Uranus, distant gas giant

Introduction
Uranus is the seventh planet from the Sun and the third of the four gas giants, revolving outside the orbit of Saturn and inside the orbit of Neptune. Uranus was accidentally discovered in 1781 by the British astronomer Sir William Herschel and was originally named the Georgium Sidus (Star of George), in honour of his royal patron King George III of Great Britain. The planet was later, for a time, called Herschel in honour of its discoverer. The name Uranus, which was first proposed by the German astronomer Johann Elert Bode, was in use by the late 19th century.

Composition
Uranus' rocky core is wrapped in a mantle of gases and ices. Surrounding the mantle is an atmosphere containing methane, which gives the planet its distinctive blue-green colour. Uranus lies in the cold outer reaches of the Solar System, and its cloud tops have a temperature of -210° C (-346°F).

Uranus' Orbit
Uranus has a diameter of 51,120 km (31,771 miles), and its mean distance from the sun is 2.87 billion km (1.78 billion miles). Uranus takes 84 years for a single revolution, or orbit, and 17 hr 15 min for a complete rotation about its axis, which is inclined 98° to the plane of the planet's orbit around the sun. The extreme tilt of Uranus results in its poles each spending 42 Earth years in darkness during a single orbital period. It is so far away from the Sun, however, that the temperature difference between summer and winter at the poles is only 2°C (3.6°F). Through a telescope, the planet appears as a small, bluish-green disk with a faint green periphery.

Mass
Compared to the Earth, Uranus has a mass 14.5 times greater, a volume 67 times greater, and a gravity 1.17 times greater. Its magnetic field, however, is only a tenth as strong as Earth's, with an axis tilted 55° from the rotational axis. The density of Uranus is approximately 1.2 times that of water.

Rings of Uranus
In 1977, while recording the occultation of a star behind the planet, the American astronomer James L. Elliot discovered the presence of five rings encircling the equator of Uranus. Named Alpha, Beta, Gamma, Delta, and Epsilon (starting from the innermost ring), they form a 9400 km (5840 miles) wide belt extending to 51,300 km (31,860 miles) from the planet's centre. Four more rings were discovered in January 1986 during the exploratory flight of Voyager 2.

Moons of Uranus
At least 21 of Uranus' 27 satellites orbit its equator in the direction of its east-west rotation. The two largest moons, Oberon and Titania, were discovered by Herschel in 1787. The next two, Umbriel and Ariel, were found in 1851 by the British astronomer William Lassell. Miranda, thought before 1986 to be the innermost moon, was discovered in 1948 by the American astronomer Gerard Peter Kuiper. The moons Caliban, Sycorax, Prospero, Setebos, Stephano, and 1986U10 were all discovered fairly recently. Cordelia is the closest of Uranus' ten 'inner moons', two new satellites have also been spotted by Hubble in conjunction with a new set or rings, suggesting an unstable planetary system.

Author: Marc Delehanty