larger image and description
Discovered byWilliam Herschel
Discovered onMarch 13, 1781
Orbital characteristics
Mean radius2,870,972,200 km
Revolution period84y 3d 15.66h
Synodic period369.7 days
Avg. Orbital Speed6.8352 km/s
Number of satellitess22
Physical characteristics
Equatorial diameter51,118 km
Surface area8,130,000,000 km2
Mass8.686×1025 kg
Mean density1.29 g/cm3
Surface gravity8.69 m/s2
Rotation period-17h17h 14m
Axial tilt97.86°
Escape Speed21.29 km/s
Cloudtop avg. temp55 K
Surface temp
59 K68 KN/A K
Atmospheric characteristics
Atmospheric pressureVaries with depth
Carbon monoxide
Hydrogen sulfide

Uranus (pronounced "YOOR-uh-nus", or "yer-ANE-us") is the seventh planet from the Sun. It is a gas giant and the third largest by diameter.

Table of contents
1 Physical characteristics
2 The exploration of Uranus
3 The moons of Uranus

Physical characteristics

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rockyy core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' cyan color is due to the adsorption of red light by atmospheric methane.

One of the most distinctive features of Uranus is its axial tilt, almost ninety degrees. As a result of this tilt, for part of its orbit one pole faces the Sun continually while the other faces away continually. At the other side of Uranus' orbit the orientation of the poles towards the sun is reversed, and at the two sections of its orbit between these two extremes the Sun rises and sets around the equator normally.

At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. Note that the labelling of this pole as "south" is actually in some dispute. Uranus can either be described as having an axial tilt of slightly more than 90°, or it can be described as having an axial tilt of slightly less than 90° and rotating in a retrograde direction; these two descriptions are exactly equivalent as physical descriptions of the planet but result in different definitions of which pole is the North Pole and which is the South Pole.

One result of this odd orientation is that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that perhaps during the formation of the planet it collided with an enormous protoplanet, resulting in the skewed orientation.

It appears that Uranus' extreme axial tilt also results in extreme seasonal variations in its weather. During the Voyager 2 flyby, Uranus' banded cloud patterns were extremely bland and faint. Recent Hubble Space Telescope observations, however, show a more strongly banded appearance now that the Sun is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus' equator.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and 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 magnetotail is twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field's source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent.

The exploration of Uranus

Uranus was the first planet discovered that was not known in ancient times, though it had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Sir William Herschel discovered the planet in 1781, and originally called it Georgium Sidus (George's Star) in honour of King George III of England. French astronomers began calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek god. The name didn't come into common usage until around 1850.

The moons of Uranus

Uranus has about twenty moons. The first two were discovered by William Herschel in 1787, and named, by his son, after characters from Shakespeare's A Midsummer Nights Dream, as Titania and Oberon. Two more moons discovered by William Lassell in 1851 were named Ariel and Umbriel; Gerard Kuiper discovered the moon Miranda in 1948. All moons of Uranus are named after characters from Shakespeare or Alexander Pope.

The flyby of the Voyager 2 space probe in January of 1986 led to the discovery of another 10 moons. Six additional moons have been discovered by telescope since the Voyager flyby. Uranus has a faint planetary ring system, composed of dark particulate matter up to 10m in diameter. The ring system was discovered in 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink.

Uranus' natural satellites

Name Diameter (km) Mass (kg) Mean orbital
radius (km)
Orbital period
Cordelia 26 4.5×1016 49,752 0.335033 days
Ophelia 32 5.4×1016 53,764 0.376409 days
Bianca 44 9.3×1016 59,165 0.434577 days
Cressida 66 3.43×1017 61,767 0.463570 days
Desdemona 58 1.78×1017 62,659 0.473651 days
Juliet 84 5.57×1017 64,358 0.493066 days
Portia 110 1.68×1018 66,097 0.513196 days
Rosalind 54 2.54×1017 69,927 0.558459 days
Belinda 68 3.57×1017 75,255 0.623525 days
S/1986 U10* ~40 Unknown ~75,000  
Puck 154 2.89×1018 86,006 0.761832 days
Miranda 472 6.3×1019 129,850 1.41 days
Ariel 1158 1.27×1021 190,930 2.52 days
Umbriel 1170 1.27×1021 265,980 4.14 days
Titania 1578 3.49×1021 436,270 8.71 days
Oberon 1523 3.03×1021 583,420 13.46 days
Caliban 60 7.3×1017 7,168,900 -579 days**
Stephano 30 6×1015 7,948,000 -676 days**
Trinculo 10 Unknown 8,578,000 759 days
Sycorax 120 5.4×1018 12,213,000 -1289 days**
Prospero 50 2.1×1016 16,568,000 -1953 days**
Setebos 40 2.1×1016 17,681,000 -2345 days**
*Awaiting confirmation and naming

**negative orbital periods indicate a retrograde orbit around Uranus (opposite to the planet's rotation)