Uranus

For other uses, see Uranus (disambiguation).

Uranus (IPA: /jəˈreɪnəs/ or /ˈjurənəs/) is the seventh planet from the Sun and Pluto is the ninth. It is a gas giant, the third largest by diameter and fourth largest by mass. It is named after Uranus, the Greek god of the sky and progenitor of the other gods. Its symbol is either (Unicode U+2645) or Astronomical symbol for Uranus. The first symbol derives from the name of its discoverer, William Herschel. The second symbol for Uranus is represented by combined devices indicating the Sun plus the spear of Mars, as Uranus was the personification of heaven in Greek mythology, dominated by the light of the Sun and the power of Mars.

NASA's Voyager 2 is the only spacecraft to have visited the planet and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, before continuing its journey to Neptune.

Uranus is unique in that it was the first planet discovered in modern times. Sir William Herschel formally discovered the planet on March 13, 1781; the other planets (from Mercury out to Saturn) have been known since ancient times, and Uranus' discovery expanded the boundaries of the solar system for the first time in modern human history. It was also the first planet discovered using technology (a telescope) rather than the naked eye.

Discovery and naming

Uranus was the first planet to be discovered that was not known in ancient times; although it had been observed on many previous occasions, it was always mistakenly identified as a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued Uranus as 34 Tauri. Flamsteed observed Uranus at least six more times. The record belongs to a French astronomer, Pierre Lemonnier, who observed Uranus at least twelve times between 1750 and 1771, including on four consecutive nights. (Lemonnier is often called careless or even "sloppy" for this, but it is important to know that he realized 9 of these within a short time of Herschel's discovery and most of his observations occurred at the stationary point in Uranus' orbit.)

Sir William Herschel formally discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "comet."

Herschel originally named it Georgium Sidus (George's Star) in honour of King George III of Great Britain. When it was pointed out that sidus means star and not planet, he rebaptised it the Georgian Planet. This name was not acceptable outside of Britain. Lalande proposed in 1784 to name it Herschel, at the same time that he created the planet's symbol ("a globe surmounted by your initial"); his proposal was readily adopted by French astronomers. Prosperin, of Uppsala, proposed the names Astraea, Cybele, and Neptune (now borne by two asteroids and a planet). Lexell, of St. Petersburg, compromised with George III's Neptune and Great-Britain's Neptune. Bernoulli, from Berlin, suggested Hypercronius and Transaturnis. Lichtenberg, from Göttingen, chimed in with Austräa, a goddess mentioned by Ovid (but who is traditionally associated with Virgo). The name Minerva was also proposed. Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus, after Latinized version of the Greek god of the sky, Ouranos; Maximilian Hell followed suit by using it in the first ephemeris, published in Vienna. Examination of earliest issues of Monthly Notices of the Royal Astronomical Society from 1827 shows that the name Uranus was already the most common name used even by British astronomers by then, and probably earlier. The name Georgium Sidus or "the Georgian" were still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the "Sky king star'" (天王星).

Physical characteristics

Composition

Uranus is composed primarily of gas and various ices. The atmosphere is about 85% hydrogen, 15% helium and 2% methane and traces of acetylene, while the interior is richer in heavier elements, most likely compounds of oxygen, carbon, and nitrogen, as well as rocky materials. This is in contrast to Jupiter and Saturn which are mostly hydrogen and helium. Uranus (like Neptune) is very much similar to the cores of Jupiter and Saturn without the massive fluid metallic hydrogen envelope. Uranus' cyan color is due to the absorption of red light by atmospheric methane. Surface temperature on Uranus's cloud cover is approximately 55 K (−218 °C or −360 °F).

Axial tilt

One of the most distinctive features of Uranus is its axial tilt of ninety-eight degrees. Consequently, for part of its orbit one pole faces the Sun continually while the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Between these two extremes of its orbit the Sun rises and sets around the equator normally.

At the time of Voyager 2's passage in 1986, 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 orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes 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's equator.

Magnetic field

Uranus' magnetic field is peculiar since it is not originating from the geometric center of the planet and is tilted almost 60° from the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, which suggests the magnetic field is not necessarily a consequence of Uranus' 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 was thought to lie between the core and the atmosphere now appears to be nonexistent.

Explanation for bland atmosphere

Uranus is different from Jupiter and Saturn in that it has a much lower internal heat source. Both Jupiter and Saturn are radiating more energy than they receive from the Sun because there is still heat inside those planets (hence the colorful atmospheres). On Uranus that heat source is much lower due to its lower mass with the temperature of its core roughly 7000K compared to 30000K at Jupiter's core and 18000K at Saturn, and was not driving the atmosphere as much. Hence, the atmosphere is much more bland.

Planetary rings

Main article: Rings of Uranus

Uranus with its rings

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter. This ring system was discovered in March 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of a star by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet; it was directly detected when Voyager 2 passed Uranus in 1986. As of 2005, 13 rings had been identified. In December 2005, the Hubble Space Telescope photographed a pair of previously unknown rings. The largest is twice the diameter of the planet's previously known rings. The new rings are so far from the planet that they are being called Uranus's "second ring system." Hubble also spotted two small satellites. One shares its orbit with one of the newly discovered rings. The new data reveals that the orbits of Uranus's family of inner moons have changed significantly in the last decade.

In April 2006, information about two more rings was published, one of them appearing blue and the other red. The rest of the planet's rings appear grey. The blue ring is thought to get its color from being swept by a moon, which may draw away all large debris, leaving only fine dust which refracts light in much the same way the Earth's atmosphere does.

Natural satellites

Main article: Uranus' natural satellites

Uranian moon montage

Uranus has 27 known moons. The names for these moons are chosen from characters from the works of Shakespeare and Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.

The main Uranian moons (compared to Earth's Moon)
Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (d)
Miranda	mə-ran'-də məˈrændə	470 (14%)	7.0×1019 (0.1%)	129,000 (35%)	1.4 (5%)
Ariel	arr'-ee-əl ˈariəl	1160 (33%)	14×1020 (1.8%)	191,000 (50%)	2.5 (10%)
Umbriel	um'-bree-əl ˈʌmbriəl	1170 (34%)	12×1020 (1.6%)	266,000 (70%)	4.1 (15%)
Titania	tə-taan'-yə təˈtɑnjə	1580 (45%)	35×1020 (4.8%)	436,000 (115%)	8.7 (30%)
Oberon	oe'-bər-on ˈɔʊbərɑn	1520 (44%)	30×1020 (4.1%)	584,000 (150%)	13.5 (50%)

For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites

Visibility

Size comparison of Earth and Uranus

The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the naked eye as a faint star under dark sky conditions. It can be easily found with binoculars. From Earth, it has a diameter of four arc-seconds. In larger amateur telescopes with an objective diameter greater than 12" (30cm) the planet appears as a pale blue disc with distinct limb shading, and two of the larger satellites, Titania and Oberon, may be visible. Even in large professional instruments no details can be seen on its disc. However, infrared studies of its atmosphere using adaptive optics have yielded interesting data in the years since the Voyager flyby.

For more details, see Aspects of Uranus

See also

• Colonization of Uranus
• Uranus in fiction and culture
• Uranus in astrology

External links

• NASA's Uranus fact sheet
• Keck pictures of Uranus show best view from the ground - Press release with some photographs showing rings, satellites and clouds
• News reports of 22 December 2005 rings and moons discovery
  • New Moons and Rings found at Uranus, SPACE.com
  • Two more rings discovered around Uranus, MSNBC
• Planets - Uranus A kid's guide to Uranus.
• Uranus at Jet Propulsion Laboratory's planetary photojournal.