earth.

Satellites:

Before the Voyager 2 fly-by 5 satellites were known. Two, Titania and Oberon, were found by Herschel in 1787. Ariel and Umbriel were discovered in 1851, and Miranda in 1948. Voyager added another 10 objects of which two are 'shepherds' for the rings.

All the satellites have impact craters on them. Ariel and Umbriel are each about 1,200 kilometres in diameter but have very different surfaces. Ariel shows signs of geological activity on a large scale, with evidence for liquid flows. The temperature is far too low for the liquid to have been water, and a mixture of liquid ammonia and water ice has been suggested.

Umbriel is covered in craters and shows almost no sign of geological activity. Its surface is probably similar to what it was shortly after the formation of the Solar System.

Titania and Oberon are each about 1,600 kilometres in diameter and are both heavily cratered. Both show signs of geological activity.

Miranda is only 500 kilometres in diameter and yet shows incredibly complex jumbled geological structures, with faults that appear to be global in size, with surface features kilometres deep. These must have been formed in a violent stage in Miranda's life, either due to tidal heating in the past or a violent collision.

Rings:

The ring system around Uranus was discovered, almost by accident, when the occultation of a bright star by Uranus was being studied. Small dips in the brightness of the star were seen while the star was some distance away from the disc of the planet, and it was deduced that this was due to material in a ring about Uranus obscuring the starlight. The presence of the rings was confirmed by the Voyager spacecraft, which added a tenth ring to the nine discovered from Earth.

The sizes of the particles making up the rings can be estimated from how they scatter sunlight and how they affected the spacecraft's radio signals. There are far less small particles (micron-sized) than in Saturn's rings, and many of the particles must measure several centimetres across. The rings are grey and reflect light poorly, and are thus likely to be made of dust. They must be younger than 100 million years old, and are likely to have been formed from the break-up of a small moon due to collision with a meteoroid or comet.

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Thu Nov 25 10:45:20 GMT 1993.

webman@mail.ast.cam.ac.uk

Updated: July 6 '97, June 25 '14

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For some illustrative images and excellent texts, link to: Uranus in Calvin J. Hamilton's Views of the Solar System

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