Uranus

Template:Short description Template:About Template:Featured article Template:Pp-semi-indef Template:Pp-move Template:Use British English Template:Use dmy dates {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Uranus | image = {{#invoke:InfoboxImage|InfoboxImage|image=Uranus Voyager2 color calibrated.png|upright={{#if:||1.1}}|alt=}} | caption = Uranus in true colour,Template:Efn as captured by Voyager 2. Its pale, muted appearance is due to a shroud of haze above its clouds | headerstyle = {{#if:LightBlue|background-color:LightBlue|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| label2 = Discovered by | data2 = [[{{#property:P61}}]] | label3 = Discovery site | data3 = | label4 = Discovery date | data4 = {{#time:j F Y|{{#property:P575}}}} | label5 = Template:Longitem | data5 =

| header10 = {{#if:|Designations|Designations}}

| label11 = Template:Longitem | data11 = | label12 = Pronunciation | data12 = Template:IPAc-en<ref name="BBCOUP" /> or Template:IPAc-en<ref name="OED" /> | label13 = Template:Longitem | data13 = the Latin form Ūranus of the Greek god Οὐρανός Ouranos | label14 = Template:Longitem | data14 = | label15 = Template:Longitem | data15 = | label16 = Adjectives | data16 = Uranian (Template:IPAc-en)<ref>Template:OED</ref> | label17 = Symbol | data17 = ⛢, ♅

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name="VSOP87" />Template:Efn

| data21 = | data22 = {{#if:J2000 |Epoch J2000}} | data23 = {{#if: | Uncertainty parameter {{{uncertainty}}}}} | label24 = Observation arc | data24 = | label25 = Earliest precovery date | data25 = | label26 = {{#switch:{{{apsis}}} |apsis|gee|barion|center|centre|(apsis)=Apo{{{apsis}}} |Ap{{#if:|{{{apsis}}}|helion}}}} | data26 = Template:Convert | label27 = Peri{{#if:|{{{apsis}}}|helion}} | data27 = Template:Convert | label28 = Peri{{#if:|{{{apsis}}}|apsis}} | data28 = | label29 = {{#switch:{{{apsis}}} |helion|astron=Ap{{{apsis}}} |Apo{{#if:|{{{apsis}}}|apsis}}}} | data29 = | label30 = Periastron | data30 = | label31 = Apoastron | data31 = | label32 = Template:Longitem | data32 = Template:Convert | label33 = Template:Longitem | data33 = | label34 = Eccentricity | data34 = Template:Val | label35 = Template:Longitem | data35 = Template:Plainlist | label36 = Template:Longitem | data36 = 369.66 days<ref name="fact" /> | label37 = Template:Longitem | data37 = 6.80 km/s<ref name="fact" /> | label38 = Template:Longitem | data38 = Template:Val | label39 = Template:Longitem | data39 = | label40 = Inclination | data40 = Template:Ubl | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = Template:Val | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = 17–19 August 2050<ref>Jean Meeus, Astronomical Algorithms (Richmond, Virginia: Willmann-Bell, 1998) p271. Bretagnon's complete VSOP87 model. It gives the 17th @ 18.283075301au. http://vo.imcce.fr/webservices/miriade/?forms Template:Webarchive IMCCE Observatoire de Paris / CNRS Calculated for a series of dates, five or ten days apart, in August 2050, using an interpolation formula from Astronomical Algorithms. Perihelion came very early on the 17th. INPOP planetary theory</ref><ref name=horizons-perihelion>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> | label45 = Template:Longitem | data45 = Template:Val | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = | label48 = Group | data48 = | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = 28 | label50 = Star | data50 = | label51 = Earth MOID | data51 = | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = T_Jupiter | data59 =

| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} degTemplate:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsecTemplate:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsecTemplate:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = | label72 = Template:Longitem | data72 = | label73 = Template:Longitem | data73 = Template:Nowrap<ref name="Seidelmann Archinal A'hearn et al. 2007" />Template:Efn | label74 = Template:Longitem | data74 = Template:Nowrap
4.007 Earths<ref name="Seidelmann Archinal A'hearn et al. 2007" />Template:Efn | label75 = Template:Longitem | data75 = Template:Nowrap
3.929 Earths<ref name="Seidelmann Archinal A'hearn et al. 2007" />Template:Efn | label76 = Flattening | data76 = Template:ValTemplate:Efn | label77 = Circumference | data77 = Template:Nowrap<ref name="nasafact" /> | label78 = Template:Longitem | data78 = Template:Val<ref name="nasafact" />Template:Efn
15.91 Earths | label79 = Volume | data79 = Template:Val<ref name="fact" />Template:Efn
63.086 Earths | label80 = Mass | data80 = Template:Val
14.536 Earths<ref name="Jacobson Campbell et al. 1992" />
GM=Template:Nowrap | label81 = Template:Longitem | data81 = Template:Val<ref name="fact" />Template:Efn | label82 = Template:Longitem | data82 = Template:Cvt<ref name="fact" />Template:Efn | label83 = Template:Longitem | data83 = Template:Val<ref name="PS15">Template:Cite book</ref> (estimate) | label84 = Template:Longitem | data84 = 21.3 km/s<ref name="fact" />Template:Efn | label85 = Template:Longitem | data85 = Template:Val
−Template:RA
(retrograde) | label86 = Template:Longitem | data86 = Template:Val
−Template:RA ± Template:RA
(retrograde)<ref name="Lamy2025"/> | label87 = Template:Longitem | data87 = 2.59 km/s | label88 = Template:Longitem | data88 = 82.23° (to orbit, retrograde).<ref name="fact"/>

97.77°(prograde, right-hand rule)

| label89 = Template:Longitem | data89 = Template:RA
257.311°<ref name="Seidelmann Archinal A'hearn et al. 2007" /><ref name="iau2015">Template:Cite journal</ref> | label90 = Template:Longitem | data90 = −15.175°<ref name="Seidelmann Archinal A'hearn et al. 2007" /><ref name="iau2015" /> | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = 0.300 (Bond)<ref name="Pearl_et_al_Uranus"/>
0.488 (geom.)<ref name="Mallama_et_al"/> | label94 = Temperature | data94 =

| data100 = {{#if:1 bar level<ref name="Podolak Weizman et al. 1995" />0.1 bar
(tropopause)<ref name="Lunine 1993" />|

{{#if:1 bar level<ref name="Podolak Weizman et al. 1995" />|}}{{#if:0.1 bar
(tropopause)<ref name="Lunine 1993" />|}}{{#if:|}}{{#if:|}}

Surface temp.	min	mean	max
1 bar level<ref name="Podolak Weizman et al. 1995" />		Template:Convert
0.1 bar (tropopause)<ref name="Lunine 1993" />	47 K	53 K	57 K
{{{temp_name3}}}
{{{temp_name4}}}

}}

| label101 = Surface absorbed dose rate | data101 = | label102 = Surface equivalent dose rate | data102 = | label103 = Template:Longitem | data103 = | label104 = Template:Longitem | data104 = | label105 = Template:Longitem | data105 = 5.38<ref name="Mallama_and_Hilton" /> to 6.03<ref name="Mallama_and_Hilton" /> | label106 = Template:Longitem | data106 = −7.2<ref name="IMCCE">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> | label107 = Template:Longitem | data107 = 3.3″ to 4.1″<ref name="fact" />

| header110 = Atmosphere<ref name="Lunine 1993" /><ref name="Lindal Lyons et al. 1987" /><ref name="Conrath Gautier et al. 1987" />Template:Efn

| label111 = Template:Longitem | data111 = | label112 = Template:Longitem | data112 = 27.7 km<ref name="fact" /> | label113 = Composition by volume | data113 = Below Template:Cvt: Template:Indented plainlist Icy volatiles: Template:Cslist

| below = {{#if:||Template:Reflist }}

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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 the lowest minimum temperature (Template:Convert) 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 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 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 Template:Convert,<ref name="Sromovsky & Fry 2005" /> variations in its polar cap, and its erratic cloud formation. The planet also has very low internal heat compared to other giant planets, 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 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 was reached that the planet be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities. As of 2025, it has been visited only once when in 1986 the Voyager 2 probe flew by the planet.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 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.<ref name="TianwenPlSoc">Template:Cite news</ref>

History

File:Discovery of Uranus1781.png

Position of Uranus (marked with a cross) on 13 March 1781, the date of its discovery

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> William Herschel first observed Uranus on 13 March 1781, leading to its discovery as 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 as far from the Sun as the planet Saturn.

Template:Anchor Discovery

File:William Herschel01.jpg

William Herschel, discoverer of Uranus

Before its recognition as a planet, Uranus had been observed many times, but was generally misidentified as 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.<ref>Template:Cite journal</ref> 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,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> including on four consecutive nights.

William Herschel observed Uranus on 13 March 1781 from the garden of his house at 19 New King Street in Bath, Somerset, England (now the Herschel Museum of Astronomy),<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and initially reported it (on 26 April 1781) as a comet.<ref>Template:Cite journal</ref> With a homemade 6.2-inch reflecting telescope, Herschel "engaged in a series of observations on the parallax of the fixed stars."<ref name="Ref-1">Journal of the Royal Society and Royal Astronomical Society 1, 30, quoted in Miner, p. 8.</ref><ref>Template:Cite journal</ref>

Herschel recorded in his journal: "In the quartile near ζ Tauri ... either [a] Nebulous star or perhaps a comet."<ref>Royal Astronomical Society MSS W.2/1.2, 23; quoted in Miner p. 8.</ref> 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."<ref>RAS MSS Herschel W.2/1.2, 24, quoted in Miner p. 8.</ref> 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:<ref name="Ref-1"/>

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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 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."<ref>RAS MSS Herschel W1/13.M, 14 quoted in Miner p. 8.</ref>

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.<ref name="lexell" /> Its nearly circular orbit suggested 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".<ref>Johann Elert Bode, Berliner Astronomisches Jahrbuch, p. 210, 1781, quoted in Miner, p. 11.</ref> Bode concluded that its near-circular orbit was more like a planet's than a comet's.<ref>Miner, p. 11.</ref>

The object was soon accepted as a new planet. By 1783, Herschel acknowledged this 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."<ref name="Dreyer" /> In recognition of his achievement, King George III gave Herschel an annual stipend of £200 (Template:Inflation)Template:Inflation/fn on condition that he moved to Windsor so that the Royal Family could look through his telescopes.<ref name="Miner12" />

Name

The name Uranus references the ancient Greek deity of the sky Uranus (Template:Langx), 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 {{#invoke:Lang|lang}} in Latin ({{#invoke:IPA|main}}).<ref name="OED" /> It is the only one of the eight planets whose English name derives from a figure of Greek mythology. The pronunciation of the name Uranus preferred among astronomers is Template:IPAc-en Template:Respell,<ref name="BBCOUP" /> with the long "u" of English and stress on the first syllable as in Latin {{#invoke:Lang|lang}}, in contrast to Template:IPAc-en Template:Respell, with stress on the second syllable and a long a, though both are considered acceptable.Template:Efn

Consensus on the name was not reached until almost 70 years after the planet's discovery. During the original discussions following discovery, Maskelyne asked Herschel to "do the astronomical world the Template: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".<ref>RAS MSS Herschel W.1/12.M, 20, quoted in Miner, p. 12</ref> In response to Maskelyne's request, Herschel decided to name the object {{#invoke:Lang|lang}} (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.<ref>Template:Cite journal</ref> He explained this decision in a letter to Joseph Banks:<ref name="Dreyer" />

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 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'.{{#if:|{{#if:|}}

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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.<ref name="Francisca" /> Swedish astronomer Erik Prosperin proposed the names Astraea, Cybele (now the names of asteroids), and Neptune, which later became 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 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.<ref name="lexell" /><ref name=":0">Template:Cite journal</ref> Daniel Bernoulli suggested Hypercronius and Transaturnis. Minerva was also proposed.<ref name=":0" />

File:Johann elert bode painting.jpg

Johann Elert Bode, the astronomer who suggested the name Uranus

In a March 1782 treatise, Johann Elert Bode proposed Uranus, the Latinised version of the Greek god of the sky, Ouranos.<ref name=Bode>Template:Harvnb: [In original German]:

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Already in the pre-read at the local Natural History Society on 12th March 1782 treatise, I have the father's name from Saturn, namely Uranos, or as it is usually with the Latin suffix, proposed Uranus, and have since had the pleasure that various astronomers and mathematicians, cited in their writings or letters to me approving this designation. In my view, it is necessary to follow the mythology in this election, which had been borrowed from the ancient name of the other planets; because in the series of previously known, perceived by a strange person or event of modern times name of a planet would very noticeable. Diodorus of Cilicia tells the story of Atlas, an ancient people that inhabited one of the most fertile areas in Africa, and looked at the sea shores of his country as the homeland of the gods. Uranus was her first king, founder of their civilized life and inventor of many useful arts. At the same time he is also described as a diligent and skilful astronomers of antiquity ... even more: Uranus was the father of Saturn and the Atlas, as the former is the father of Jupiter.{{#if:|{{#if:|}}

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{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}</ref> Bode argued that 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.<ref name=Bode/> 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.<ref name="Miner12" /><ref name=Bode/><ref name="planetsbeyond" /><ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> However, he was apparently unaware that Uranus was only 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="planetsbeyond" />

Uranus has two astronomical symbols. The first to be proposed, ⛢,Template:Efn was proposed by Johann Gottfried Köhler at Bode's request in 1782.<ref name=platinum>Astronomisches Jahrbuch für das Jahr 1785. George Jacob Decker, Berlin, p. 191.</ref> 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 the symbols for the other planets while remaining distinct.<ref name=platinum/> This symbol predominates in modern astronomical use in the rare cases that symbols are used at all.<ref>Template:Cite journal</ref><ref>Solar System Symbols Template:Webarchive, NASA/JPL</ref> The second symbol, ♅,Template:Efn was suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "{{#invoke:Lang|lang}}" ("a globe surmounted by the first letter of your surname").<ref name="Francisca" /> The second symbol 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".<ref>Template:Cite news</ref>

Uranus is called by a variety of names in other languages. Uranus's name is literally translated as the "Heavenly King star" in Chinese (Template:Lang-zh), Japanese (天王星), Korean (천왕성), and Vietnamese (sao Thiên Vương).<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref> In Thai, its official name is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), as in English. Its other name in Thai is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Star of Mṛtyu), after the Sanskrit word for 'death', {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}). In Mongolian, its name is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), translated as 'King of the Sky', reflecting its namesake god's role as the ruler of the heavens. In Hawaiian, its name is {{#invoke:Lang|lang}}, the Hawaiian rendering of the name 'Herschel'.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Formation

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It is argued that the differences between the ice giants and the gas giants arise from their formation history.<ref name="Thommes1999" /><ref name="Brunini1999" /><ref name="dangelo2021">Template:Cite journal</ref> 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 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.<ref name="Thommes1999" /><ref name="Brunini1999" /><ref name="dangelo_bodenheimer_2013">Template:Cite journal</ref> 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.<ref name="dl2018">Template:Cite book</ref> The ice giants, with only a few Earth masses of nebular gas, never reached that critical point.<ref name="Thommes1999" /><ref name="Brunini1999" /><ref name="Sheppard Jewitt Kleyna 2006" /> 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).<ref name="Thommes1999" />

Orbit and rotation

Uranus orbits the Sun once every 84 years. As viewed against the background of stars, since being discovered in 1781,<ref>Template:Cite news</ref> the planet has returned to 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Its average distance from the Sun is roughly Template:Convert. 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.<ref name=AA>Jean Meeus, Astronomical Algorithms (Richmond, VA: Willmann-Bell, 1998) p 271. From the 1841 aphelion to the 2092 one, perihelia are always 18.28 and aphelia always 20.10 astronomical units</ref> The intensity of sunlight varies inversely with 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The orbital elements of Uranus were first calculated in 1783 by Pierre-Simon Laplace.<ref name="georgeforbes" /> 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 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The rotational period of the interior of Uranus is 17 hours, 14 minutes, and 52 seconds<ref name="Lamy2025">Template:Cite journal</ref> which was determined by tracking the rotational motion of Uranus's aurorae.<ref>Template:Cite news</ref> As on all giant planets, its upper atmosphere experiences strong winds in 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Axial tilt

File:Uranus orientation 1985-2030.gif

Simulated Earth view of Uranus from 1986 to 2030, from southern summer solstice in 1986 to equinox in 2007 and northern summer solstice in 2028.

The Uranian axis of rotation is approximately parallel to the plane of the Solar System, with an axial tilt that can be described either as 82.23° or as 97.77°, depending on which pole is considered north.Template:Efn The former follows 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 97.77°, which reverses which pole is considered north and which is considered south and giving the planet prograde rotation.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 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 cause of Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.<ref>Template:Cite book</ref> 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.<ref>Template:Cite news</ref> Uranus's south pole was pointed almost directly at the Sun at the time of Voyager 2Template:'s flyby in 1986.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

List of solstices and equinoxes<ref>Template:Cite conference</ref>

Northern hemisphere	Year	Southern hemisphere
Winter solstice	1902, 1986, 2069	Summer solstice
Vernal equinox	1923, 2007, 2092	Autumnal equinox
Summer solstice	1944, 2028	Winter solstice
Autumnal equinox	1965, 2050	Vernal equinox

Visibility from Earth

File:UranusAndMoon 20221108 fromJP (crop).jpg

Uranus seen through an amateur telescope, shortly after lunar occultation, during the November 2022 lunar eclipse

The mean apparent magnitude of Uranus is 5.68 with a standard deviation of 0.17, while the extremes are 5.38 and 6.03.<ref name="Mallama_and_Hilton" /> 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.<ref name="Schmude_et_al" /> 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.<ref name="ephemeris" /> At opposition, Uranus is visible to the naked eye in dark skies, and becomes an easy target even in urban conditions with binoculars.<ref name="fact" /> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Internal structure

File:Uranus, Earth size comparison 2.jpg

Size comparison of Earth and Uranus

Uranus's mass 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/cm³ makes Uranus the second least dense planet, after Saturn.<ref name="Seidelmann Archinal A'hearn et al. 2007" /><ref name="Jacobson Campbell et al. 1992" /> This value indicates that it is made primarily of various ices, such as water, ammonia, and methane.<ref name="Podolak Weizman et al. 1995" /> The total mass of ice in Uranus's interior is not precisely known, because different figures emerge depending on the model chosen; it must be between 9.3 and 13.5 Earth masses.<ref name="Podolak Weizman et al. 1995" /><ref name="Podolak Podolak et al. 2000" /> Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses.<ref name="Podolak Weizman et al. 1995" /> The remainder of the non-ice mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.<ref name="Podolak Weizman et al. 1995" />

The standard model of Uranus's structure 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.<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> 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 the last 20% of Uranus's radius.<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> Uranus's core density is around 9 g/cm³, with a pressure in the centre of 8 million bars (800 GPa) and a temperature of about 5000 K.<ref name="Podolak Podolak et al. 2000" /><ref name="Faure2007" /> 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.<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.<ref name="Atreya2006" />

File:Uranus diagram.svg

Diagram of the interior of Uranus, listing each layer's composition

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref> This phenomenon is similar to diamond rains that are theorised by scientists to exist on Jupiter, Saturn, and Neptune.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite news</ref> Very-high-pressure experiments at 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.<ref name="Eggert">Template:Cite journal</ref><ref name="Bland, Eric">Template:Cite news</ref><ref>Template:Cite journal</ref>

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 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.<ref>Template:Cite news</ref>

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 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.<ref name="Podolak Podolak et al. 2000" /> The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.<ref name="Podolak Weizman et al. 1995" /> 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 Template:Convert and Template:Convert, respectively.<ref name="Seidelmann Archinal A'hearn et al. 2007" /> This surface is 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.<ref name="Sromovsky & Fry 2005" /><ref name="Hanel Conrath et al. 1986" /> 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,<ref name="Sromovsky & Fry 2005" /> but Uranus 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 Template:Val times the solar energy absorbed in its atmosphere.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> Uranus's heat flux is only Template:Val, which is lower than the internal heat flux of Earth of about Template:Val.<ref name="Pearl Conrath et al. 1990" /> The lowest temperature recorded in Uranus's tropopause is Template:Convert, making Uranus the coldest planet in the Solar System.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" />

One of the hypotheses for this discrepancy suggests 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.<ref>Template:Cite magazine</ref> Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface.<ref name="Podolak Weizman et al. 1995" /> For example, convection may take place in a set of compositionally different layers, which may inhibit upward heat transport;<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> perhaps double diffusive convection is a limiting factor.<ref name="Podolak Weizman et al. 1995" />

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 the planet's low temperature.<ref>Template:Cite journal</ref>

Atmosphere

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} 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.<ref name="Lunine 1993" /> Remote-sensing capability extends down to roughly 300 km below the Template:Convert level, with a corresponding pressure around Template:Convert and temperature of Template:Convert.<ref name="de Pater Romani et al. 1991" /> The tenuous thermosphere extends over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar.<ref name="Herbert Sandel et al. 1987" /> The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of Template:Convert and pressures from 100 to 0.1 bar (10 MPa to 10 kPa); the stratosphere, spanning altitudes between Template:Convert and pressures of between Template:Nowrap (10 kPa to 10 μPa); and the thermosphere extending from 4,000 km to as high as 50,000 km from the surface.<ref name="Lunine 1993" /> There is no mesosphere.

Composition

File:Tropospheric profile Uranus new.svg

Diagram of the Uranus atmosphere's composition and layers, along with the graph of its pressure

The composition of Uranus's atmosphere is different from its bulk, consisting mainly of molecular hydrogen and helium.<ref name="Lunine 1993" /> The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is Template:Val<ref name="Conrath Gautier et al. 1987" /> in the upper troposphere, which corresponds to a mass fraction Template:Val.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> This value is close to the protosolar helium mass fraction of Template:Val,<ref name="Lodders 2003" /> indicating that helium has not settled in its centre as it has in the gas giants.<ref name="Lunine 1993" /> The third-most-abundant component of Uranus's atmosphere is methane (Template:Chem2).<ref name="Lunine 1993" /> Methane has prominent absorption bands in the visible and near-infrared (IR), making Uranus aquamarine or cyan in colour.<ref name="Lunine 1993" /> Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of Template:Convert; this represents about 20 to 30 times the carbon abundance found in the Sun.<ref name="Lunine 1993" /><ref name="Lindal Lyons et al. 1987" /><ref name="Tyler 1986" />

The mixing ratioTemplate:Efn 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.<ref name="Bishop Atreya et al. 1990" /> The abundances of less volatile compounds such as ammonia, water, and hydrogen sulfide in the deep atmosphere are poorly known. They are probably also higher than solar values.<ref name="Lunine 1993" /><ref name="de Pater Romani et al. 1989" /> 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.<ref name="Summers & Strobel 1989" /> They include ethane (Template:Chem2), acetylene (Template:Chem2), methylacetylene (Template:Chem2), and diacetylene (Template:Chem2).<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz 2003" /> Spectroscopy has also uncovered traces of water vapour, carbon monoxide, and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.<ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz 2003" /><ref name="Encrenaz Lellouch et al. 2004" />

Troposphere

The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude.<ref name="Lunine 1993" /> The temperature falls from about Template:Convert at the base of the nominal troposphere at −300 km to Template:Convert at 50 km.<ref name="de Pater Romani et al. 1991" /><ref name="Tyler 1986" /> The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between Template:Convert depending on planetary latitude.<ref name="Lunine 1993" /><ref name="Hanel Conrath et al. 1986" /> The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of Template:Convert.<ref name="Hanel Conrath et al. 1986" /><ref name="Pearl Conrath et al. 1990" />

The troposphere is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of Template:Convert, ammonium hydrosulfide clouds in the range of Template:Convert, ammonia or hydrogen sulfide clouds at between Template:Convert and finally directly detected thin methane clouds at Template:Convert.<ref name="Lunine 1993" /><ref name="Lindal Lyons et al. 1987" /><ref name="de Pater Romani et al. 1991" /><ref name="Atreya Wong 2005" /> The troposphere is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds, and seasonal changes.<ref name="Sromovsky & Fry 2005" />

Upper atmosphere

File:Adding to Uranus's legacy.tif

CitationClass=web }}</ref>

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from Template:Convert in the tropopause to between Template:Convert at the base of the thermosphere.<ref name="Herbert Sandel et al. 1987" /> The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,<ref name="Young et al. 2001" /> which form in this part of the atmosphere as a result of methane photolysis.<ref name="Summers & Strobel 1989" /> Heat is also conducted from the hot thermosphere.<ref name="Young et al. 2001" /> 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 Template:Convert.<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" />

The most abundant hydrocarbons are methane, acetylene, and ethane with mixing ratios of around Template:10^ relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.<ref name="Bishop Atreya et al. 1990" /><ref name="Burgdorf Orton et al. 2006" /><ref name="Encrenaz Lellouch et al. 2004" /> Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.<ref name="Burgdorf Orton et al. 2006" /> The abundance ratio of water is around 7Template:E.<ref name="Encrenaz 2003" /> Ethane and acetylene tend to condense in the colder lower part of the stratosphere and tropopause (below 10 mBar level) forming haze layers,<ref name="Summers & Strobel 1989" /> which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.<ref name="Bishop Atreya et al. 1990" /><ref name="Herbert & Sandel 1999" />

File:PIA25951-Uranus-NorthPole-Cyclone-October2021.jpg

Planet Uranus – North Pole – Cyclone (VLA; October 2021)

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature of around Template:Convert to Template:Convert.<ref name="Lunine 1993" /><ref name="Herbert & Sandel 1999" /> The heat sources necessary to sustain such a high level are not understood, as neither the solar UV nor 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.<ref name="Herbert Sandel et al. 1987" /><ref name="Herbert & Sandel 1999" /> 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 Template:Convert, or two Uranian radii, from its surface.<ref name="Herbert Sandel et al. 1987" /><ref name="Herbert & Sandel 1999" />

This extended corona is a unique feature of Uranus.<ref name="Herbert & Sandel 1999" /> Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.<ref name="Herbert Sandel et al. 1987" /> The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.<ref name="Tyler 1986" /> Observations show that the ionosphere occupies altitudes from Template:Convert.<ref name="Tyler 1986" /> The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.<ref name="Herbert & Sandel 1999" /><ref name="Trafton Miller et al. 1999" /> The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.<ref name="Encrenaz Drossart et al. 2003" /> Auroral activity is insignificant as compared to Jupiter and Saturn.<ref name="Herbert & Sandel 1999" /><ref name="Lam Miller et al. 1997" />

Climate

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

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.<ref name="Sromovsky & Fry 2005" /> When Voyager 2 flew by Uranus in 1986, it observed a total of 10 cloud features across the entire planet.<ref name="Smith Soderblom et al. 1986" /><ref name="planetary" /> 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.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" />

Banded structure, winds and clouds

File:Uranus-timelapse.gif

Voyager 2's timelapse of Uranus's dynamic atmosphere

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.<ref name="Smith Soderblom et al. 1986" /> Their boundary 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.<ref name="Smith Soderblom et al. 1986" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> 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.<ref name="Rages Hammel et al. 2004" /> Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.<ref name="Smith Soderblom et al. 1986" /> 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 a polar cap in the northern hemisphere.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> In 2007, when Uranus passed its equinox, the southern collar almost disappeared, and a faint northern collar emerged near 45° of latitude.<ref name="Sromovsky Fry et al. 2009" /> In 2023, a team employing the Very Large Array observed a dark collar at 80° latitude, and a bright spot at the north pole, indicating the presence of a polar vortex.<ref>Template:Cite journal</ref>

File:Uranus Dark spot.jpg

The first dark spot observed on Uranus. Image obtained by the HST ACS in 2006.

In the 1990s, the number of the observed bright cloud features grew considerably, partly because new high-resolution imaging techniques became available.<ref name="Sromovsky & Fry 2005" /> Most were found in the northern hemisphere as it started to become visible.<ref name="Sromovsky & Fry 2005" /> An early explanation—that bright clouds are easier to identify in its dark part, whereas in the southern hemisphere the bright collar masks them—was shown to be incorrect.<ref name="Karkoschka ('Uranus') 2001" /><ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> Nevertheless, there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> They appear to lie at a higher altitude.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours; at least one southern cloud may have persisted since the Voyager 2 flyby.<ref name="Sromovsky & Fry 2005" /><ref name="planetary" /> Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.<ref name="Sromovsky & Fry 2005" /> 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.<ref name="DarkSpot" /> The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.<ref name="Hammel2007" />

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.<ref name="Sromovsky & Fry 2005" /> At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from Template:Convert.<ref name="Sromovsky & Fry 2005" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.<ref name="Sromovsky & Fry 2005" /><ref name="Hanel Conrath et al. 1986" /> Closer to the poles, the winds shift to a prograde direction, flowing with Uranus's rotation. Wind speeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.<ref name="Sromovsky & Fry 2005" /> Wind speeds at −40° latitude range from Template:Convert. Because the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.<ref name="Sromovsky & Fry 2005" /> In contrast, in the northern hemisphere maximum speeds as high as Template:Convert are observed near +50° latitude.<ref name="Sromovsky & Fry 2005" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /><ref name="Hammel Rages et al. 2001" />

In 1986, the Voyager 2 Planetary Radio Astronomy (PRA) experiment observed 140 lightning flashes, or Uranian electrostatic discharges with a frequency of 0.9-40 MHz.<ref name="Atmospheric Electricity at the Ice">Template:Cite journal</ref><ref name="Radio detection of uranian lightnin">Template:Cite journal</ref> The UEDs were detected from 600,000 km of Uranus over 24 hours, most of which were not visible .<ref name="Atmospheric Electricity at the Ice"/> However, microphysical modelling suggests that Uranian lightning occurs in convective storms occurring in deep troposphere water clouds.<ref name="Atmospheric Electricity at the Ice"/><ref name="ReferenceA">Template:Cite journal</ref> If this is the case, lightning will not be visible due to the thick cloud layers above the troposphere.<ref name="Radio detection of uranian lightnin"/> The UEDs were detected from 600,000 km of Uranus, most of which were not visible .<ref name="Atmospheric Electricity at the Ice"/> Uranian lightning has a power of around 10⁸ W, emits 1×10^7 J - 2×10^7 J of energy, and lasts an average of 120 ms. There is a possibility that the power of Uranian lightning varies greatly with the seasons caused by changes in convection rates in the clouds<ref name="Radio detection of uranian lightnin"/> The UEDs were detected from 600,000 km of Uranus, most of which were not visible.<ref name="Atmospheric Electricity at the Ice"/> Uranian lightning is much more powerful than lightning on Earth and comparable to Jovian lightning.<ref name="Radio detection of uranian lightnin"/> During the Ice Giant flybys, "Voyager 2" detected lightning more clearly on Uranus than on Neptune due to the planet's lower gravity and possible warmer deep atmosphere.<ref name="ReferenceA"/>

Seasonal variation

File:Uranus clouds.jpg

Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).

For a short period from March to May 2004, large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.<ref name="NYT-20240104">Template:Cite news</ref><ref name="Hammel de Pater et al. Uranus in 2004, 2005" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Observations included record-breaking wind speeds of Template:Convert and a persistent thunderstorm referred to as "Fourth of July fireworks".<ref name="planetary" /> On 23 August 2006, researchers at the Space Science Institute (Boulder, Colorado) and the University of Wisconsin observed a dark spot on Uranus's surface, giving scientists more insight into Uranus atmospheric activity.<ref name="DarkSpot" /> Why this sudden upsurge in activity occurred is not fully known, but it appears that Uranus's extreme axial tilt results in extreme seasonal variations in its weather.<ref name="weather" /><ref name="Hammel2007" /> Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere has existed for less than 84 years, or one full Uranian year. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.<ref name="Lockwood & Jerzykiewicz 2006" /> A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.<ref name="Klein & Hofstadter 2006" /> Stratospheric temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.<ref name="Young et al. 2001" /> The majority of this variability is thought to occur owing to changes in viewing geometry.<ref name="Karkoschka ('Uranus') 2001" />

There are some indications that physical seasonal changes are happening in Uranus. Although Uranus is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.<ref name="Hammel2007" /> During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.<ref name="Lockwood & Jerzykiewicz 2006" /> This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.<ref name="Hammel2007" /> Detailed analysis of the visible and microwave data revealed that the periodical changes in brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.<ref name="Hammel2007" />

In the 1990s, as Uranus moved away from its solstice, Hubble and ground-based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),<ref name="Rages Hammel et al. 2004" /> whereas the northern hemisphere demonstrated increasing activity,<ref name="planetary" /> such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> This indeed happened in 2007 when it passed an equinox: a faint northern polar collar arose, and the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.<ref name="Sromovsky Fry et al. 2009" />

The mechanism of these physical changes is still not clear.<ref name="Hammel2007" /> Near the summer and winter solstices, Uranus's hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere.<ref name="Rages Hammel et al. 2004" /> The bright collar at −45° latitude is also connected with methane clouds.<ref name="Rages Hammel et al. 2004" /> Other changes in the southern polar region can be explained by changes in the lower cloud layers.<ref name="Rages Hammel et al. 2004" /> The variation of the microwave emission from Uranus is probably caused by changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection.<ref name="Hofstadter & Butler 2003" /> Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.<ref name="planetary" /><ref name="Hofstadter & Butler 2003" />

Magnetosphere

File:PIA23683-Uranus-MagneticField-20200325.gif

The magnetic field of Uranus
(animated; 25 March 2020)

Before the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, scientists had expected the magnetic field of Uranus to be in line with the solar wind, because it would then align with Uranus's poles that lie in the ecliptic.<ref name="Ness Acuña et al. 1986" />

VoyagerTemplate:'s observations revealed that Uranus's magnetic field is peculiar, both because it does not originate from its geometric centre, and because it is tilted at 59° from the axis of rotation.<ref name="Ness Acuña et al. 1986" /><ref name="Russell993" /> In fact, the magnetic dipole is shifted from Uranus's centre towards the south rotational pole by as much as one-third of the planetary radius.<ref name="Ness Acuña et al. 1986" /> This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 μT), whereas in the northern hemisphere it can be as high as 1.1 gauss (110 μT).<ref name="Ness Acuña et al. 1986" /> The average field at the surface is 0.23 gauss (23 μT).<ref name="Ness Acuña et al. 1986" />

File:Uranian Magnetic field.gif

A diagram showing Uranus's asymmetric magnetosphere

Studies of Voyager 2 data in 2017 suggest that this asymmetry causes Uranus's magnetosphere to connect with the solar wind once a Uranian day, opening the planet to the Sun's particles.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator.<ref name="Russell993" /> The dipole moment of Uranus is 50 times that of Earth.<ref name="Ness Acuña et al. 1986" /><ref name="Russell993" /> Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.<ref name="Russell993" /> One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giants, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.<ref name="Atreya2006" /><ref>Template:Cite journal</ref> Another possible explanation for the magnetosphere's alignment is that there are oceans of liquid diamond in Uranus's interior that would deter the magnetic field.<ref name="Bland, Eric" />

It is, however, unclear whether the observed asymmetry of Uranus's magnetic field represents the typical state of the magnetosphere, or a coincidence of observing it during unusual space weather conditions. A post-analysis of Voyager data from 2024 suggests that the strongly asymmetric shape of the magnetosphere observed during the fly-by represents an anomalous state, as the measured values of solar wind density at the time were unusually high, which could have compressed Uranus's magnetosphere. The interaction with the solar wind event could also explain the apparent paradox of presence of strong electron radiation belts despite the otherwise low magnetospheric plasma density measured. Such conditions are estimated to occur less than 5% of the time.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail, and radiation belts.<ref name="Ness Acuña et al. 1986" /><ref name="Russell993" /><ref name="Krimigis Armstrong et al. 1986" /> Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's.<ref name="Ness Acuña et al. 1986" /><ref name="Russell993" /> Uranus's magnetotail trails behind it into space for millions of kilometres and is twisted by its sideways rotation into a long corkscrew.<ref name="Ness Acuña et al. 1986" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web

}}</ref>

File:Alien aurorae on Uranus (remastered).jpg

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Uranus's magnetosphere contains charged particles: mainly protons and electrons, with a small amount of H₂⁺ ions.<ref name="Russell993" /><ref name="Krimigis Armstrong et al. 1986" /> Many of these particles probably derive from the thermosphere.<ref name="Krimigis Armstrong et al. 1986" /> The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.<ref name="Krimigis Armstrong et al. 1986" /> The density of low-energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm⁻³.<ref name="Bridge1986" /> The particle population is strongly affected by the Uranian moons, which sweep through the magnetosphere, leaving noticeable gaps.<ref name="Krimigis Armstrong et al. 1986" /> The particle flux is high enough to cause darkening or space weathering of their surfaces on an astronomically rapid timescale of 100,000 years.<ref name="Krimigis Armstrong et al. 1986" /> This may be the cause of the uniformly dark colouration of the Uranian satellites and rings.<ref name="summary" />

Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.<ref name="Herbert & Sandel 1999" /> Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere.<ref name="Lam Miller et al. 1997" /> They, or rather their trihydrogen cations' infrared spectral emissions, have been studied in-depth as of late 2023.<ref name="Thomas Melin Stallard Chowdhury 2023 pp. 1473–1480">Template:Cite journal</ref>

In March 2020, NASA astronomers reported the detection of a large atmospheric magnetic bubble, also known as a plasmoid, released into outer space from the planet Uranus, after reevaluating old data recorded by the Voyager 2 space probe during a flyby of the planet in 1986.<ref name="NASA-20200325">Template:Cite news</ref><ref name="NYT-20200327">Template:Cite news</ref>

Moons

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File:Uranian moon montage, albedo corrected.png

Major moons of Uranus in order of increasing distance (left to right), at their proper relative sizes and albedos. From left to right, they are Miranda, Ariel, Umbriel, Titania, and Oberon. (collage of Voyager 2 photographs)

File:Annotated Moons of Uranus.png

Uranus along with its five major moons and nine inner moons as taken by the James Webb Space Telescope's NIRCam.

Uranus has 28 known natural satellites.<ref name="Sheppardmoons2024" /> The names of these satellites are chosen from characters in the works of William Shakespeare and Alexander Pope.<ref name="Faure2007" /><ref name="Nineplanets" /> The five main satellites are Miranda, Ariel, Umbriel, Titania, and Oberon.<ref name="Faure2007" /> The Uranian satellite system is the least massive among those of the giant planets; the combined mass of the five major satellites would be less than half that of Triton (largest moon of Neptune) alone.<ref name="Jacobson Campbell et al. 1992" /> The largest of Uranus's satellites, Titania, has a radius of only Template:Convert, or less than half that of the Moon, but slightly more than Rhea, the second-largest satellite of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus's satellites have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light).<ref name="Smith Soderblom et al. 1986" /> They are ice–rock conglomerates composed of roughly 50% ice and 50% rock. The ice may include ammonia and carbon dioxide.<ref name="summary" /><ref name="Hussmann2006" />

Among the Uranian satellites, Ariel appears to have the youngest surface, with the fewest impact craters, and Umbriel the oldest.<ref name="Smith Soderblom et al. 1986" /><ref name="summary" /> Miranda has fault canyons Template:Convert deep, terraced layers, and a chaotic variation in surface ages and features.<ref name="Smith Soderblom et al. 1986" /> Miranda's past geologic activity is thought to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a former 3:1 orbital resonance with Umbriel.<ref name="Tittemore Wisdom 1990" /> Extensional processes associated with upwelling diapirs are the likely origin of Miranda's 'racetrack'-like coronae.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Ariel is thought to have once been held in a 4:1 resonance with Titania.<ref name="Tittemore 1990" />

Uranus has at least one horseshoe orbiter occupying the Sun–Uranus Template:L3 Lagrangian point—a gravitationally unstable region at 180° in its orbit, 83982 Crantor.<ref name="coorbital1">Template:Cite journal</ref><ref name="coorbital2">Template:Cite journal</ref> Crantor moves inside Uranus's co-orbital region on a complex, temporary horseshoe orbit. Template:Mpl is also a promising Uranus horseshoe librator candidate.<ref name="coorbital2" />

Rings

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File:Annotated Uranian rings.png

Uranus's rings, inner moons, and atmosphere as imaged by the James Webb Space Telescope's near-infrared camera.

The Uranian rings are composed of extremely dark particles, which vary in size from micrometres to a fraction of a metre.<ref name="Smith Soderblom et al. 1986" /> Thirteen distinct rings are presently known, the brightest being the ε ring. All except the two rings of Uranus are extremely narrow—they are usually a few kilometres wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as a result of those impacts, only a few particles survived, in stable zones corresponding to the locations of the present rings.<ref name="summary" /><ref name="Esposito2002" />

William Herschel described a possible ring around Uranus in 1789. This sighting is generally considered doubtful, because the rings are quite faint, and in the two following centuries none were noted by other observers. Still, Herschel made an accurate description of the epsilon ring's size, its angle relative to Earth, its red colour, and its apparent changes as Uranus travelled around the Sun.<ref>Template:Cite news</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The ring system was definitively discovered on 10 March 1977 by James L. Elliot, Edward W. Dunham, and Jessica Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 (also known as HD 128598) by Uranus to study its atmosphere. When their observations were analysed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind Uranus. They concluded that there must be a ring system around Uranus.<ref name="Elliot1977" /> Later, they detected four additional rings.<ref name="Elliot1977" /> The rings were directly imaged when Voyager 2 passed Uranus in 1986.<ref name="Smith Soderblom et al. 1986" /> Voyager 2 also discovered two additional faint rings, bringing the total number to eleven.<ref name="Smith Soderblom et al. 1986" />

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located twice as far from Uranus as the previously known rings. These new rings are so far from Uranus that they are called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In April 2006, images of the new rings from the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other one red.<ref name="dePater2006" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.<ref name="dePater2006" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In contrast, Uranus's inner rings appear grey.<ref name="dePater2006" />

Although the Uranian rings are very difficult to directly observe from Earth, advances in digital imaging have allowed several amateur astronomers to successfully photograph the rings with red or infrared filters; telescopes with apertures as small as Template:Convert may be able to detect the rings with proper imaging equipment.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Exploration

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File:Uranus seen from Saturn by Cassini.jpg

Uranus as seen from the Cassini spacecraft at Saturn

Launched in 1977, Voyager 2 made its closest approach to Uranus on 24 January 1986, coming within Template:Convert of the cloudtops, before continuing its journey to Neptune. The spacecraft studied the structure and chemical composition of Uranus's atmosphere,<ref name="Tyler 1986" /> including its unique weather, caused by its extreme axial tilt. It made the first detailed investigations of its five largest moons and discovered 10 new ones. Voyager 2 examined all nine of the system's known rings and discovered two more.<ref name="Smith Soderblom et al. 1986" /><ref name="summary" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail caused by Uranus's sideways orientation.<ref name="Ness Acuña et al. 1986" />

No other spacecraft has flown by Uranus since then, though there have been many proposed missions to revisit the Uranus system. The possibility of sending the Cassini spacecraft from Saturn to Uranus was evaluated during a mission extension planning phase in 2009, but was ultimately rejected in favour of destroying it in the Saturnian atmosphere,<ref name="spilker" /> as it would have taken about twenty years to get to the Uranian system after departing Saturn.<ref name="spilker" /> A Uranus entry probe could use Pioneer Venus Multiprobe heritage and descend to 1–5 atmospheres.<ref name="uop" /> A Uranus orbiter and probe was recommended by the 2013–2022 Planetary Science Decadal Survey published in 2011; the proposal envisaged launch during 2020–2023 and a 13-year cruise to Uranus.<ref name="uop" /> The committee's opinion was reaffirmed in 2022, when a Uranus probe/orbiter mission was placed at the highest priority, due to the lack of knowledge about ice giants.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Most recently, the CNSA's Tianwen-4 Jupiter orbiter, launching in 2029, is planned to have a subprobe that will detach and get a gravity assist instead of entering orbit, flying by Uranus in March 2045 before heading to interstellar space.<ref name="TianwenPlSoc"/> China also has plans for a potential Tianwen-5 that may orbit either Uranus or Neptune that have yet to come to fruition.<ref name="TianwenPlSoc"/>

In culture

As well as being a popular subject in fiction, Uranus has inspired artistic works including Lydia Sigourney's 1827 poem Template:Ws and a movement in Gustav Holst's orchestral suite The Planets, written between 1914 and 1916. Herschel's discovery of the planet is also referenced in the lines "Then felt I like some watcher of the skies/When a new planet swims into his ken", from John Keats's poem "On First Looking into Chapman's Homer".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The planet's discovery also inspired the naming of the chemical element uranium, itself discovered in 1789 by the German chemist Martin Heinrich Klaproth.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In modern astrology, the planet Uranus (symbol File:Uranus monogram.svg) is the ruling planet of Aquarius; prior to the discovery of Uranus, the ruling planet of Aquarius was Saturn. Because Uranus is cyan and Uranus is associated with electricity, the colour electric blue, which is close to cyan, is associated with the sign Aquarius.<ref>Template:Cite book</ref>

Operation Uranus was the successful military operation in World War II by the Red Army to take back Stalingrad and marked the turning point in the land war against the Wehrmacht. It was part of a series of operations named after planets, including Mars and Saturn.Template:Citation needed

See also

• Template:Mpl and Template:Mpl, the only two known Uranus trojans
• Colonisation of Uranus
• Extraterrestrial diamonds (thought to be abundant in Uranus)
• Outline of Uranus
• Statistics of planets in the Solar System
• Uranus in astrology
• Uranus in fictionTemplate:Div col end

Further reading

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• Template:Cite book
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External links

• Uranus at European Space Agency
• Uranus at NASA's Solar System Exploration site
• Uranus at Jet Propulsion Laboratory's planetary photojournal (photos)
• Voyager at Uranus Template:Webarchive (photos)
• Uranian system montage (photo)
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• Interactive 3D gravity simulation of the Uranian system Template:Webarchive
• Template:Citation