Uranus is the seventh planet from the Sun and the third largest of the gas giants (narrowly beating out Neptune) with a diameter about four times that of Earth.

 

mean distance from Sun	2,869 million km (1,783 million mi, 19.2 AU)
equatorial diameter	51,108 km (31,764 mi); 4.007 × Earth
mass (Earth = 1)	14.6
mean density	1.3 g/cm3
axial period	17.9 hours
axial inclination	97.9º
orbital period	84 years
orbital inclination	0.8º
orbital eccentricity	0.047
number of moons	27
atmospheric composition	83% hydrogen, 15% helium, 2% methane
mean temperature (cloud tops)	-197ºC (-323ºF)
gravity at cloud tops (Earth = 1)	1.17
escape velocity	22.5 km/s (81,000 km/h; 50,342 mph)
albedo	0.35

 

Discovery

Uranus was discovered by William Herschel in 1781. Others had seen it earlier – at its brightest it is just visible to the naked eye – but had taken it to be a star because it doesn't move perceptibly from one night to the next. The earliest recorded sighting was in 1690 when John Flamsteed catalogued it as 34 Taurus. William Herschel originally called it Georgium Sidus (George's Star) in honor of King George III, while French astronomers began calling it Herschel. It was Johann Bode who proposed the name Uranus, after the Greek god of the heavens, but this didn't come into common usage until around 1850.

 

Strange spin

Uranus's most extraordinary feature is the tilt of its axis – almost 98° (or 82° if it is taken to be retrograde), so that the planet effectively spins around on its side. As a result, for part of its orbit, one pole continually faces the Sun while the other is in total darkness. Half an orbit later, the roles (and poles) are reversed. In between, the Sun rises and sets around the equator normally. For Uranus to be in such a position, it was almost certainly struck a formidable blow by another massive object. Since the plane containing its 13 rings and 27 known moons is similarly tilted, this impact presumably took place during or shortly after the accretionary phase of the planets.

 

Composition

Uranus probably has a solid, rocky core, surrounded by a deep layer of ammonia and water that has condensed into an icy slush. The outer layer is made up primarily of liquid hydrogen and helium.

 

Atmosphere

The atmosphere of Uranus is composed of hydrogen (83%), helium (15%), methane (2%), and trace amounts of acetylene and other hydrocarbons. Its bluish hue stems from an upper methane haze that absorbs strongly at red wavelengths – leaving a featureless blue planet in our telescopes. During Voyager 2's flyby in 1986, Uranus's banded cloud patterns were extremely bland and faint; more recent Hubble Space Telescope observations, however, have shown a more strongly banded appearance now that the Sun is getting closer to being directly overhead at Uranus' equator, a position it will assume in 2007.

 

A single image of Uranus taken in 2004 shows 18 distinct cloud systems – eight more than Voyager saw during its entire months-long flyby. And one set of images taken with the Keck II telescope in Hawaii in summer 2004 shows an extremely bright cloud reaching up high above the planet's opaque methane layers in its southern hemisphere.

 

Until 10 years ago, the only pictures sharp enough to show any cloud features were those from Voyager. But then images from the Hubble Space Telescope began to exceed Voyager's resolution, and over the past four years images from the 10-meter Keck telescope, using its new adaptive optics system and an infrared camera, have done even better. These observations have shown that some storm systems on Uranus come and go within days, while others can persist for years. The fastest winds ever recorded, seen in late 2003, were up to 420 kilometers per hour.

 

Magnetic field

Uranus's magnetic field is odd in that it isn't centered on the center of the planet and that is tilted almost 60° with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. (Neptune has a similarly displaced magnetic field, suggesting that this is not necessarily a result of Uranus's axial tilt.) The magnetosphere is twisted by the planet's rotation into a long corkscrew shape behind the planet.

 

Moons

Uranus has 27 known moons, placing it third, by this measure, behind Saturn (83) and Jupiter (80). The first two were discovered by William Herschel in 1787, and named, by his son, after characters from Shakespeare's A Midsummer Nights Dream, Titania and Oberon. Two more moons were found by William Lassell in 1851 and named Ariel and Umbriel; G. Kuiper discovered Miranda in 1948. All the moons of Uranus are named after characters from Shakespeare or Alexander Pope. Voyager 2's flyby in January 1986 led to the discovery of another 10. Twelve additional moons have since been discovered by telescope.

 

In an unusual move, in late 2001, the International Astronomical Union stripped the title of "moon" from a small body seen in images taken by Voyager and previously reported to be orbiting Uranus. The IAU concluded that there wasn't yet enough data to confirm that the object, designated S/1986 U10, is a satellite. It appeared to be in nearly the same orbit as another moon, Belinda, about 75,000 kilometers from Uranus, and, based on its brightness in the Voyager images, to have a diameter of about 40 kilometers. S/1986 U 10 has not been seen since the Voyager 2 discovery, despite observing efforts that have discovered several other small moons orbiting the planet.

 

Two of the most recently discovered satellites, Cupid (S/2003 U1) and Mab (S/2003 U2), belong to the inner collection of satellites, each orbiting Uranus in less than a day. Indeed, it's hard to understand how the inner swarm of 13 Uranian moons, which are so close together that they must be gravitationally unstable, can coexist. All the moons from S2001/U3 out move in retrograde orbits.

 

moon	semimajor axis (km)	orbital period (d)	orbital eccen.	orbital inclin (o)	diameter (km)
Cordelia	49,770	0.335	0.0003	0.085	50 × 36
Ophelia	53,790	0.376	0.0099	0.104	54 × 38
Bianca	59,170	0.435	0.0009	0.193	64 × 46
Cressida	61,780	0.464	0.0004	0.006	92 × 74
Desdemona	62,680	0.474	0.0001	0.113	90 × 54
Juliet	64,350	0.493	0.0007	0.065	150 × 74
Portia	66,090	0.513	0.0001	0.060	156 × 126
Rosalind	69,940	0.558	0.0001	0.279	72±12
Cupid	74,800	0.618	0.0013	0.1?	~18
Belinda	75,260	0.624	0.0001	0.031	128 × 64
Perdita	76,400	0.638	0.0012	0.0	30±6
Puck	86,010	0.762	0.0001	0.319	162±4
Mab	97,700	0.923	0.0025	0.134	~25
Miranda	129,300	1.413	0.0013	4.232	471.6±1.4
Ariel	191,020	2.520	0.0012	0.260	1,1578±1.2
Umbriel	266,300	4.144	0.0039	0.205	1,169.4±5.6
Titania	435,910	8.706	0.0011	0.340	1,578.8±1.2
Oberon	583,520	13.463	0.0014	0.058	1,522.8±5.2
Francisco	4,282,900	267.09 R	0.1324	147.460	~22
Caliban	7,231,100	579.7 R	0.1812	139.885	~42
Stephano	8,007,400	677.5 R	0.2248	141.874	~32
Trinculo	8,505,200	749.4 R	0.2194	166.25	~18
Sycorax	12,179,400	1288.4 R	0.5219	152.447	~157
Margaret	14,146,700	1661.0 R	0.6672	51.456	~20
Prospero	16,276,800	1978.4 R	0.4445	146.17	~50
Setebos	17,420,400	2225.1 R	0.5908	145.884	~48
Ferdinand	20,430,100	2790.0 R	0.3993	167.346	~20

 

Rings

Uranus has a faint system of 13 narrow, elliptical-shaped rings, composed of dark dust and rocks up to about 10 meters in diameter. The ring system was discovered in 1977, during an occultation of a star by the planet. As the star first approached and then receded behind the planet it appear to blink off and on – a blinking found to be due to a system of rings.

 

ring	distance from center of Uranus (km)	width of ring (km)	eccentricity	inclination (°)
R/2003 U2	97,700
R/2003 U1	66,000
Epsilon	51,149	20 to 96	0.00794
Lambda	50,024	~2	0.	0.
Delta	48,300	3 to 7	0.00004	0.001
Gamma	47,627	1 to 4	0.00109	0.000
Eta	47,176	1.6
Beta	45,661	5 to 11	0.00044	0.005
Alpha	44,720	4 to 10	0.00076	0.015
Ring 4	42,571	~2	0.001065	0.032
Ring 5	42,234	~2	0.00190	0.054
Ring 6	41,837	1.5	0.00101	0.062
R/1986 U2	38,000