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For other uses, see Uranus (disambiguation).
Uranus
Image taken by the Voyager 2 spacecraft
Uranus, as seen by Voyager 2
Discovery
Discovered byWilliam Herschel
Discovery dateMarch 13, 1781
Designations
AdjectivesUranian
SymbolAstronomical symbol of Uranus
Orbital characteristics
Epoch J2000
Aphelion3,006,389,405 km
20.096 471 90 AU
1,868,088,249 mi
Perihelion2,735,555,035 km
18.286 055 96 AU
1,699,799,169 mi
2,870,972,220 km
19.191 263 93 AU
1,783,943,710 mi
Eccentricity0.046 167 71
369.65 day
6.795 km/s
Inclination0.769 86°
(6.48° to Sun's equator)
74.229 88°
Known satellites27
Physical characteristics
Equatorial radius
25,559 km
(4.007 Earths)
Polar radius
24,973 km
(3.929 Earths)
8.084×109 km2
(15.849 Earths)
Volume6.834×1013 km3
(63.086 Earths)
Mass8.6832×1025 kg
(14.536 Earths)
Mean density
1.270 g/cm3[1]
8.69 m/s2
(0.886 g)
21.29 km/s
13.23 mi/s
−0.718 33 day (17 h 14 min 24 s by convention)[2]
Equatorial rotation velocity
2.59 km/s = 9320 km/h
97.77°
North pole right ascension
17 h 9 min 15 s
(257.311°)
North pole declination
-15.175°
Albedo0.51
Surface temp. min mean max
Surface 59 K 68 K N/A
Cloudtop 55 K
Atmosphere
Surface pressure
120 kPa (at the cloud level)
Composition by volume83% Hydrogen
15% Helium
1.99% Methane
0.01% Ammonia
0.00025% Ethane
0.00001% Acetylene
trace Carbon monoxide
trace Hydrogen sulphide

Uranus (/jʊˈreɪ.nəs, ˈjʊr.ə.nəs/) is the seventh planet from the Sun, and the first discovered in modern times. Although, like the five classical planets, Uranus is visible to the naked eye, its dimness and slow motion across the sky meant that it was never recognised as a planet by ancient observers. The planet remained unknown Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. Uranus was also the first planet discovered using technology (a telescope) rather than the naked eye.

Uranus is named after the ancient Greek deity of the sky (Uranus, [[wiktionary:οὐρανός|Οὐρανός]]), Its astronomical symbol is Astronomical symbol for Uranus. The symbol is a combination of the symbols for the Sun and Mars, as Uranus was the personification of heaven in Greek mythology, dominated by the light of the Sun and the power of Mars. It is also the alchemical symbol of platinum. The element uranium was named in its honour.

The adjective of Uranus is "Uranian". Although it is a gas giant, like Jupiter and Saturn, it is far less massive than either and has a markedly different internal composition, with much less hydrogen and helium and a far greater abundance of "ices", such as water, ammonia, and methane. Neptune is of a similar mass and composition to Uranus, and astronomers have begun to employ a new category to describe them collectively: "Uranian planets" or "ice giants" as distinct from the Jovian planets Jupiter and Saturn.

Like the other giant planets, Uranus possesses a ring system, a magnetosphere, and numerous moons. Uranus is unique though in that it, its moons, and its rings orbit the sun virtually sideways; their north and south poles lie where the other planets have their equators. Seen from Earth, Uranus's rings appear to circle the planet like an archery target, while its moons revolve around it like the hands of a clock. When Voyager 2 first imaged Uranus in 1986, it found it virtually featureless, with none of the cloud bands or storms associated with the other giants. Now, as Uranus approaches its equinox, ground-based observations are beginning to see signs of weather activity on its surface.

Discovery and naming

Although Uranus had been observed on many previous occasions, it was often mistaken for 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 1769,[3] including on four consecutive nights.

Sir William Herschel discovered the planet on March 13, 1781, but reported it on April 26, 1781 as a "comet".[4]

On the 13th of March, 1781, between ten and eleven o'clock at night, while Herschel was examining the small stars near H Geminorum with a seven-foot telescope, bearing a magnifying power of two hundred and twenty-seven times, one of these stars seemed to have an unusual diameter; and it was, therefore, thought to be a comet. It was under this denomination that it was discussed at the Royal Society of London. But the researches of Herschel and of Laplace showed later that the orbit of the new body was nearly circular, and Uranus was consequently elevated to the rank of a planet.[5]

Herschel originally named it Georgium Sidus (George's Star) in honour of King George III of Great Britain (c.f. American poet Elizabeth Graeme Fergusson's "Upon the Discovery of the Planet..." about the event). When it was pointed out that sidus means star and not planet, Herschel rebaptised it the Georgian Planet. This name was not accepted outside of Britain. Lalande proposed in 1784 to name it Herschel, at the same time that he created the planet's astrological symbol (, "a globe surmounted by your [i.e. Herschel's] 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 another 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.[6]

William Herschel, discoverer of Uranus

Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus,[7] a name he had proposed as early as 1781,[8] after the Latinized version of the Greek god of the sky, Ouranos; Maximilian Hell followed suit by using it in the first ephemeris, published in Vienna and computed by the Benedictine priest Placidus Fixlmillner. The earliest publication to include Uranus in its title was in 1823.[9][10] The name was in use in Germany at least as far back as 1791, however.[11] 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" was still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.[7]

Jupiter (or Zeus) was the father of Mars (or Ares), and Saturn (or Cronos) was the father of Jupiter, thus, it was deemed most logical to continue the tradition and name the planet beyond Saturn after Uranus, since he, according to mythology, was the father of Saturn.[12]

The stressed syllable in the name is properly the first, antepenultimate syllable, since in Latin the penultimate vowel a is short (ūrănŭs) and in an open syllable, and such syllables are never stressed in Latin.[13] The historically correct pronunciation of the name by English-speakers is therefore [ˈjʊ.rə.nəs]. The historically incorrect pronunciation [jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūrānŭs) has become very common.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).[14][15]

Orbit and rotation

Image showing Uranus, its bands, rings and moons clearly outlining its sideways pose

Uranus's orbit is roughly 84 Earth years long. Its average distance from the Sun is roughly 3 billion km; at that distance, sunlight is barely 1/400 that of Earth.[16] Its orbit was first calculated in 1792, however, almost immediately discrepancies began to appear between the orbit predicted and the actual orbit as observed. In 1841, John Couch Adams, then an undergraduate at Cambridge, first proposed that the discrepancy might be due to the gravitational tug of an unseen planet beyond it. In 1845, Urbain Le Verrier, working at the Paris Observatory, began his own independent research into Uranus's uncertain orbit. On September 23, 1846, German astronomer Johann Gottfried Galle, working from information supplied by Le Verrier, located a new planet, later named Neptune, at nearly the position predicted. Le Verrier gained credit for its discovery.[17]

Uranus's rotational period, its "day", lasts roughly 17 hours, 14 minutes. Like all gas giant planets, its core spins more slowly than its upper atmosphere. The fastest region of Uranus's atmosphere, around its south pole, makes one rotation in only 14 hours. Windspeeds in that region reach upwards of 720 kilometres per hour.[18]

Axial tilt

One of the most unusual features of Uranus is its axial tilt of ninety-eight degrees. Effectively, Uranus is lying on its side. Consequently, for part of its orbit one pole faces the Sun continually while the other pole faces away. At the other side of Uranus's orbit the orientation of the poles towards the Sun is reversed. This gives each pole 42 years of continuous sunlight, followed by 42 years of darkness.[19] Between these two extremes of its orbit, particularly at the equinoxes, the Sun rises and sets around the equator normally. Uranus will reach its next equinox around December 2007, and not again until 2049.[20]

Season, Northern Hemisphere Year Season, Southern Hemisphere
Winter Solstice 1902, 1986 Summer Solstice
Vernal Equinox 1923, 2007 Autumnal Equinox
Summer Solstice 1944, 2028 Winter Solstice
Autumnal Equinox 1965, 2049 Vernal Equinox

At the time of Voyager 2's passage in 1986, Uranus' south pole was pointed almost directly at the Sun. The labelling of this pole as "south" uses the coordinate definitions currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite shall be the pole which points above the invariable plane of the solar system (regardless of the direction the planet is spinning).[21][22] A different system is sometimes used, defining a body's north and south poles according to the right-hand rule in relation to the direction of rotation.[23] In terms of this latter coordinate system it was Uranus' north pole which was in sunlight in 1986. Astronomer Patrick Moore, commenting on the issue, sums it up by saying "Take your pick!"[24]

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 during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing the skewed orientation.[25]

Physical characteristics

Much of what we know of Uranus's physical properties derives from a single encounter with the spacecraft Voyager 2 on January 24 1986. No further missions to the planet are planned, and all subsequent discoveries have been made through ground-based observations.

Visibility

From 1995 to 2006, Uranus's apparent magnitude fluctuated between +5.6 and +5.9, placing it just above the limit of naked eye visibility at +6.0.[26]. Its angular diameter is between 3.4 and 3.7 arcseconds, compared with between roughly 16-20 arcseconds for Saturn and 32 and 45 arcseconds for Jupiter.[26] At opposition, Uranus is visible to the naked eye in dark, un-light polluted skies, and becomes an easy target even in urban skies with binoculars.[1] In larger amateur telescopes with an objective diameter of between 15 and 22.5 cm (6 and 9 inches), the planet appears as a pale green disk with distinct limb darkening. With a large telescope of 25 cm (10 inches) or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.[27]

Composition

Size comparison of Earth and Uranus

Uranus's mass is roughly 14 times that of the Earth, making it the least massive of the giant planets. It is also, at 1.270g/cm³, the second least dense planet after Saturn. Though of a similar diameter to Neptune (roughly four times Earth's), it is less massive, and therefore less dense.

Physically and chemically, Uranus bears a far closer resemblance to Neptune than it does to the giants Jupiter and Saturn; like Neptune, it is roughly a tenth the mass of Jupiter and has far less elemental hydrogen and helium.[28] Astronomers have therefore begun to refer to both collectively as belonging to a separate category: "ice giants", because they are primarily made of "ices" like water, ammonia, hydrogen sulphide and methane.[29]

Many argue that the differences between the ice giants and the gas giants extend to their formation. The Solar System is believed to have formed from a giant rotating ball of gas and dust known as the presolar nebula. As it condensed, it formed into a disc with a slowly collapsing Sun in the middle. Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, while the dust grains collected together to form the first protoplanets. As the planets grew, some of them eventually accreted enough matter for their gravity to hold onto the nebula's leftover gas. The more gas they held onto, the larger they became; the larger they became, the more gas they held onto until a critical point was reached, and their size began to increase exponentially. The ice giants, with only a few Earth masses of nebular gas, never reached that critical point.[28][30]

Measuring by numbers of atoms, rather than by mass,[31] Uranus's atmosphere consists of about 83% hydrogen, 15% helium, 2% methane, traces of heavier hydrocarbons such as acetylene, ethane, methylacetylene, diacetylene,[32] and traces of carbon monoxide and dioxide.[32][33] The atmosphere of Uranus appears to be enriched in deuterium as compared with the solar composition (D/H ratio about 5.5×10−5).[34] Uranus' cyan colour is mainly due to the absorption of red light by atmospheric methane.[35] The minimal temperature in Uranus' atmosphere is approximately 55 K (−218 °C or −360 °F).[36] It is reached in the tropopause approximately 30 km above the cloud cover at the pressure about 0.1 bar. The interior is richer in heavier elements, most likely compounds of oxygen, carbon, and nitrogen,[37] as well as rocky materials.[38] This is in contrast to Jupiter and Saturn which are mostly hydrogen and helium. Uranus (like Neptune) has a core similar to those of Jupiter and Saturn but without the massive fluid metallic hydrogen envelope.[39] Instead, it is thought that much of Uranus' interior between the core and the atmosphere may consist of superheated liquid water under very high pressure.[40]

Current theories of solar system formation cannot account for the presence of Uranus and Neptune so far out from Jupiter and Saturn. They are too large to have formed from the amount of material expected at that distance. Rather, some scientists expect that both formed closer to the Sun but were scattered outward by Jupiter.[41]

Atmosphere

File:Uranuscolour.gif
Uranus in approximate natural colour (left) and in higher wavelengths (right), showing its faint cloud bands and atmospheric "hood"

Uranus's atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles. When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[42] One proposed explanation for this dearth of cloud features is that Uranus's internal heat is lower than that of Jupiter and Saturn. Both Jupiter and Saturn radiate more energy than they receive from the Sun. This causes many powerful convection currents to form in the atmosphere. On Uranus that heat source is much lower due to its lower mass, with the temperature of its core roughly 7000 K[43] compared to 24 000 K at Jupiter's core[44] and 12000 K at Saturn.[45] The convection currents formed in the Uranian atmosphere are not as strong and hence it lacks the atmosphere banding of the larger gas giants.

Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.6 times as much energy into space as it receives from the Sun. Uranus, by contrast, radiates hardly any excess heat at all.[46] Hypotheses for this discrepancy include that when it was "knocked over" early in its history by the supermassive impactor which caused Uranus's extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface.[46]

The sunlit pole is circled by a high-altitude haze-layer, and to radiate extensively in the ultraviolet, a phenomenon astronomers termed "dayglow". The average temperature is about 60 Kelvins (−350 degrees Fahrenheit). Despite their orientation towards and away from the Sun, both poles, light and dark, have nearly equal temperatures at the cloud tops.[47]

Uranus has an ionosphere which is located at the heights from 1000 km and extends up to 10,000 km. It has been observed from the ground by infrared emission of H+3 ions.[48] The temperature of the ionosphere is 550-750 K and varies from year to year.[49][50] The ionosphere is mainly sustained by the solar short wavelength radiation. The charged particles flux from the magnetosphere is not significant in contrast to Jupiter and Saturn.[51]

Seasonal variation

File:Uranusfireworks.jpg
'Fireworks' on Uranus observed by the Keck telescope in 2004

For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[52] Observations included record-breaking wind speeds and a persistent thunderstorm referred to as "Fourth of July fireworks".[42] On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity.[53]

Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' extreme axial tilt results in extreme seasonal variations in its weather.[20] When Voyager 2 visited Uranus, it was during the height of the planet's southern summer. In summer/winter near the solstices, Uranus's hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. This arrangement leads to the formation of a high atmosphere haze layer, or "hood", that inhibits convection. Now that the Spring/Autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again. This is, however, still largely speculation.[42]

Planetary rings

Main article: Rings of Uranus
Uranus with its rings in false colour

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter. It was the first ring system to be discovered in the Solar System after Saturn's. The first mention of Uranus's ring system comes from William Herschel's notes detailing his observations of Uranus. These include the following passage: "February 22, 1789: A ring was suspected".[54] Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case. Herschel's notes were published in a Royal Society journal in 1797. Between 1797 and 1977 the rings are rarely mentioned, if at all; this has led some to conclude that Herschel erred in claiming to have found the ring. However, Dr Stuart Eves, of Surrey Satellite Technology Limited, claims that Herschel actually gave accurate descriptions of the ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour. He theorises that the rings were missed for so long because over the course of the intervening two centuries they have darkened and expanded outward.[55] The ring system was rediscovered on March 10, 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 the star SAO 158687 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;[56] it was directly imaged when Voyager 2 passed Uranus in 1986.

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet than the previously known rings. The new rings are so far from the planet that they are being called Uranus' "outer" ring system. Hubble also spotted two small satellites. One, Mab, shares its orbit with the outermost newly discovered ring. These two rings bring the total number of Uranus rings to 13.[57]

In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: one was blue and the other red.[58] One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice taken from the surface of Mab that are small enough to scatter visible blue light.[59] The planet's inner rings appear grey.[35]

Uranus’s rings are probably quite young; gaps in their circumference as well as differences in their opacity suggest that they did not form with Uranus. The rings may once have been a moon which was shattered by a high-speed impact or tidal forces.[47]

Magnetic field

Prior to the arrival of Voyager 2, no measurements of Uranus's magnetosphere had been taken, so its nature remained a mystery. Voyager's observations revealed that Uranus's magnetic field is peculiar, both because it does not originate from the planet's geometric centre, and because it is tilted at 59° from the axis of rotation. Neptune has a similarly displaced magnetic field, suggesting that it is not due to Uranus's extreme axial tilt, which Neptune does not share. One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated deep within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths.[60]

Astronomers had expected Uranus's magnetic field to be in line with the solar wind, since it would line up with Uranus's horizontal poles. Due to its lopsided nature, the field in fact lies in a position similar to those of the terrestrial planets.[61] Like those of other planets, Uranus's magnetosphere possesses a magnetotail that trails into space for millions of miles behind it; however, Uranus's is twisted by the planet's sideways rotation into a long corkscrew.[62] The magnetic fields of Earth and Jupiter are dipoles; they are roughly as strong at either pole, and their "magnetic equators" are roughly parallel with their physical equators. Uranus and Neptune, by contrast, have magnetic equators entirely within their southern hemispheres, and their southern magnetic poles are far stronger than their northern poles.[63]

Uranus's radiation belts (similar in strength to those of Saturn) are strong enough to darken any methane trapped in the icy surfaces of the inner moons and ring particles. This may have been responsible for the uniformly dark coloration of the moons and rings.[47]

Moons

Main article: Uranus' natural satellites
See also: Timeline of discovery of Solar System planets and their natural satellites
Major moons of Uranus compared, at their proper relative sizes.

Uranus has 27 known natural satellites. The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.[64] The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.[65]

The Uranian satellite system is the least massive among the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton alone. The largest of the satellites, Titania, has a radius of only 788.9 km, or less than half that of the Moon. The moons are low-albedo, ice-rock conglomerates composed roughly of 50% ice, twenty percent carbon/nitrogen compounds and thirty percent rock.[47] Ariel appears to have the youngest surface, with the fewest impact craters, while Umbriel's appears oldest.[47]

Miranda possesses fault canyons 20 kilometres (12 miles) deep, terraced layers, and a chaotic variation in surface ages and features. One hypothesis holds that Miranda may have been blasted completely apart by a massive impact some time in its past, and then reformed haphazardly.[47]

Exploration

Main article: Exploration of Uranus
A picture of Uranus taken by Voyager 2 as it headed to Neptune.

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, coming within 81,500 kilometres (50,600 miles) of the planet's cloud tops, before continuing its journey to Neptune. Voyager 2 discovered 10 new moons and studied the planet's unique atmosphere, caused by its axial tilt of 97.77°; and examined its ring system.[66]

Although it had been suspected, the presence of a magnetic field at Uranus was not known until Voyager's arrival.[47] Voyager 2 studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus's sideways orientation. It made the first detailed investigations of its five largest moons, and studied all nine of the system's known rings, discovering two new ones.[47]

See also

References

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