Uranus
Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a supercritical phase of matter, which astronomy calls "ice" or volatiles. The planet's atmosphere has a complex layered cloud structure and has the lowest minimum temperature 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 a thick hydrogen and helium atmosphere surrounds it. Trace amounts of hydrocarbons and carbon monoxide along with carbon dioxide have been detected in the upper atmosphere. There are many unexplained climate phenomena in Uranus's atmosphere, such as its peak wind speed of, 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 29 natural satellites include 19 known regular moons, of which 14 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 moves very slowly relative to the background stars 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, 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. 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.
History
Like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. William Herschel first observed Uranus on 13 March 1781, leading to its discovery as 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.Discovery
Before its recognition as a planet, Uranus had been observed many times, but was generally misidentified as a star. It was apparently observed by Hipparchus, who measured the positions of stars in 128 BC for his catalog that was later incorporated into Ptolemy's Almagest. The catalog gives the positions of four stars forming a quadrilateral in Virgo. One of the four stars does not exist, but Uranus was at that position in April of 128 BC. The earliest definite sighting was in 1690, when John Flamsteed observed it at least six times, cataloguing it as 34 Tauri. James Bradley observed it 3 times, in 1748, 1750, and 1753. Tobias Mayer, observed it once in 1756. The French astronomer Pierre Charles Le Monnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights.William Herschel observed Uranus on 13 March 1781 from the garden of his house at 19 New King Street in Bath, Somerset, England, and initially reported it as a comet. With a homemade 6.2-inch reflecting telescope, Herschel "engaged in a series of observations on the parallax of the fixed stars."
Herschel recorded in his journal: "In the quartile near ζ Tauri ... either Nebulous star or perhaps a comet." On 17 March he noted: "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place." When he reported his discovery to the Royal Society, he continued to assert that he had found a comet, but also implicitly compared it to a planet:
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."
Although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell, working in Russia, was the first to compute the orbit of the new object. Its nearly circular orbit 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". Bode concluded that its near-circular orbit was more like a planet's than a comet's.
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." In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on condition that he moved to Windsor so that the Royal Family could look through his telescopes.
Name
The name Uranus references the ancient Greek deity of the sky Uranus, known as Caelus in Roman mythology, the father of Cronus, grandfather of Zeus and the great-grandfather of Ares, which was rendered as Uranus in Latin. 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 , with the long "u" of English and stress on the first syllable as in Latin Uranus, in contrast to , with stress on the second syllable and a long a, though both are considered acceptable.Consensus on the name was not reached until almost 70 years after the planet's discovery. During the original discussions following discovery, Maskelyne asked Herschel to "do the astronomical world the to give a name to your planet, which is entirely your own, which we are so much obliged to you for the discovery of". In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus, or the "Georgian Planet", in honour of his new patron, King George III. He explained this decision in a letter to Joseph Banks:
Herschel's proposed name was not popular outside Britain and Hanover, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed that it be named Herschel in honour of its discoverer. Swedish astronomer Erik Prosperin proposed the names Astraea, ''Cybele, and Neptune, which later became the name of the next planet to be discovered. Georg Lichtenberg from Göttingen also supported Astraea, 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. Daniel Bernoulli suggested Hypercronius and Transaturnis. Minerva was also proposed.
In a March 1782 treatise, Johann Elert Bode proposed Uranus, the Latinised version of the Greek god of the sky, Ouranos. Bode argued that the name should follow the mythology so as not to stand out as different from the other planets, and that Uranus was an appropriate name as the father of the first generation of the Titans. He also noted the elegance of the name in that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn. However, he was apparently unaware that Uranus was only the Latinised form of the deity's name, and the Roman equivalent was Caelus. In 1789, Bode's Royal Academy colleague Martin Klaproth named his newly discovered element uranium in support of Bode's choice. Ultimately, Bode's suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus''.
Uranus has two astronomical symbols. The first to be proposed, ⛢, was proposed by Johann Gottfried Köhler at Bode's request in 1782. Köhler suggested that the new planet be given the symbol for platinum, which had been described scientifically only 30 years before. As there was no alchemical symbol for platinum, he suggested ⛢ or ⛢, a combination of the planetary-metal symbols ☉ and ♂, as platinum is found mixed with iron. Bode thought that an upright orientation, ⛢, fit better with the symbols for the other planets while remaining distinct. This symbol predominates in modern astronomical use in the rare cases that symbols are used at all. The second symbol, ♅, was suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom". 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".
Uranus is called by a variety of names in other languages. Uranus's name is literally translated as the "Heavenly King star" in Chinese, Japanese, Korean, and Vietnamese. In Thai, its official name is Dao Yurenat, as in English. Its other name in Thai is Dao Maruettayu, after the Sanskrit word for 'death', Mrtyu. In Mongolian, its name is Tengeriin Van, translated as 'King of the Sky', reflecting its namesake god's role as the ruler of the heavens. In Hawaiian, its name is Hele, the Hawaiian rendering of the name 'Herschel'.