# Uranus <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Uranus in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Uranus&embed=1" data-title="Uranus in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Uranus`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 10 years/s and watch Uranus complete a lap in a little over eight seconds while its tipped axis keeps pointing the same way in space; drag the year slider from 1986 to 2028 to carry the planet half an orbit, from the solstice that turned its south pole to the Sun to the one that turns its north pole there; then switch the scale to true to see the planets at their real distances, with Uranus about 19 AU from the Sun and the inner planets crowded into the centre.* **Uranus** is the seventh planet from the [[Sun]] and the lighter of the two [[Ice_giant|ice giants]]: a world of 14.5 Earth masses whose bulk is a hot, dense fluid of [[Water|water]], ammonia and methane beneath an envelope of [[Hydrogen|hydrogen]] and [[Helium|helium]].[^nasa-fs-u][^podolak1995] It orbits at a mean distance of 19.2 [[Astronomical_unit|AU]] once every 84 years, and its spin axis lies almost in the plane of that orbit, tipped 97.77° from the orbit's pole, so that each pole spends about 42 years in daylight and then 42 years in darkness.[^nasa-fs-u][^sromovsky2006] William Herschel identified it in 1781, the first planet found with a telescope, and it was named after the Greek sky god whose Latin form is Uranus.[^herschel1781][^gingerich1958] Unlike the other giants it gives off almost no heat of its own; its tropopause, at 49 K, is the lowest minimum temperature of any planet.[^pearl1990] It has a faint, dark ring system, 29 known moons, and a magnetic field tilted 59° from its axis and offset far from its centre.[^smith1986][^sheppard2024][^ness1986] The only spacecraft to visit was Voyager 2, in January 1986.[^nasa-explore] The explorer at the top of this page is locked on Uranus. It draws the planet's orbit from JPL elements, around 19 AU from the Sun between [[Saturn]] and [[Neptune]], and shows the axis tilted 97.8°, the geometry that gives the planet its extreme seasons.[^jpl-t1][^nasa-fs-u] ## History Uranus is just bright enough to see without a telescope, but it is faint and creeps along the sky so slowly that no ancient observer singled it out from the stars.[^mira] Its recognition in 1781 was the first addition to the planets known since antiquity, and because its orbit is about twice the size of [[Saturn]]'s it roughly doubled the known radius of the Solar System.[^herschel1781][^nasa-fs-u] ### Discovery A star in Hipparchus's catalogue of 128 BC that does not exist matches the planet's position at that time.[^bourtembourg2013] The first definite sighting was by John Flamsteed in 1690, who catalogued it as a star, 34 Tauri; James Bradley, Tobias Mayer and Pierre Charles Le Monnier, who logged it at least twelve times between 1750 and 1769, all missed its motion.[^alexander1965] William Herschel, surveying stars with a home-built 6.2-inch reflector in the garden of his house in Bath, noticed it on 13 March 1781.[^herschel1781] Its disc and its slow shift from night to night marked it as something other than a star, and he reported it to the Royal Society as a comet.[^herschel1781] The Astronomer Royal, Nevil Maskelyne, was unsure: it could as well be a planet on a nearly circular orbit as a comet on a very eccentric one, and he had seen no coma or tail.[^miner1998] Anders Johan Lexell computed the first orbit and found it nearly circular, and Johann Elert Bode reached the same conclusion; by 1783 Herschel accepted that he had found a primary planet.[^lexell1783][^dreyer1912] King George III granted him a stipend of £200 a year on condition that he move to Windsor.[^miner1998] ### Name The name took about seventy years to settle. Herschel proposed *Georgium Sidus*, "George's Star", after his patron.[^dreyer1912] Jérôme Lalande suggested "Herschel", and others offered Neptune, Cybele and Astraea.[^herschel1917][^gingerich1958] Bode argued in 1782 for Uranus, the sky god: since Saturn was the father of Jupiter, the next planet out should bear the name of Saturn's father.[^littmann2004] Martin Klaproth named his new element [[Uranium|uranium]] after the planet in 1789, and the last holdout, HM Nautical Almanac Office, adopted "Uranus" in 1850.[^littmann2004][^hobart2013] Two symbols exist: ⛢, devised by Johann Gottfried Köhler at Bode's request,[^jahrbuch1785] and ♅, a globe topped by Herschel's initial, proposed by Lalande in 1784.[^herschel1917] ## Formation The split between the gas giants and the ice giants is thought to reflect how fast each grew. In the core-accretion picture, a solid core built from dust and ice in the [[Protoplanetary_disk|protoplanetary disc]] begins to hold nebular gas once it is massive enough, and beyond a critical mass its gas envelope grows in a runaway.[^dangelo2018][^dangelo2013] [[Jupiter]] and Saturn crossed that threshold. Uranus and Neptune gathered only a few Earth masses of gas before the nebula dispersed, so ices and rock dominate their bulk.[^thommes1999][^brunini1999] Simulations place the birth of both ice giants closer in, in the region between Jupiter and Saturn, followed by an outward migration to their present orbits; this migration is part of the [[Nice_model|Nice model]] of the early [[Formation_and_evolution_of_the_Solar_System|Solar System]].[^thommes1999] The planet's tilt is usually attributed to a later event: an impact by a body of about Earth's mass or larger, which in simulations by Jacob Kegerreis and colleagues occurs 3–4 billion years ago.[^bergstralh1991][^borenstein2018] ## Orbit and rotation Uranus's [[Orbit|orbit]] has a semi-major axis of 2,867 million km (19.2 AU) and an eccentricity of 0.047. Its distance from the Sun therefore ranges from 2,733 to 3,001 million km, or about 18.3 to 20.1 AU, a spread of 1.8 AU that is the largest of any planet, though smaller than that of the dwarf planet [[Pluto]].[^nasa-fs-u][^meeus1998] One orbit takes 30,685 days, or 84.0 years, as [[Kepler's_laws_of_planetary_motion|Kepler's third law]] requires for that distance (19.19^1.5 ≈ 84.1, derived); since the discovery the planet has twice returned to its 1781 position near Zeta Tauri, in 1865 and 1949, and will do so again in 2033.[^nasa-fs-u][^fahad2022] Sunlight there averages 3.69 W/m², about 1/370 of what reaches [[Earth]], as the inverse-square law predicts for 19.2 AU (1/19.2² ≈ 1/369, derived).[^nasa-fs-u] Irregularities in the orbit led to the next planet. Pierre-Simon Laplace computed Uranus's orbital elements in 1783, but over the following decades the planet drifted from its predicted track. John Couch Adams in 1841 and Urbain Le Verrier in 1845 independently attributed the discrepancy to an unseen outer planet, and Johann Gottfried Galle found Neptune close to Le Verrier's position on 23 September 1846.[^forbes1909][^oconnor1996] The interior rotates once every 17 h 14 min 52 s, a period refined in 2025 by tracking Uranus's aurorae with the Hubble Space Telescope.[^lamy2025][^nasa-hubble2025] The atmosphere does not rotate rigidly: near 60° S, cloud features circle the planet in as little as 14 hours.[^gierasch2004] ### Axial tilt The rotation axis is nearly parallel to the orbital plane. Under the IAU convention, which puts the north pole on the same side of the invariable plane as Earth's, the tilt is 82.23° and the rotation retrograde; defining north by the right-hand rule instead gives 97.77° and prograde rotation.[^nasa-fs-u][^masl2003] Near a solstice one pole points almost at the Sun and the other stays in night, and only a narrow equatorial strip sees a quick day–night cycle; near an equinox the Sun is over the equator and days and nights alternate as on other planets.[^sromovsky2006] The last southern summer solstice fell in 1986, during the Voyager 2 flyby; the equinox came in 2007, and the northern summer solstice falls in 2028.[^hammel2006] Laplace had already inferred the strange orientation in 1805, from the inclination of the orbits of Titania and Oberon.[^laplace1839] ### Visibility from Earth The planet's mean apparent magnitude is 5.68, varying between 5.38 and 6.03 depending partly on which latitudes face the Sun and Earth, so it sits at the limit of naked-eye visibility under dark skies.[^mallama2018][^schmude2015] Its disc spans only 3.3–4.1 arcseconds; a telescope of 25 cm aperture or more can show cloud patterns and the moons Titania and Oberon.[^nasa-fs-u][^nowak2006] ## Internal structure With a mass of 8.68 × 10²⁵ kg and a mean radius of 25,362 km, Uranus has a mean [[Density|density]] of 1,270 kg/m³, the second lowest of the planets after Saturn's.[^nasa-fs-u] That density is too high for a mostly hydrogen planet and too low for a rocky one, and the standard model fits it with three layers: a small core of silicate and iron–nickel rock of about 0.55 Earth masses, a mantle of about 13.4 Earth masses of hot, dense fluid made of water, ammonia and methane, and an outer envelope of hydrogen and helium of roughly 0.5 Earth masses occupying the last fifth of the radius.[^podolak1995][^faure2007] At the centre the model gives a pressure of about 800 GPa and a temperature near 5,000 K.[^podolak2000] The "ices" of the mantle are a conducting fluid, sometimes described as a water–ammonia ocean.[^atreya2006] The solution is not unique: mixing more hydrogen and rock into the mantle lowers the ice fraction, and the data allow between 9.3 and 13.5 Earth masses of ice.[^podolak2000] Laboratory work suggests that methane breaks down under mantle conditions and that its [[Carbon|carbon]] may form diamond; shock-compressed hydrocarbons have produced nanodiamonds, and at still higher pressure diamond may melt into liquid carbon.[^kraus2017][^eggert2010] Water deep inside may also become ionic, or superionic, with [[Oxygen|oxygen]] fixed in a lattice and hydrogen ions moving through it.[^shiga2010] The planet has no solid surface. By convention the level where the pressure is 1 bar (100 kPa) serves as a reference; there the equatorial radius is 25,559 km and the polar radius 24,973 km.[^seidelmann2007] ### Internal heat Uranus's [[Thermal_radiation|thermal emission]] is only about 1.06 times the solar energy it absorbs, whereas [[Neptune]], its near twin, emits 2.61 times as much.[^pearl1990][^sromovsky2005] The internal heat flux is about 0.042 W/m², less than the 0.075 W/m² that leaks from [[Earth]]'s interior.[^pearl1990] One proposal links this to the tilt-producing impact, which may have released much of the primordial heat. Another is that layers of different composition in the interior block [[Convection|convection]], so that heat escapes only slowly by diffusion.[^hawksett2005][^podolak1995] High-pressure experiments show that magnesium dissolves readily in hot water, and a magnesium-rich layer could act as insulation.[^kim2021] ## Atmosphere What is called the atmosphere is the part of the fluid envelope reachable by remote sensing, down to about 300 km below the 1 bar level, where the pressure is near 100 bar and the temperature 320 K.[^depater1991] It has a troposphere up to about 50 km, a stratosphere to about 4,000 km and a thermosphere extending to some 50,000 km, with no mesosphere between them.[^lunine1993] ### Composition By volume the gas is about 82.5% molecular hydrogen, 15.2% helium and 2.3% methane.[^nasa-fs-u] The helium mass fraction, 0.26, is close to the protosolar 0.275, so helium has not rained out toward the interior as it has in Saturn.[^conrath1987][^lodders2003] Methane absorbs red and near-infrared light, as [[Spectroscopy|spectroscopy]] shows, which gives the planet its cyan colour; below its cloud deck at 1.3 bar it corresponds to 20–30 times the solar carbon abundance.[^lunine1993][^lindal1987] Ultraviolet light breaks methane into ethane, acetylene and other hydrocarbons in the stratosphere.[^summers1989] Carbon monoxide, carbon dioxide and water vapour are also present high up, and must have arrived from outside, in dust or [[Comet|comets]].[^encrenaz2004] ### Troposphere Temperature falls from about 320 K at the base of the observable troposphere to 49–57 K at the tropopause, depending on latitude.[^lunine1993][^hanel1986] Most of the planet's thermal emission comes from this layer, which sets its effective temperature near 59 K.[^pearl1990] Models place water clouds at 50–100 bar, ammonium hydrosulfide at 20–40 bar and ammonia or hydrogen sulfide at 3–10 bar; thin methane clouds at 1–2 bar have been observed directly.[^atreya2005][^lindal1987] ### Upper atmosphere In the stratosphere the temperature climbs from 53 K to about 800–850 K at the base of the thermosphere, heated by methane and other hydrocarbons absorbing sunlight.[^herbert1987][^young2001] Hazes of condensed ethane and acetylene may help give the planet its featureless look.[^summers1989] The thermosphere's high temperature cannot be explained by sunlight or aurorae alone.[^herbert1999] Its atomic hydrogen forms a corona reaching two planetary radii, whose drag clears dust from the rings.[^herbert1987] ## Climate At visible wavelengths Uranus looked almost blank to Voyager 2, which counted only about ten cloud features in 1986.[^smith1986] Telescopes have since shown an active but slow-changing atmosphere, and its low internal heat is one proposed reason it is quieter than [[Neptune]].[^sromovsky2005][^pearl1990] ### Banded structure, winds and clouds In 1986 the sunlit southern hemisphere showed a bright polar cap bounded near −45° latitude by an even brighter band, the "collar", both thought to be thick methane cloud at 1.3–2 bar.[^smith1986][^rages2004] As the north came into view after 2000, it at first showed no matching cap.[^hammel2005a] Around the 2007 equinox the southern collar faded and a northern one appeared near 45° N.[^sromovsky2009] In 2023 Very Large Array radio images showed a polar cyclone at the north pole.[^akins2023] Tracking of clouds gives the wind pattern. At the equator the winds blow against the rotation at 50–100 m/s; they fall to zero near ±20° and turn prograde closer to the poles, peaking near ±60° latitude, with speeds of up to about 240 m/s measured near 50° N.[^sromovsky2005][^hammel2005a] Voyager 2's radio receiver detected about 140 lightning discharges, thought to come from convective storms in deep water clouds.[^zarka1986][^aplin2020] ### Seasonal variation The weather changes with the long seasons. In 2004 large clouds briefly gave Uranus a Neptune-like look, with winds of 229 m/s and a persistent storm nicknamed the "Fourth of July fireworks", and in 2006 Hubble imaged the first dark spot seen on the planet.[^lakdawalla2004][^sromovsky-darkspot] Photometry since the 1950s shows the brightness peaking near the solstices and dipping near the equinoxes, and microwave measurements of the deep troposphere since the 1960s show the same rhythm.[^lockwood2006][^klein2006] Much of this is the changing viewing geometry, but the north was brighter at the 1944 solstice than geometry alone predicts, a sign of real change.[^karkoschka2001][^hammel2007] The thickening of methane cloud on the sunlit hemisphere is one suggested mechanism.[^rages2004] ## Magnetosphere Before 1986 the field was expected to line up with the rotation axis, as on the other planets. Voyager 2 found instead a dipole tilted about 59° to the axis and displaced from the centre toward the south pole by roughly a third of the planet's radius.[^ness1986] The field at the cloud tops therefore ranges from about 0.1 gauss (10 μT) in the south to 1.1 gauss (110 μT) in the north, averaging 0.23 gauss.[^ness1986] Neptune's field is similarly skewed, and one explanation is that ice giant fields are generated in a thin convecting shell of conducting fluid, not deep in a core.[^stanley2004][^russell1993] Otherwise the structure resembles [[Saturn]]'s more than [[Jupiter]]'s: a bow shock about 23 Uranian radii upstream, a magnetopause at 18 radii, radiation belts and a magnetotail that the planet's rotation twists into a long corkscrew.[^ness1986][^russell1993] The trapped [[Plasma_(physics)|plasma]], mainly protons and electrons, is swept by the moons and can darken their surfaces in about 100,000 years.[^krimigis1986] A 2024 reanalysis found that Voyager 2 arrived during an unusual surge of [[Solar_wind|solar wind]] that compressed the magnetosphere, a state estimated to occur less than 5% of the time.[^jasinski2024] Aurorae glow around both magnetic poles, and in 2020 re-examined Voyager data revealed a plasmoid, a bubble of plasma, breaking away down the tail.[^herbert1999][^nasa2020] ## Moons Uranus has 29 known moons, named after characters from Shakespeare and Alexander Pope.[^sheppard2024][^nasa-webb2025][^faure2007] There are five major moons, Miranda, Ariel, Umbriel, Titania and Oberon, but their combined mass is less than half that of Neptune's [[Triton_(moon)|Triton]].[^jacobson1992] Titania, the largest, has a radius of 788.9 km, less than half the [[Moon]]'s.[^nasa-fs-usat] The major moons are roughly half ice and half rock, with albedos from 0.20 for Umbriel to 0.35 for Ariel.[^smith1986][^hussmann2006] Their surfaces differ widely. Ariel shows the fewest craters and Umbriel the most.[^smith1986] Miranda has canyons 20 km deep and patchwork terrain, and its past activity is attributed to [[Tide|tidal]] heating while an old 3:1 [[Resonance|resonance]] with Umbriel kept its orbit eccentric.[^tittemore1990a] The asteroid 83982 Crantor, a [[Centaur_(small_Solar_System_body)|centaur]], currently follows a temporary horseshoe orbit in Uranus's co-orbital region.[^delafuente2013] ## Rings Herschel reported a possible ring in 1789, a sighting usually considered doubtful, though his description of the brightest ring's size and angle is accurate.[^bbc2007] The rings were confirmed only on 10 March 1977. James Elliot, Edward Dunham and Jessica Mink, observing a star pass behind Uranus from the Kuiper Airborne Observatory, saw it blink out five times before and after the planet, the signature of narrow rings.[^elliot1977] Voyager 2 imaged them in 1986 and added two, bringing the total to eleven.[^smith1986] In 2005 the Hubble Space Telescope found two faint outer rings, one sharing an orbit with the small moon Mab, for a total of 13.[^hubble2005] Keck images showed the outermost ring to be blue, possibly fine ice from Mab, and the next one red, while the inner rings are grey.[^depater2006] The ring material is very dark: the narrow rings have albedos of about 0.015, reflecting roughly 1.5% of the light that falls on them.[^nasa-fs-urings] ## Exploration Voyager 2, launched by [[NASA]] in 1977, passed 81,500 km from the cloud tops on 24 January 1986, on its way to Neptune.[^smith1986] It measured the atmosphere, found ten moons and two rings, imaged the major moons and mapped the tilted field.[^smith1986][^ness1986] It remains the only visit to the planet.[^nasa-explore] A diversion of Cassini from Saturn was considered in 2009 but rejected, since the trip would have taken about twenty years.[^spilker2008] The US decadal survey of 2011 recommended a Uranus orbiter and probe, and the 2022 survey made it the highest-priority new flagship mission.[^ssb2019][^nas2022] China's Tianwen-4 mission, planned for launch in 2029, is to release a subprobe that would fly past Uranus in 2045.[^jones2023] ## In culture Herschel's discovery supplied an image for John Keats's sonnet "On First Looking into Chapman's Homer": "Then felt I like some watcher of the skies / When a new planet swims into his ken".[^melani2009] Gustav Holst gave the planet a movement of his suite *The Planets* (1914–1916), and Klaproth's naming of [[Uranium|uranium]] gave the planet a permanent place in chemistry.[^melani2009][^hobart2013] In Western astrology Uranus is the ruling planet of Aquarius, a role it took over from Saturn.[^parker1996] ## See also - [[Neptune]] · [[Ice_giant]] · [[Gas_giant]] - [[Formation_and_evolution_of_the_Solar_System]] · [[Nice_model]] - [[Triton_(moon)]] · [[Centaur_(small_Solar_System_body)]] - [[Timeline_of_Solar_System_exploration]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived values: the ratio of sunlight at Uranus to that at Earth follows from the inverse-square law, 1/19.2² ≈ 1/369, consistent with the fact sheet's 3.69 W/m² against 1,361 W/m². The time for one lap at 10 years/s in the explorer is 84.0/10 ≈ 8.4 s. Ring albedos are from the NASA Uranus rings fact sheet (about 15 × 10⁻³ for the narrow rings). ## References [^nasa-fs-u]: Williams, D. R. "Uranus Fact Sheet". NASA NSSDCA (last updated 2 October 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html (fetched 2026-09-18). [^nasa-fs-usat]: Williams, D. R. "Uranian Satellite Fact Sheet". NASA NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/uraniansatfact.html (fetched 2026-09-18). [^nasa-fs-urings]: Williams, D. R. "Uranus Rings Fact Sheet". NASA NSSDCA (last updated 14 October 2015). https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranringfact.html (fetched 2026-09-18). [^jahrbuch1785]: Bode, J. E. (ed.) (1782). *Astronomisches Jahrbuch für das Jahr 1785*. George Jacob Decker, Berlin, p. 191. [^bbc2007]: BBC News (19 April 2007). 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University of Arizona Press. - Alexander, A. F. O'D. (1965). *The Planet Uranus: A History of Observation, Theory, and Discovery*. American Elsevier. ## External links - NASA Science: Uranus. https://science.nasa.gov/uranus/ - NASA NSSDCA Uranus fact sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html - ESA: Uranus. https://www.esa.int/Science_Exploration/Space_Science/Uranus - JPL Photojournal: Uranus. https://photojournal.jpl.nasa.gov/target/uranus ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Uranus) : [Wikitube](https://en.wikitube.io/wiki/Uranus) · pinned revision [1374546941](https://en.wikipedia.org/w/index.php?oldid=1374546941) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-017 · explorer state `?obj=Uranus`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->