140-498: Uranus is the seventh planet from the Sun. Uranus may also refer to: Uranus Uranus is the seventh planet from the Sun . It is a gaseous cyan -coloured ice giant . Most of the planet is made of water , ammonia , and methane in a supercritical phase of matter , which astronomy calls "ice" or volatiles . The planet's atmosphere has a complex layered cloud structure and has
280-460: A G-type main-sequence star . The largest objects that orbit the Sun are the eight planets . In order from the Sun, they are four terrestrial planets ( Mercury , Venus , Earth and Mars ); two gas giants ( Jupiter and Saturn ); and two ice giants ( Uranus and Neptune ). All terrestrial planets have solid surfaces. Inversely, all giant planets do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of
420-517: A planetary nebula , returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium . Astronomers sometimes divide the Solar System structure into separate regions. The inner Solar System includes Mercury, Venus, Earth, Mars, and the bodies in the asteroid belt . The outer Solar System includes Jupiter, Saturn, Uranus, Neptune, and
560-404: A bright spot at the north pole, indicating the presence of a polar vortex . In the 1990s, the number of the observed bright cloud features grew considerably, partly because new high-resolution imaging techniques became available. Most were found in the northern hemisphere as it started to become visible. An early explanation—that bright clouds are easier to identify in its dark part, whereas in
700-597: A comet." On 17 March he noted: "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place." When he presented his discovery to the Royal Society , he continued to assert that he had found a comet, but also implicitly compared it to a planet: 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
840-408: A diameter of about 250 km (160 mi) and is one of the few minor planets possessing a ring system. Beyond the orbit of Neptune lies the area of the " trans-Neptunian region ", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than
980-459: A few Earth masses of nebular gas, never reached that critical point. Recent simulations of planetary migration have suggested that both ice giants formed closer to the Sun than their present positions, and moved outwards after formation (the Nice model ). Uranus orbits the Sun once every 84 years. As viewed against the background of stars, since being discovered in 1781, the planet has returned to
1120-415: A few meters to hundreds of kilometers in size. Many asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them , that is, asteroids that orbit larger asteroids. The asteroid belt occupies a torus-shaped region between 2.3 and 3.3 AU from the Sun, which lies between the orbits of Mars and Jupiter. It
1260-423: A much greater distance from Uranus are the ten known irregular moons . The planet's magnetosphere is highly asymmetric and has many charged particles , which may be the cause of the darkening of its rings and moons. Uranus is visible to the naked eye, but it is very dim and was not classified as a planet until 1781, when it was first observed by William Herschel . About seven decades after its discovery, consensus
1400-516: A planet, expanding the known boundaries of the Solar System for the first time in history and making Uranus the first planet classified as such with the aid of a telescope . The discovery of Uranus also effectively doubled the size of the known Solar System because Uranus is around twice the distance from the Sun as the planet Saturn . Before its recognition as a planet, Uranus had been observed on numerous occasions, albeit generally misidentified as
1540-459: A polar cap in the northern hemisphere. So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar. In 2007, when Uranus passed its equinox, the southern collar almost disappeared, and a faint northern collar emerged near 45° of latitude. In 2023, a team employing the Very Large Array observed a dark collar at 80° latitude, and
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#17328697912231680-493: 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." Although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell , working in Russia, was the first to compute the orbit of the new object. Its nearly circular orbit led him to
1820-565: A relationship between these orbital distances, like the Titius–Bode law and Johannes Kepler's model based on the Platonic solids , but ongoing discoveries have invalidated these hypotheses. Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called orreries )—whereas others extend across cities or regional areas. The largest such scale model,
1960-477: 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 km (15,881.6 ± 2.5 mi) and 24,973 ± 20 km (15,518 ± 12 mi), respectively. This surface is used throughout this article as a zero point for altitudes. Uranus's internal heat appears markedly lower than that of
2100-532: A shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant . Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (4,220 °F) at its coolest) than it is on the main sequence. The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable (possibly destroying Earth as well). Eventually,
2240-413: A small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed
2380-537: A star. The earliest possible known observation was by Hipparchus , who in 128 BC might have recorded it as a star for his star catalogue that was later incorporated into Ptolemy 's Almagest . The earliest definite sighting was in 1690, when John Flamsteed observed it at least six times, cataloguing it as 34 Tauri . The French astronomer Pierre Charles Le Monnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights. William Herschel observed Uranus on 13 March 1781 from
2520-427: A total of 10 cloud features across the entire planet. One proposed explanation for this dearth of features is that Uranus's internal heat is markedly lower than that of the other giant planets, being the coldest planet in the Solar System. In 1986, Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands. Their boundary
2660-639: Is Dao Yurenat ( ดาวยูเรนัส ), as in English. Its other name in Thai is Dao Maruettayu ( ดาวมฤตยู , Star of Mṛtyu), after the Sanskrit word for 'death', Mrtyu ( मृत्यु ). In Mongolian , its name is Tengeriin Van ( Тэнгэрийн ван ), translated as 'King of the Sky', reflecting its namesake god's role as the ruler of the heavens. In Hawaiian , its name is Heleʻekala ,
2800-417: Is 30 AU from the Sun. With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine
2940-504: Is 5.68 with a standard deviation of 0.17, while the extremes are 5.38 and 6.03. This range of brightness is near the limit of naked eye visibility. Much of the variability is dependent upon the planetary latitudes being illuminated from the Sun and viewed from the Earth. Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter. At opposition , Uranus
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#17328697912233080-457: Is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds, and seasonal changes. The middle layer of the Uranian atmosphere is the stratosphere , where temperature generally increases with altitude from 53 K (−220 °C; −364 °F) in the tropopause to between 800 and 850 K (527 and 577 °C; 980 and 1,070 °F) at the base of the thermosphere. The heating of
3220-409: Is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but
3360-412: Is a limiting factor. In a 2021 study, the ice giants' interior conditions were mimicked by compressing water that contained minerals such as olivine and ferropericlase , thus showing that large amounts of magnesium could be dissolved in the liquid interiors of Uranus and Neptune. If Uranus has more of this magnesium than Neptune, it could form a thermal insulation layer, thus potentially explaining
3500-428: Is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today. The Sun's main-sequence phase, from beginning to end, will last about 10 billion years for
3640-411: Is a unique feature of Uranus. Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings. The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus. Observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km (1,200 to 6,200 mi). The Uranian ionosphere
3780-404: Is close to the protosolar helium mass fraction of 0.275 ± 0.01 , indicating that helium has not settled in its centre as it has in the gas giants. The third-most-abundant component of Uranus's atmosphere is methane ( CH 4 ). Methane has prominent absorption bands in the visible and near-infrared (IR), making Uranus aquamarine or cyan in colour. Methane molecules account for 2.3% of
3920-399: Is considered north and which is considered south and giving the planet prograde rotation. This gives it seasonal changes completely unlike those of the other planets. Pluto and asteroid 2 Pallas also have extreme axial tilts. Near the solstice , one pole faces the Sun continuously and the other faces away, with only a narrow strip around the equator experiencing a rapid day–night cycle, with
4060-537: Is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere. The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity . Auroral activity is insignificant as compared to Jupiter and Saturn. At ultraviolet and visible wavelengths, Uranus's atmosphere is bland in comparison to the other giant planets, even to Neptune, which it otherwise closely resembles. When Voyager 2 flew by Uranus in 1986, it observed
4200-444: Is located at about −45° of latitude . A narrow band straddling the latitudinal range from −45 to −50° is the brightest large feature on its visible surface. It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar. Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to
4340-515: Is nearly universal in astrology. In English-language popular culture , humour is often derived from the common pronunciation of Uranus's name, which resembles that of the phrase "your anus ". Uranus is called by a variety of names in other languages. Uranus's name is literally translated as the "sky king star" in Chinese ( 天王星 ; Tiānwángxīng ), Japanese (天王星), Korean (천왕성), and Vietnamese ( sao Thiên Vương ). In Thai , its official name
Uranus (disambiguation) - Misplaced Pages Continue
4480-412: Is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System
4620-404: Is not precisely known, because different figures emerge depending on the model chosen; 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 non-ice mass (0.5 to 3.7 Earth masses) is accounted for by rocky material . The standard model of Uranus's structure
4760-463: Is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27 g/cm makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior
4900-429: Is similar at these altitudes. Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower. The abundance ratio of water is around 7 × 10 . Ethane and acetylene tend to condense in the colder lower part of the stratosphere and tropopause (below 10 mBar level) forming haze layers, which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in
5040-411: Is sometimes called a water–ammonia ocean. The extreme pressure and temperature deep within Uranus may break up the methane molecules, with the carbon atoms condensing into crystals of diamond that rain down through the mantle like hailstones. This phenomenon is similar to diamond rains that are theorised by scientists to exist on Jupiter , Saturn , and Neptune . Very-high-pressure experiments at
5180-464: Is strong consensus among astronomers that five members of the Kuiper belt are dwarf planets . Many dwarf planet candidates are being considered, pending further data for verification. The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by
5320-464: Is that it consists of three layers: a rocky ( silicate / iron–nickel ) core in the centre, 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% of the planet; the mantle comprises its bulk, with around 13.4 Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for
5460-400: Is that, averaged over the Uranian year, the near-polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism that causes this is unknown. The reason for Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of
5600-430: Is the gravitationally bound system of the Sun and the objects that orbit it. It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc . The Sun is a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core , releasing this energy from its outer photosphere . Astronomers classify it as
5740-430: Is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses ), which comprises 99.86% of all the mass in the Solar System, produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium. This releases an enormous amount of energy , mostly radiated into space as electromagnetic radiation peaking in visible light . Because
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5880-468: Is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident. Below are
6020-436: Is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 20 to 40 bar (2 to 4 MPa), ammonia or hydrogen sulfide clouds at between 3 and 10 bar (0.3 and 1 MPa) and finally directly detected thin methane clouds at 1 to 2 bar (0.1 to 0.2 MPa). The troposphere
6160-435: Is visible to the naked eye in dark skies, and becomes an easy target even in urban conditions with binoculars. On larger amateur telescopes with an objective diameter of between 15 and 23 cm, Uranus appears as a pale cyan disk with distinct limb darkening . With a large telescope of 25 cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon , may be visible. Uranus's mass
6300-533: The International Astronomical Union definition that the north pole is the pole which lies on Earth's North's side of the invariable plane of the Solar System . Uranus has retrograde rotation when defined this way. Alternatively, the convention in which a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation, Uranus's axial tilt may be given instead as 97.8°, which reverses which pole
6440-492: The Lawrence Livermore National Laboratory suggest that an ocean of metallic liquid carbon, perhaps with floating solid 'diamond-bergs', may comprise the base of the mantle. The bulk compositions of Uranus and Neptune are different from those of Jupiter and Saturn , with ice dominating over gases, hence justifying their separate classification as ice giants . There may be a layer of ionic water where
6580-489: The Milky Way galaxy. The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud . This initial cloud was likely several light-years across and probably birthed several stars. As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As
6720-608: The Sweden Solar System , uses the 110-meter (361-foot) Avicii Arena in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere at Stockholm Arlanda Airport , 40 km (25 mi) away, whereas the farthest current object, Sedna , is a 10 cm (4 in) sphere in Luleå , 912 km (567 mi) away. At that scale, the distance to Proxima Centauri would be roughly 8 times further than
6860-488: The Voyager ;2 flyby. Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune. For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed Uranus Dark Spot was imaged. The speculation is that Uranus is becoming more Neptune-like during its equinoctial season. Solar System The Solar System
7000-449: The asteroid belt (between Mars's and Jupiter's orbit) and the Kuiper belt (just outside Neptune's orbit). Six planets, seven dwarf planets, and other bodies have orbiting natural satellites , which are commonly called 'moons'. The Solar System is constantly flooded by the Sun's charged particles , the solar wind , forming the heliosphere . Around 75–90 astronomical units from the Sun,
7140-497: The auroral activity can provide the necessary energy to maintain these temperatures. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure levels may contribute too. In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass and high temperatures explain why the corona extends as far as 50,000 km (31,000 mi), or two Uranian radii, from its surface. This extended corona
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#17328697912237280-446: The frost line , and it lies at roughly five times the Earth's distance from the Sun. The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic . Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane. Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of
7420-419: The fusor stars in the Milky Way . The Sun is a population I star , having formed in the spiral arms of the Milky Way galaxy. It has a higher abundance of elements heavier than hydrogen and helium (" metals " in astronomical parlance) than the older population II stars in the galactic bulge and halo . Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so
7560-528: The grand tack hypothesis suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the Late Heavy Bombardment of the inner planets. The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun. Although the Solar System has been fairly stable for billions of years, it is technically chaotic , and may eventually be disrupted . There
7700-427: The heliosphere , which spans much of the Solar System. Along with light , the Sun radiates a continuous stream of charged particles (a plasma ) called the solar wind . This stream spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph), filling the vacuum between the bodies of the Solar System. The result is a thin , dusty atmosphere, called
7840-554: The interplanetary medium , which extends to at least 100 AU . Activity on the Sun's surface, such as solar flares and coronal mass ejections , disturbs the heliosphere, creating space weather and causing geomagnetic storms . Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near
7980-421: The magnetic poles . The largest stable structure within the heliosphere is the heliospheric current sheet , a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium. The inner Solar System is the region comprising the terrestrial planets and the asteroids . Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to
8120-566: The pre-solar nebula collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings. As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in
8260-451: The "Georgian Planet" in honour of his new patron, King George III. He explained this decision in a letter to Joseph Banks: In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to
8400-545: The Earth-sized impactor theorised to be behind Uranus's axial tilt left the planet with a depleted core temperature, as the impact caused Uranus to expel most of its primordial heat. Another hypothesis is that some form of barrier exists in Uranus's upper layers that 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 upward heat transport ; perhaps double diffusive convection
8540-450: The Hawaiian rendering of the name 'Herschel'. In Māori , its name is Whērangi . It is argued that the differences between the ice giants and the gas giants arise from their formation history. The Solar System is hypothesised to have formed from a rotating disk of gas and dust known as the presolar nebula . Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, and
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#17328697912238680-470: The Kuiper belt. The entire region is still largely unexplored . It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System. The Kuiper belt
8820-640: The Latinised form of the deity's name, and the Roman equivalent was Caelus. In 1789, Bode's Royal Academy colleague Martin Klaproth named his newly discovered element uranium in support of Bode's choice. Ultimately, Bode's suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office , the final holdout, switched from using Georgium Sidus to Uranus . Uranus has two astronomical symbols . The first to be proposed, [REDACTED] ,
8960-557: The Moon is from Earth. If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale. Besides solar energy,
9100-462: The Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components. Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets. The radius of
9240-545: The Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane, than planets of the inner Solar System because their lower temperatures allow these compounds to remain solid, without significant sublimation . The four outer planets, called giant planets or Jovian planets, collectively make up 99% of
9380-457: The Solar System stands out in lacking planets interior to the orbit of Mercury. The known Solar System lacks super-Earths , planets between one and ten times as massive as the Earth, although the hypothetical Planet Nine , if it does exist, could be a super-Earth orbiting in the edge of the Solar System. Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there
9520-523: The Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn. There is a strong consensus among astronomers that the Solar System has at least nine dwarf planets : Ceres , Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , Eris , and Sedna . There are a vast number of small Solar System bodies , such as asteroids , comets , centaurs , meteoroids , and interplanetary dust clouds . Some of these bodies are in
9660-659: The Solar System's planets. Based on current models, inside its volatile mantle layer is a rocky core, and surrounding it is a thick hydrogen and helium atmosphere. Trace amounts of hydrocarbons (thought to be produced via hydrolysis ) and carbon monoxide along with carbon dioxide (thought to have been originated from comets ) have been detected in the upper atmosphere. There are many unexplained climate phenomena in Uranus's atmosphere , such as its peak wind speed of 900 km/h (560 mph), variations in its polar cap, and its erratic cloud formation. The planet also has very low internal heat compared to other giant planets,
9800-429: The Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation. Research by Jacob Kegerreis of Durham University suggests that the tilt resulted from a rock larger than Earth crashing into the planet 3 to 4 billion years ago. Uranus's south pole was pointed almost directly at the Sun at the time of Voyager 2 's flyby in 1986. The mean apparent magnitude of Uranus
9940-400: The Solar System, created by heat and light pressure from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. The boundary in the Solar System beyond which those volatile substances could coalesce is known as
10080-406: The Sun by the outer planets, and are expected to become comets or be ejected out of the Solar System. While most centaurs are inactive and asteroid-like, some exhibit cometary activity, such as the first centaur discovered, 2060 Chiron , which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun. The largest known centaur, 10199 Chariklo , has
10220-455: The Sun compared to around two billion years for all other subsequent phases of the Sun's pre- remnant life combined. The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along
10360-495: The Sun fuses hydrogen at its core, it is a main-sequence star. More specifically, it is a G2-type main-sequence star , where the type designation refers to its effective temperature . Hotter main-sequence stars are more luminous but shorter lived. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs , make up about 75% of
10500-422: The Sun is 0.0047 AU (700,000 km; 400,000 mi). Thus, the Sun occupies 0.00001% (1 part in 10 ) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly 1 millionth (10 ) that of the Sun. Jupiter, the largest planet, is 5.2 AU from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune,
10640-403: The Sun is growing brighter; early in its main-sequence life its brightness was 70% that of what it is today. The temperature, reaction rate , pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away
10780-421: The Sun low over the horizon. On the other side of Uranus's orbit, the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness. Near the time of the equinoxes , the Sun faces the equator of Uranus, giving a period of day–night cycles similar to those seen on most of the other planets. One result of this axis orientation
10920-472: The Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU. Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated 1992 QB 1 , (and has since been named Albion); they are still in near primordial, low-eccentricity orbits. Currently, there
11060-499: The Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the frost line , which is a little less than 5 AU from the Sun. The four terrestrial or inner planets have dense, rocky compositions, few or no moons , and no ring systems . They are composed largely of refractory minerals such as silicates —which form their crusts and mantles —and metals such as iron and nickel which form their cores . Three of
11200-400: The Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets. The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature of around 800 K (527 °C) to 850 K (577 °C). The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor
11340-478: The ancient Greek deity of the sky Uranus ( Ancient Greek : Οὐρανός ), known as Caelus in Roman mythology, the father of Cronus ( Saturn ), grandfather of Zeus ( Jupiter ) and the great-grandfather of Ares ( Mars ), which was rendered as Uranus in Latin ( IPA: [ˈuːranʊs] ). It is the only one of the eight planets whose English name derives from a figure of Greek mythology . The pronunciation of
11480-565: The astronomical sense (chemical compounds with melting points of up to a few hundred kelvins such as water, methane, ammonia, hydrogen sulfide , and carbon dioxide . ) Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune's orbit. The centaurs are icy, comet-like bodies whose semi-major axes are longer than Jupiter's and shorter than Neptune's (between 5.5 and 30 AU). These are former Kuiper belt and scattered disc objects (SDOs) that were gravitationally perturbed closer to
11620-463: 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 is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out. The abundances of less volatile compounds such as ammonia, water, and hydrogen sulfide in
11760-402: The bodies in the Kuiper belt . Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune . The principal component of the Solar System is the Sun, a G-type main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies,
11900-453: The cause of which remains unclear. Like the other giant planets, Uranus has a ring system , a magnetosphere , and many natural satellites . The extremely dark ring system reflects only about 2% of the incoming light. Uranus's 28 natural satellites include 18 known regular moons , of which 13 are small inner moons . Further out are the larger five major moons of the planet: Miranda , Ariel , Umbriel , Titania , and Oberon . Orbiting at
12040-463: The coldest upper region of the troposphere (the tropopause ) actually vary in the range between 49 and 57 K (−224 and −216 °C; −371 and −357 °F) depending on planetary latitude. The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K (−214.1 ± 0.3 °C; −353.3 ± 0.5 °F). The troposphere
12180-410: The collisions caused their destruction and ejection. The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller orbital eccentricity . Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined. The Sun
12320-471: The conclusion that it was a planet rather than a comet. 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". Bode concluded that its near-circular orbit was more like a planet's than a comet's. The object was soon universally accepted as a new planet. By 1783, Herschel acknowledged this to Royal Society president Joseph Banks : "By
12460-432: The core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf , half the original mass of the Sun but only the size of Earth. The ejected outer layers may form
12600-529: The course of the American Revolutionary War by calling the new planet either Neptune George III or Neptune Great Britain , a compromise Lexell suggested as well. Daniel Bernoulli suggested Hypercronius and Transaturnis . Minerva was also proposed. In a March 1782 treatise, Johann Elert Bode proposed Uranus , the Latinised version of the Greek god of the sky, Ouranos . Bode argued that
12740-559: The deep atmosphere are poorly known. They are probably also higher than solar values. Along with 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. They include ethane ( C 2 H 6 ), acetylene ( C 2 H 2 ), methylacetylene ( CH 3 C 2 H ), and diacetylene ( C 2 HC 2 H ). Spectroscopy has also uncovered traces of water vapour, carbon monoxide , and carbon dioxide in
12880-413: The descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets , a precursor stage before becoming a fully-formed planet (see List of exceptional asteroids ): Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits. They lie in three linked clusters between Jupiter and
13020-423: The direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours. The Uranian axis of rotation is approximately parallel to the plane of the Solar System, with an axial tilt of 82.23°. Depending on which pole is considered north, the tilt can be described either as 82.23° or as 97.8°. The former follows
13160-412: 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 on to 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
13300-441: The early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover minor bodies . Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the frost line ). They would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because these refractory materials only comprised
13440-449: The first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets formed from the accretion of "metals". The region of space dominated by the Solar magnetosphere is
13580-407: The four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous , refractory rocky and metallic minerals, with some ice. They range from
13720-527: The garden of his house at 19 New King Street in Bath, Somerset , England (now the Herschel Museum of Astronomy ), and initially reported it (on 26 April 1781) as a comet . With a homemade 6.2-inch reflecting telescope, Herschel "engaged in a series of observations on the parallax of the fixed stars." Herschel recorded in his journal: "In the quartile near ζ Tauri ... either [a] Nebulous star or perhaps
13860-441: The giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids , and comets) together comprise less than 0.002% of the Solar System's total mass. The Sun is composed of roughly 98% hydrogen and helium, as are Jupiter and Saturn. A composition gradient exists in
14000-566: The gravitational influence of Neptune's early outward migration . Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects". Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with
14140-482: The gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On 23 September 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. As on all giant planets , its upper atmosphere experiences strong winds in
14280-422: 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 a scientific determination of 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. For the sake of convenience,
14420-415: The larger moons orbit their planets in prograde direction, matching the direction of planetary rotation; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner. Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde. To a good first approximation, Kepler's laws of planetary motion describe
14560-491: The largest natural satellites are in synchronous rotation , with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison. As a result of the formation of the Solar System , planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole. There are exceptions, such as Halley's Comet . Most of
14700-410: The last 20% of Uranus's radius. Uranus's core density is around 9 g/cm , with a pressure in the centre 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,
14840-422: The level of cosmic-ray penetration in the Solar System varies, though by how much is unknown. The zone of habitability of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water . Habitability might be possible in subsurface oceans of various outer Solar System moons. Compared to many extrasolar systems,
14980-450: The lowest minimum temperature (49 K (−224 °C; −371 °F)) of all the Solar System 's planets. It has a marked axial tilt of 82.23° with a retrograde rotation period of 17 hours and 14 minutes. This means that in an 84-Earth-year orbital period around the Sun, its poles get around 42 years of continuous sunlight, followed by 42 years of continuous darkness. Uranus has the third-largest diameter and fourth-largest mass among
15120-443: The main asteroid belt. Trojans are bodies located within another body's gravitationally stable Lagrange points : L 4 , 60° ahead in its orbit, or L 5 , 60° behind in its orbit. Every planet except Mercury and Saturn is known to possess at least 1 trojan. The Jupiter trojan population is roughly equal to that of the asteroid belt. After Jupiter, Neptune possesses the most confirmed trojans, at 28. The outer region of
15260-400: The mass orbiting the Sun. All four giant planets have multiple moons and a ring system, although only Saturn's rings are easily observed from Earth. Jupiter and Saturn are composed mainly of gases with extremely low melting points, such as hydrogen, helium, and neon , hence their designation as gas giants . Uranus and Neptune are ice giants , meaning they are largely composed of 'ice' in
15400-475: The name Uranus preferred among astronomers is / ˈ jʊər ə n ə s / YOOR -ə-nəs , with the long "u" of English and stress on the first syllable as in Latin Uranus , in contrast to / j ʊ ˈ r eɪ n ə s / yoo- RAY -nəs , with stress on the second syllable and a long a , though both are considered acceptable. Consensus on the name was not reached until almost 70 years after
15540-454: The name should follow the mythology so as not to stand out as different from the other planets, and that Uranus was an appropriate name as the father of the first generation of the Titans . He also noted the elegance of the name in that just as Saturn was the father of Jupiter , the new planet should be named after the father of Saturn. However, he was apparently unaware that Uranus was only
15680-488: The names Astraea , Cybele (now the names of asteroids), and Neptune , which would become the name of the next planet to be discovered. Georg Lichtenberg from Göttingen also supported Astraea (as Austräa ), but she is traditionally associated with Virgo instead of Taurus. Neptune was supported by other astronomers who liked the idea of commemorating the victories of the British Royal Naval fleet in
15820-452: The nominal surface, which is defined to lie at a pressure of 1 bar. The Uranian atmosphere can be divided into three layers: the troposphere , between altitudes of −300 and 50 km (−186 and 31 mi) and pressures from 100 to 0.1 bar (10 MPa to 10 kPa); the stratosphere , spanning altitudes between 50 and 4,000 km (31 and 2,485 mi) and pressures of between 0.1 and 10 bar (10 kPa to 10 μPa ); and
15960-468: The north from the collar. In all other respects, Uranus looked like a dynamically dead planet in 1986. Voyager 2 arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor
16100-606: 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." In recognition of his achievement, King George III gave Herschel an annual stipend of £200 (equivalent to £30,000 in 2023) on condition that he moved to Windsor so that the Royal Family could look through his telescopes. The name Uranus references
16240-402: The orbits of objects around the Sun. These laws stipulate that each object travels along an ellipse with the Sun at one focus , which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its perihelion , whereas its most distant point from the Sun is called its aphelion . With the exception of Mercury, the orbits of
16380-413: 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, but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of
16520-400: The planet's discovery. During the original discussions following discovery, Maskelyne asked Herschel to "do the astronomical world the faver [ sic ] to give a name to your planet, which is entirely your own, [and] which we are so much obliged to you for the discovery of". In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or
16660-512: The planet's low temperature. 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 (47 °C; 116 °F). The tenuous thermosphere extends over two planetary radii from
16800-464: The planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using numerical models , but the planetary system can change chaotically over billions of years. The angular momentum of
16940-542: The point of its discovery northeast of the binary star Zeta Tauri twice—in March 1865 and March 1949—and will return to this location again in April 2033. Its average distance from the Sun is roughly 20 AU (3 billion km ; 2 billion mi ). The difference between its minimum and maximum distance from the Sun is 1.8 AU, larger than that of any other planet, though not as large as that of dwarf planet Pluto . The intensity of sunlight varies inversely with
17080-408: 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
17220-415: The primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called cosmic rays . The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so
17360-471: The proposed Uranus Orbiter and Probe mission a top priority in the 2023–2032 survey, and the CNSA 's proposal to fly by the planet with a subprobe of Tianwen-4 . Like the classical planets , Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. William Herschel first observed Uranus on 13 March 1781, leading to its discovery as
17500-461: The remaining gas and dust from the protoplanetary disc into interstellar space. Following the dissipation of the protoplanetary disk , the Nice model proposes that gravitational encounters between planetisimals and the gas giants caused each to migrate into different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period,
17640-667: The same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, 'In the reign of King George the Third'. Herschel's proposed name was not popular outside Britain and Hanover, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed that it be named Herschel in honour of its discoverer. Swedish astronomer Erik Prosperin proposed
17780-404: The solar wind is halted, resulting in the heliopause . This is the boundary of the Solar System to interstellar space . The outermost region of the Solar System is the theorized Oort cloud , the source for long-period comets , extending to a radius of 2,000–200,000 AU . The closest star to the Solar System, Proxima Centauri , is 4.25 light-years (269,000 AU) away. Both stars belong to
17920-399: The southern hemisphere the bright collar masks them—was shown to be incorrect. Nevertheless, there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter. They appear to lie at a higher altitude. The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours; at least one southern cloud may have persisted since
18060-441: The spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere . Uranus's heat flux is only 0.042 ± 0.047 W / m , which is lower than the internal heat flux of Earth of about 0.075 W / m . The lowest temperature recorded in Uranus's tropopause is 49 K (−224.2 °C; −371.5 °F), making Uranus the coldest planet in the Solar System. One of the hypotheses for this discrepancy suggests
18200-408: The square of the distance—on Uranus (at about 20 times the distance from the Sun compared to Earth), it is about 1/400 the intensity of light on Earth. The orbital elements of Uranus were first calculated in 1783 by Pierre-Simon Laplace . With time, discrepancies began to appear between predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to
18340-488: 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 23 April 1781: "I don't know what to call it. It is as likely to be
18480-651: The stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons , which form in this part of the atmosphere as a result of methane photolysis . Heat is also conducted from the hot thermosphere. The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300 km corresponding to a pressure range of 1,000 to 10 Pa and temperatures of between 75 and 170 K (−198 and −103 °C; −325 and −154 °F). The most abundant hydrocarbons are methane, acetylene , and ethane with mixing ratios of around 10 relative to hydrogen. The mixing ratio of carbon monoxide
18620-423: The symbols for the other planets while remaining distinct. This symbol predominates in modern astronomical use in the rare cases that symbols are used at all. The second symbol, [REDACTED] , was 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 surname"). The second symbol
18760-480: The terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud. Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion . As helium accumulates at its core,
18900-436: The thermosphere extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere . The composition of Uranus's atmosphere is different from its bulk, consisting mainly of molecular hydrogen and helium. The helium molar fraction , i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03 in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05 . This value
19040-456: The total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth. Many Kuiper belt objects have satellites, and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic. The Kuiper belt can be roughly divided into the " classical " belt and the resonant trans-Neptunian objects . The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around
19180-447: The upper atmosphere, which can only originate from an external source such as infalling dust and comets . The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude. The temperature falls from about 320 K (47 °C; 116 °F) at the base of the nominal troposphere at −300 km to 53 K (−220 °C; −364 °F) at 50 km. The temperatures in
19320-402: The water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions move freely within the oxygen lattice. Although the model considered above is reasonably standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in
19460-464: Was proposed by Johann Gottfried Köhler at Bode's request in 1782. Köhler suggested that the new planet be given the symbol for platinum , which had been described scientifically only 30 years before. As there was no alchemical symbol for platinum, he suggested ⛢ or ⛢ , a combination of the planetary-metal symbols ☉ (gold) and ♂ (iron), as platinum (or 'white gold') is found mixed with iron. Bode thought that an upright orientation, ⛢, fit better with
19600-447: Was reached that the planet be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities . As of 2024, it had been visited up close only once when in 1986 the Voyager 2 probe flew by the planet. Though nowadays it can be resolved and observed by telescopes, there is much desire to revisit the planet, as shown by Planetary Science Decadal Survey 's decision to make
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