Uranus was discovered in 1781 by astronomer William Herschel, who at first believed it to be a comet. This seventh planet from the Sun is so distant that it takes 84 years to complete an orbit.
A false-color image of Uranus reveals the distant planet's clouds and ring system.
Uranus is classified as a "gas giant" planet because it has no solid surface. The atmosphere of Uranus is hydrogen and helium, with a small amount of methane and traces of water and ammonia. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of "icy" materials (water, methane, and ammonia), with higher-density material at depth.
In 1986, Voyager 2 observed faint cloud markings in the southern latitudes blowing westward between 100 and 600 km/hr. In 1998, the Hubble Space Telescope observed as many as 20 bright clouds at various altitudes in Uranus� atmosphere. The bright clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the atmosphere of Uranus.
Uranus currently moves around the Sun with its rotation axis nearly horizontal with respect to the ecliptic plane. This unusual orientation may be the result of a collision with a planet-sized body early in the planet�s history, which apparently changed Uranus� rotation radically. Uranus� magnetic field is unusual in that the magnetic axis is tilted 60 degrees from the planet�s axis of rotation and is offset from the center of the planet by one-third of the planet�s radius.
Uranus is so far from the Sun that, even though tipped on its side and experiencing seasons that last over twenty years, the temperature differences on the summer and winter sides of the planet do not differ that greatly. Near the cloudtops, the temperature of Uranus is near -215 �C.
A near-infrared view of Uranus and moons Miranda (top) and Ariel.
Uranus� rings were first discovered in 1977. The rings are in the planet�s equatorial plane, perpendicular to its orbit about the Sun. The 10 outer rings are dark, thin, and narrow, while the 11th ring is inside the other ten and is broad and diffuse. The rings of Uranus are very different from those surrounding Jupiter and Saturn. When viewed with the Sun behind the rings, fine dust can be seen scattered throughout all of the rings.
Uranus has at least 20 moons, named mostly for characters from the works of Shakespeare and Alexander Pope. Miranda is the strangest Uranian moon. The high cliffs and winding valleys of the moon may indicate partial melting of the interior, with icy material occasionally drifting to the surface
When two teams of scientists set up to watch Uranus pass in front of star SAO 158687 in 1977, they expected a rare chance to observe a distant planet. Instead, they made a major disovery: Uranus, like Saturn, is encircled with a band of rings.
Voyager 2 found lanes of fine dust particles between the Uranian rings.
As the observers in the Kuiper Airborne Observatory and the Perth Observatory in Australia watched, the star appeared to blink out briefly several times. The blinking was caused by the rings blocking the starlight. The Australian team was so surprised they missed three rings as they tried to figure out why the starlight signal kept disappearing.
The Kuiper team had a better vantage point and were first to publish the surprising news that Uranus was encircled by five narrow rings, which they named Alpha, Beta, Gamma, Delta and Epsilon in order of increasing distance from the planet. The Perth team identified six distinct dips in the starlight, which they named rings 1 through 6. More rings were discovered in 1986.
Observing distant planets is difficult, especially with complications due to daylight arriving before the planet completed its transit in front of the star. But, after careful analysis and a closer view courtesy of the Voyager 2 spacecraft in 1986, scientists have now identified 11 known rings around Uranus. In order of increasing distance from the planet, they are 1986U2R, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda and Epsilon. Some of the larger rings are surrounded by belts of fine dust.
2,870,972,200 km
19.19126393 A.U.
Mean Equatorial Radius
25,559 km
(4.007 of Earth's radius)
Mass
86.849 � 10-27 g
moons:
1. Cordelia2. Ophelia3. Bianca4. Cressida5. Desdemona6. Juliet7. Portia8. Rosalind9. Belinda10. Puck11. Miranda12. Ariel13. Umbriel14. Titania15. Oberon16. Caliban17. Stephano18. Trinculo19. Sycorax20. Prospero21. Setebos