Uranus

For the Greek god, see Uranus (mythology). For other uses, see Uranus (disambiguation) and Uranian (disambiguation).

Uranus is the seventh planet from the Sun. It is a gas ice giant with a cyan colour. Most of the planet is made of water, ammonia, and methane in a supercritical phase of matter. Astronomers call these materials "ice" or "volatiles."

The planet's atmosphere has layers of clouds. It is the coldest of all the planets in the Solar System, at about 49 K (−224 °C; −371 °F). Uranus is tipped over on its side, with an axial tilt of 82.23°. It spins in a retrograde direction, and one day on Uranus lasts about 17 hours and 14 minutes. It takes 84 Earth years to go around the Sun. Because of its tilt, each pole gets about 42 years of sunlight, followed by 42 years of darkness.

Uranus is the third-largest planet by diameter and the fourth-largest by mass. Scientists think there is a rocky core inside a thick mantle layer. Above that is a thick atmosphere of hydrogen and helium. Small amounts of hydrocarbons, which may form through hydrolysis, along with carbon monoxide and carbon dioxide, have been found in the upper atmosphere. The carbon dioxide may have come from comets. Many of the weather patterns in Uranus's atmosphere are still not understood. These include its very strong winds, which can reach 900 km/h (560 mph), changes in its polar cap, and its uneven clouds. The planet also gives off very little internal heat compared to other giant planets. Scientists do not yet know why.

Like the other giant planets, Uranus has a ring system, a magnetosphere, and many natural satellites. Its rings are very dark and reflect only about 2% of the light that reaches them. Uranus has 29 known natural satellites. Of these, 19 are regular moons, and 14 of those are small inner moons. Farther out are the five larger major moons: Miranda, Ariel, Umbriel, Titania, and Oberon. Much farther away are the ten known irregular moons. The planet's magnetosphere is very uneven and holds many charged particles. These particles may help explain why its rings and moons are dark.

Uranus can be seen with the naked eye, but it is very dim and moves slowly against the background stars. It was not called a planet until 1781, when William Herschel first saw it. About seven decades later, it was decided that the planet would be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities. As of 2026, Uranus has been visited only once, in 1986, when the Voyager 2 probe flew past it. Today telescopes can resolve and observe it, but scientists still want to visit it again. The Planetary Science Decadal Survey made the proposed Uranus Orbiter and Probe mission a top priority for 2023–2032. The CNSA has also proposed flying past the planet with a smaller probe from Tianwen-4.

History

Like the classical planets, Uranus can be seen with the naked eye. Ancient observers never recognised it as a planet, because it is dim and moves slowly. William Herschel first observed Uranus on 13 March 1781. This led to its discovery as a planet, which expanded the known edges of the Solar System for the first time in history. Uranus was the first planet found with the aid of a telescope. Its discovery also roughly doubled the known size of the Solar System, because Uranus is about twice as far from the Sun as Saturn is.

Before it was recognised as a planet, Uranus had been seen many times but was usually mistaken for a star. It may have been observed by Hipparchus in 128 BC, whose star positions were later included in Ptolemy's Almagest. The earliest sure sighting was in 1690, when John Flamsteed recorded it as 34 Tauri. Others who saw it include James Bradley, Tobias Mayer, and the French astronomer Pierre Charles Le Monnier.

Herschel observed Uranus from the garden of his house at 19 New King Street in Bath, Somerset, England (now the Herschel Museum of Astronomy). He first reported it as a comet, while studying the parallax of the fixed stars with a homemade telescope. When he told the Royal Society about it, he still called it a comet. The Astronomer Royal, Nevil Maskelyne, was unsure what it was. Other astronomers soon thought it might be a planet. Anders Johan Lexell found its nearly circular orbit, and Johann Elert Bode described it as a planet-like object beyond Saturn. By 1783, Herschel accepted that it was a planet, and he told Joseph Banks so. To honour this, King George III gave Herschel a yearly stipend on condition that he move to Windsor.

The name Uranus comes from the ancient Greek sky god Uranus (Ancient Greek), known as Caelus in Roman myth. He was the father of Cronus (Saturn), grandfather of Zeus (Jupiter), and great-grandfather of Ares (Mars). It is the only one of the eight planets whose English name comes from Greek mythology. Astronomers prefer a pronunciation with the long u and stress on the first syllable. Pamela Gay, an astronomer at Southern Illinois University Edwardsville, noted that the other pronunciation can sound like a rude word to English speakers.

Agreement on the name took almost 70 years. Herschel first wanted to call the planet Georgium Sidus (George's Star), in honour of King George III. Other names were proposed. Jérôme Lalande suggested Herschel, and Erik Prosperin suggested Astraea, Cybele, and Neptune, the last of which later became the name of the next planet to be discovered. Georg Lichtenberg from Göttingen also supported Astraea. Some astronomers supported Neptune to honour the victories of the Royal Naval fleet in the American Revolutionary War. Daniel Bernoulli and Minerva were also suggested.

In 1782, Bode proposed the name Uranus, the Latin form of the Greek sky god, arguing that it fit with the mythology of the other planets. Bode's Royal Academy colleague Martin Klaproth named his newly found element uranium in support. Bode's suggestion became the most widely used, and in 1850 the HM Nautical Almanac Office switched to Uranus, the last holdout.

Uranus has two astronomical symbols. The first was proposed by Johann Gottfried Köhler in 1782. The second was suggested by Lalande in 1784 and is nearly universal in astrology.

Uranus has many names in other languages. Its name is translated as the "Heavenly King star" in Chinese, Japanese, Korean, and Vietnamese. In Thai, another name comes from the Sanskrit word Mrtyu, meaning 'death'. In Mongolian, it means 'King of the Sky'. In Hawaiian, its name is a rendering of 'Herschel'.

Formation

Main article: Formation and evolution of the Solar System

For details of the evolution of Uranus's orbit, see Nice model.

Some scientists think the differences between the ice giants and the gas giants come from how they formed. The Solar System is thought to have formed from a spinning disk of gas and dust called the presolar nebula. Most of the gas, mainly hydrogen and helium, became the Sun. Dust grains stuck together to form the first protoplanets.

As the planets grew, some became big enough that their gravity could hold on to the leftover gas. The more gas a planet held, the bigger it grew. Then it grew very quickly. The ice giants had only a few Earth masses of gas, so they never grew this fast. Computer simulations of planetary migration suggest that both ice giants formed closer to the Sun than they are today. They later moved outward (the Nice model).

Uranus orbits the Sun once every 84 years. It was discovered in 1781. Since then, the planet has returned to the spot where it was first seen, northeast of the double star Zeta Tauri, two times: in March 1865 and March 1949. It will return there again in April 2033.

Because its orbit takes about 84 years, its position in the stars is very close to where it was 84 years before. Its average distance from the Sun is about 20 AU (3 billion km; 2 billion mi). Its distance from the Sun changes more than that of any other planet, though less than that of dwarf planet Pluto. Astronomers first worked out Uranus's orbit in 1783, with help from Pierre-Simon Laplace. Later, John Couch Adams and Urbain Le Verrier suggested that an unseen planet might be pulling on Uranus. On 23 September 1846, Johann Gottfried Galle found a new planet, later named Neptune, close to where Le Verrier had said it would be.

Uranus spins once in about 17 hours, 14 minutes, and 52 seconds. Scientists measured this by tracking its aurorae and the winds in its atmosphere. Its axis is tilted so far over that its seasons are unlike those of the other planets. Near the solstice, one pole faces the Sun all the time, while the other faces away. Each pole gets about 42 years of sunlight, followed by 42 years of darkness. Scientists think a large object may have hit Uranus long ago, but the cause of its tilt is not known for certain. Scientists first suggested the unusual tilt after Titania and Oberon were discovered.

From Earth, Uranus is at the edge of what the unaided eye can see. It can be spotted with binoculars in dark skies. Through a telescope, it looks like a pale cyan disk. A large telescope may show some of its cloud patterns and its larger moons.

Internal structure

Uranus is the least massive of the giant planets. It has about 14.5 times the mass of Earth. Its diameter is about four times Earth's and is slightly larger than Neptune's. Its dense material is mostly ices such as water, ammonia, and methane. It also has some hydrogen and helium, and a small amount of rocky material. The standard model describes three layers. At the centre is a small rocky core made of silicate and iron–nickel. Above it is a thick, icy mantle. The outer layer is a envelope of hydrogen and helium gas. The mantle is not solid ice as we usually think of it. It is a hot, dense fluid of water, ammonia, and other volatiles, sometimes called a water–ammonia ocean. The core reaches a pressure of 8 million bars (800 GPa) and a temperature of about 5000 K. Scientists are not sure of the exact amounts of each material, because other models also fit the observations.

Extreme heat and pressure deep inside Uranus may break apart methane. This could form diamond crystals that fall through the mantle like hailstones. Experiments at the Lawrence Livermore National Laboratory suggest that a liquid carbon ocean may lie at the base of the mantle. Uranus has no solid surface, so scientists use a gas-to-liquid boundary as a reference point. At this boundary, the air pressure equals 1 bar. Uranus is also unusually cold inside. Neptune, its near twin, gives off 2.61 times as much energy as it gets from the Sun. Uranus gives off almost no extra heat. Its tropopause has recorded temperatures as low as 49 K, which makes Uranus the coldest planet in the Solar System. Scientists have proposed a few reasons. A large impact may have drained much of Uranus's internal heat. Or a barrier in its upper layers may keep heat from rising, possibly through heat transport such as double diffusive convection. A 2021 study found that magnesium dissolved in the liquid interior could form a thermal insulation layer, which might explain the low temperature. The study used minerals such as olivine and ferropericlase to copy these conditions. Uranus's heat flux is 0.042 W/m². This is lower than Earth's, and the reason is still not fully understood.

Atmosphere

Main article: Atmosphere of Uranus

Uranus has no solid surface inside. The outer part of its gas layers that scientists can study from far away is called its atmosphere. It has three layers. The lowest and densest is the troposphere, where the temperature gets colder the higher you go. Above it is the stratosphere, where the temperature generally gets warmer and the air becomes very thin. Its pressure can drop as low as 10⁻¹⁰ bar (about 10 μPa). The outermost layer is the thermosphere, which reaches far out from the planet. Uranus does not have a mesosphere.

Most of the atmosphere is molecular hydrogen and helium. Helium makes up a small part of the gas by molar fraction. This is about the same amount as in the early Sun, so it has not sunk into the planet's centre as it has in the gas giants. Methane (CH₄) is the third most common gas. Its absorption bands in visible and near-infrared light give Uranus its aquamarine or cyan colour. Small amounts of hydrocarbons, such as ethane, methylacetylene, and diacetylene, form when sunlight's ultraviolet rays break down methane through photolysis. Traces of carbon monoxide and carbon dioxide are also found high up. They probably come from outside sources such as dust and comets. Deeper down, hydrogen sulphide and other compounds are not well known. In the coldest part of the troposphere, heat escaping from the planet as far infrared light sets its effective temperature. Clouds there include ammonium hydrosulfide. The amounts of hydrocarbons in the stratosphere are measured as mixing ratios. The outer corona reaches about two Uranian radii out from the surface. Its ionosphere is denser than those of Saturn or Neptune. It is mostly powered by sunlight and changes with solar activity. Auroral activity is weak compared with Jupiter and Saturn. The corona's effects include drag on small particles, which thins the dust in the rings.

Climate

Main article: Climate of Uranus

In visible and ultraviolet light, Uranus's atmosphere looks plain compared with the other giant planets, even Neptune. In 1986, Voyager 2 saw only 10 cloud features across the whole planet. One idea is that Uranus is the coldest planet in the Solar System. It gives off much less inner heat than the other giant planets. Voyager 2 found that the southern half had a bright polar cap and dark bands. The boundary was at about −45° of latitude. A bright band called a southern "collar" lies between −45 and −50°. It is the brightest large feature there.

Voyager 2 could not see the northern half in 1986. Early in the 2000s, the Hubble Space Telescope and Keck telescope did not see a collar or cap there. In 2023, a team using the Very Large Array saw a dark collar at 80° latitude. They also saw a bright spot at the north pole, which suggests a polar vortex. Until 2006, scientists had not seen dark spots on Uranus like those on Neptune. In that year, the first one, called Uranus Dark Spot, was photographed. Scientists tracked clouds to measure zonal winds. At the equator, these winds blow opposite to the planet's spin. Near the north, they can reach speeds of 860 km/h. In 1986, the Voyager 2 instrument recorded 140 lightning flashes. Uranus also has extreme seasons because of its tilt. Scientists have studied its brightness over time using two spectral bands. They also used microwave measurements of its deeper atmosphere. In 2007, when the planet passed an equinox, a faint northern collar appeared. Scientists think the changes in brightness may be linked to meridional patterns. The changes in microwave emission are probably caused by changes in deep circulation.

Magnetosphere

Before Voyager 2 arrived, no one had measured the magnetosphere of Uranus, so scientists did not know much about it. Before 1986, scientists had expected the magnetic field of Uranus to line up with the solar wind. Voyager showed that the magnetic field of Uranus is unusual. It does not start from the centre of the planet, and it is tilted at 59° from the axis of rotation. The field on the surface in the southern half can be very weak, while in the northern half it can be much stronger.

Studies of Voyager 2 data in 2017 suggest that this unusual shape causes the magnetosphere of Uranus to connect with the solar wind once every Uranian day. This lets particles from the Sun reach the planet. Neptune has a similarly shifted and tilted magnetic field, so this may be common among ice giants. It is still unclear whether this unusual shape is normal for Uranus or was caused by unusual space weather when it was observed.

Moons

Main article: Moons of Uranus

See also: Timeline of discovery of Solar System planets and their moons

Uranus has 29 known natural satellites, or moons. Their names come from characters in the works of William Shakespeare and Alexander Pope. The five main moons are Miranda, Ariel, Umbriel, Titania, and Oberon. Together, the five main moons would have less mass than Triton, the largest moon of Neptune, by itself. Titania is the largest of Uranus's moons. It is less than half the size of the Moon in radius, but it is still a little bigger than Rhea, the second-largest moon of Saturn. This makes Titania the eighth-largest moon in the Solar System. Uranus's moons are quite dark and reflect little light. They are a mix of ice and rock, about half of each. The ice may include ammonia and carbon dioxide.

Ariel seems to have the youngest surface, with the fewest impact craters. Umbriel seems to have the oldest. Miranda has deep canyons, terraced layers, and a mix of different surface ages and features. Long ago, Miranda's orbit was more stretched out than it is now, and this may have heated the moon inside through tidal heating. This was probably caused by a former 3:1 orbital resonance with Umbriel. Extensional processes linked to upwelling diapirs are the likely origin of Miranda's 'racetrack'-like coronae. Ariel is thought to have once been held in a 4:1 resonance with Titania. Uranus also has at least one horseshoe orbiter in the Sun–Uranus L₃ Lagrangian point, a region at 180° in its orbit that is not very stable. This object is 83982 Crantor, which moves inside Uranus's co-orbital region on a complex, temporary horseshoe orbit. 2010 EU65 is also a promising Uranus horseshoe librator candidate.

Main article: Rings of Uranus

The Uranian rings are made of very dark particles, ranging in size from tiny specks to a fraction of a metre. Thirteen distinct rings are currently known, and the brightest is the ε ring. Most of the rings are only a few kilometres wide. The rings are probably young and did not form along with Uranus. They may have come from one or more moons that were broken apart by fast impacts, with only a few particles staying in stable zones where the present rings are located.

William Herschel described a possible ring around Uranus in 1789, but scientists generally doubt this sighting because the rings are very faint. The ring system was confirmed on 10 March 1977 by James L. Elliot, Edward W. Dunham, and Jessica Mink using the Kuiper Airborne Observatory. They had planned to study Uranus's atmosphere during an occultation of a star. When they studied their observations, they found the star had briefly disappeared several times, which led them to conclude there was a ring system. The rings were directly photographed when Voyager 2 passed Uranus in 1986. In December 2005, the Hubble Space Telescope found a pair of previously unknown outer rings, bringing the total to 13. One of Hubble's small moons, Mab, shares its orbit with the outermost of these new rings.

Exploration

Main article: Exploration of Uranus

Voyager 2 was launched in 1977. On 24 January 1986, it flew closest to Uranus, about 81,500 km (50,600 mi) from the cloud tops. Then it continued on to Neptune. The spacecraft studied the planet's air and its unusual weather, which is caused by its extreme tilt. It took the first close-up looks at its five largest moons and found 10 new ones. It also studied all nine of the known rings and found two more. It also studied the planet's magnetic field and its unusual shape.

No other spacecraft has flown by Uranus since then. Many proposed missions have been suggested to return to the Uranus system. One idea was to send the Cassini spacecraft from Saturn to Uranus. This was rejected because the trip would have taken about twenty years. Instead, it was planned to be destroyed in Saturn's atmosphere. A probe that enters Uranus's atmosphere could use ideas from the Pioneer Venus Multiprobe. The Uranus Orbiter and Probe was recommended by the Planetary Science Decadal Survey in 2011. In 2022, a Uranus mission was given the highest priority because scientists know little about ice giants. China's Tianwen-4 mission, planned to launch in 2029, will include a smaller probe. It is expected to fly by Uranus in March 2045. China also has plans for a possible Tianwen-5 mission that may orbit either Uranus or Neptune, but these plans have not happened yet.

In culture

See also: Uranus in fiction

Uranus is a popular subject in fiction, and it has inspired artworks too. Lydia Sigourney wrote a poem about the planet in 1827 called "The Georgian Planet." Gustav Holst wrote a part about Uranus in his orchestral suite The Planets, which he wrote between 1914 and 1916. John Keats mentioned Herschel's discovery of the planet in his poem "On First Looking into Chapman's Homer". The planet also gave its name to the chemical element uranium. The German chemist Martin Heinrich Klaproth discovered uranium in 1789.

In modern astrology, Uranus is the ruling planet of Aquarius. Before Uranus was discovered, Saturn was the ruling planet of Aquarius. Uranus is linked with the colour cyan and with electricity. The colour electric blue, which is close to cyan, is linked with the Aquarius sign.