Uranus

Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός) the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit.[14] 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. This was also the first discovery of a planet made using a telescope.
Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus's atmosphere, while similar to Jupiter's and Saturn's in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with traces of hydrocarbons.[10] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.[10] In contrast the interior of Uranus is mainly composed of ices and rock.[9]
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators.[15] Seen from Earth, Uranus's rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.[15] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[16]

Discovery

Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,[17] including on four consecutive nights.
Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),[18] but initially reported it (on 26 April 1781) as a "comet".[19] Herschel "engaged in a series of observations on the parallax of the fixed stars",[20] using a telescope of his own design.
He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[21] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[22] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[23]
“ The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. ”
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".[24]
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.[25] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".[26] Bode concluded that its near-circular orbit was more like a planet than a comet.[27]
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."[28] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so that the Royal Family could have a chance to look through his telescopes.[29]


Naming
Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."[30] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.[31] He explained this decision in a letter to Joseph Banks:[28]

Nomenclature


The pronunciation of the name Uranus preferred among astronomers is with stress on the first syllable as in Latin Ūranus;[37] in contrast to the colloquial / with stress on the second syllable and a long a, though both are considered acceptable. Because, in the English-speaking world, ū·rā′·nəs sounds like "your anus", the former pronunciation also saves embarrassment: as Dr. Pamela Gay, an astronomer at Southern Illinois University, noted on her podcast, so as avoid "being made fun of by any small schoolchildren ... when in doubt, don't emphasise anything and just say ūr′·ə·nəs. And then run, quickly.
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is "Uranian". Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.[40] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star


Orbit and rotation
Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that on Earth.[43] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[25] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.
The rotational period of the interior of Uranus is 17 hours, 14 minutes
However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

Internal structure
Uranus's mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after Saturn. Though having a diameter slightly larger than Neptune's (roughly four times Earth's), it is less massive. These values indicate that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses. The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material
The standard model of Uranus's structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus's; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus's radius. Uranus's core density is around 9 g/cm³, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K.The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.
While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow science to determine which model is correct. The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[9] However, for the sake of convenience, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar (100 kPa) is conditionally designated as a "surface". It has equatorial and polar radii of 25 559 ± 4 and 24 973 ± 20 km, respectively. This surface will be used throughout this article as a zero point for altitudes.

Internal heat


Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.[16] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m².[58] The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.
Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature.[59] Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.
Atmosphere
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere.Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.[60] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.[61] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa), the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10–10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.
Composition
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.[10] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[12] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[10][58] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,[62] indicating that helium has not settled in the center of the planet as it has in the gas giants.[10] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[10] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[10] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.The mixing ratio[e] is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[64] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.In addition to methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[66] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).[64][67][68] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets