Uranus is the seventh planet in the Solar System in order of distance from the Sun. It orbits it at a distance of about 19.2 astronomical units (2.87 billion kilometers), with a period of revolution of 84.05 Earth years. It is the fourth most massive planet in the Solar System and the third largest in size.
| Orbital characteristics | |
|---|---|
| Semi-major axis | 2,870,700,000 km (19.189 au) |
| Aphelion | 3,006,300,000 km (20,096 au) |
| Perihelion | 2,735,000,000 km (18,282 3 au) |
| Orbital circumference | 18,027,000,000 km (120,502 au) |
| Eccentricity | 0,047 26 |
| Period of revolution | 30 698 days (≈ 84.05 a) |
| Synodic period | 369.597 days |
| Average orbital speed | 6.796 7 km/s |
| Maximum orbital speed | 7.129 87 km/s |
| Minimum orbital speed | 6.486 4 km/s |
| Tilt on the ecliptic | 0,773° |
| Ascending node | 74,02° |
| Perihelion argument | 96,9° |
| Known satellites | 27 |
| Known rings | 13 |
| Physical characteristics | |
| Equatorial radius | 25,559 ± 4 km (4,007 Earths) |
| Polar radius | 24,973 ± 20 km (3,929 Earths) |
| Volumetric mean radius |
25,362 ± 7 km (3,981 Earths) |
| Flattening | 0,022 93 |
| Equatorial perimeter | 159,354.1 km (3,980 9 Earths) |
| Area | 8,083 1 × 109 km2 (15,847 Earths) |
| Volume | 6,833 44 × 1013 km3 (63,085 Earths) |
| Mass | 8,681 0 × 1025 kg (14,536 Earths) |
| Global density | 1,270 kg/m3 |
| Surface gravity | 8.87 m/s2 (0.904 g) |
| Release speed | 21.3 km/s |
| Rotation period (sidereal day) |
-0.718 days (17.23992 h (retrograde)) |
| Rotational speed (at the equator) |
9,320 km/h |
| Axis tilt | 97,8° |
| Right ascension of the North Pole | 77,43° |
| Declination of the North Pole | 15,10° |
| Visual geometric albedo | 0.51 |
| Bond’s Albedo | 0.300 |
| Solar irradiance | 3.71 W/m2 (0.003 Earth) |
| Blackbody equilibrium temperature |
57 K (−216 °C) |
| Surface temperature | |
| • Temperature at 10 kPa | 53 K (−220 °C) |
| • Temperature at 100 kPa | 76 K (−197 °C) |
| Characteristics of the atmosphere | |
| Density at 100 kPa |
0.42 kg/m3 |
| Ladder height | 27.7 km |
| Average molar mass | 2.64 g/mol |
| Hydrogen H2 | 83 % |
| Helium He | 15 % |
| Methane CH4 | 2,3 % |
| Ammonia NH3 | 0,01% |
| Ethane C2H6 | 2.5 ppm |
| Acetylene C2H2 | 100 ppb |
| Carbon monoxide CO | trail |
| Hydrogen sulfide H2S | trail |
| History | |
| Discovered by | William Herschel |
| Discovered on | March 13, 1781 |
It is the first planet discovered in modern times with a telescope and not known since ancient times. Although it is visible to the naked eye, its planetary character is not identified because of its very faint brightness and its apparent movement in the very slow sky. William Herschel first observed it on March 13, 1781, and the confirmation that it is a planet and not a comet is made during the months that follow.
Like Jupiter and Saturn, Uranus atmosphere is composed mainly of hydrogen and helium with traces of hydrocarbons. However, like Neptune, it contains a higher proportion of “ice” in the physical sense, i.e. volatile substances such as water, ammonia and methane, while the interior of the planet is mainly composed of ice and rocks, hence their name “ice giants”. In addition, methane is the main cause of the aquamarine hue of the planet. Its planetary atmosphere is the coldest in the Solar System, with a minimum temperature of 49 K (−224 °C) at the tropopause, and has a layered cloud structure.
Like other giant planets, Uranus has a ring system and many natural satellites: it is known to have 13 narrow rings and 27 moons. Unique in the Solar System, its axis of rotation is practically in its plane of revolution around the Sun — giving the impression that it is “rolling” in its orbit, at least at some point in its revolution — and its north and south poles are therefore where most other planets have their equator. The planet is provided with a corkscrew-shaped magnetosphere due to this tilt of the axis.
The distance of the planet from the Earth gives it a very small apparent size, its study is difficult with telescopes located on Earth. Uranus was visited only once during the Voyager 2 mission, which flew by it on January 24, 1986. The spacecraft’s images then show a planet almost featureless in visible light, without the cloud bands or storms associated with other giant planets. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics then allowed for additional detailed observations revealing seasonal change, increased weather activity, and winds on the order of 250 m/s as Uranus approached its equinox in 2007.
Its name comes from Ouranos, the Greek deity of heaven (Uranus in Roman mythology), father of Cronus (Saturn) and grandfather of Zeus (Jupiter).
History of Uranus
Initial observations
Unlike other planets with orbits closer to the Sun — Mercury, Venus, Mars, Jupiter and Saturn — Uranus was not discovered in ancient times. Because of its distance from the Sun, it is observed on many occasions but is considered a simple star until the eighteenth century because of its very faint brightness – its apparent magnitude being at the limit of visibility to the naked eye – and its apparent very slow movement in the terrestrial sky.
The first known observation may have been that of Hipparchus, who in 128 BC could have recorded it as a fixed star in his star catalog. Indeed, an asterism mentioned in Claudius Ptolemy’s Almagest, taking up the works of Hipparchus, can only be resolved by the presence of Uranus at that time. In addition, Uranus in April 128 BC. was under very favorable observation conditions: close to its perihelion with a magnitude of 5.4 and 33° from the zenith.
The earliest proven mention dates from 1690 when John Flamsteed observed it at least six times and cataloged it as a star as 34 Tauri. The French astronomer Pierre Charles Le Monnier observed Uranus at least twelve times between 1750 and 1769, including four consecutive nights John Bevis may also have observed Uranus in 1738, clues consistent with an observation but without definitive proof.
Discovery of the planet Uranus
William Herschel is an English amateur musician. Not having the financial means to buy a telescope, he polished a mirror himself to build his own. He discovers the planet on March 13, 1781during a systematic search for stars using his telescope from the garden of his home at 19 New King Street in Bath, Somerset, England (now the Herschel Museum of Astronomy).
Specifically, Herschel had undertaken cataloging of stars according to their magnitude. At the border of the constellations Gemini and Taurus, Herschel notice in the middle of the fixed stars a small spot: he then successively changes eyepieces, gradually increasing the magnification. This increases the size of the object each time while the stars around, very far away, do not vary in size and remain simple bright spots. Thus, it cannot be a star and so he writes in his diary the March 13: “In the quartile near ζ Tauri, (…) is a curious object, either a nebula or perhaps a comet”. He notes the position of the star then, a few days later, resumes his observation: “I observed the comet or the nebula and found that it was a comet, because it had changed place”.
He then decided to warn the scientific community of his discovery and sent a letter with the details of the observation of the comet to the director of the Oxford Observatory, Thomas Hornsby. He also informed Astronomer Royal Nevil Maskelyne of Greenwich Observatory. He received a bewildered response from him on April 23, 1781″ I don’t know what to call him.
It is as likely that it is a regular planet moving in an almost circular orbit relative to the Sun as a comet moving in a very eccentric ellipse. I have not yet seen hair or tail”. The latter could not decide, he spread the news to other scientists and advised Herschel to write to the Royal Society. On April 26, 1781, when William Herschel presented his discovery to the Royal Society, he continued to claim that he had found a comet, but also implicitly compared it to a planet.
Confirmation of its existence
Although Herschel continues to call this new object a comet as a precaution, other astronomers are already beginning to suspect its true nature. The Finnish-Swedish astronomer Anders Lexell, working in Russia, is the first to calculate the orbit of a new object by applying the model of a planet. Its almost circular orbit corresponding to the model applied leads him to conclude that it is a planet rather than a comet because he estimates its distance at eighteen times the Earth-Sun distance and that no comet with a perihelion greater than four times the Earth-Sun distance has ever been observed.
Berlin astronomer Johann Elert Bode describes Herschel’s discovery as “a moving star that can be thought of as a hitherto unknown planet-like object circulating beyond Saturn’s orbit”. Bode also concluded that its near-circular orbit resembled that of a planet more than a comet. The French astronomer Charles Messier also noted that with its disk-like appearance, it resembled Jupiter more than the eighteen other comets he had observed before.
The object is thus quickly unanimously accepted as a planet. In 1783, Herschel himself acknowledged this to the president of the Royal Society, Joseph Banks: “From the observation of the most eminent astronomers in Europe, it seems that the new star, which I had the honor to point out to them in March 1781, is a primary planet of our solar system”. King George III of England rewarded Herschel for his discovery by awarding him an annual annuity of £200 (
or £24,000 in 2022), on condition that he move to Windsor so that the royal family could look through his telescopes. This pension allowed Herschel to stop his work as a musician and devote himself fully to his passion for astronomy. He then had a son, John Herschel (also an astronomer), became director of the Royal Astronomical Society in 1820 and died in 1822 at nearly 84 years old — which corresponds to the period of revolution of Uranus, coincidence noted by Ellis D. Mine.
As a result, this discovery widened the known boundaries of the Solar System for the first time in history — where Saturn previously marked the limit — and made Uranus the first planet classified as such using a telescope.
Name
The name Uranus refers to the Greek sky deity Ouranos (Ancient Greek: Οὐρανός, Uranus in Roman mythology), the father of Cronus (Saturn) and grandfather of Zeus (Jupiter). The adjectival form of Uranus is “Uranian” but the adjective “Uranian” is also sometimes used as in the Urano-cruiser asteroid.
The consensus on its name was not reached until nearly 70 years after the discovery of the planet. During the original discussions that followed the discovery, Nevil Maskelyne proposed to Herschel to name the planet, this right belonging to him as the discoverer. In response to Maskelyne’s request, Herschel decided to name the object Georgium Sidus (“George’s Star” or “Georgian Planet”), in honor of his new patron, King George III.
He explained this decision in a letter to Joseph Banks by stating that in antiquity, the planets were named after the names of the main deities and that in the present era, it would hardly be acceptable according to him to use the same method to name this new celestial body. Also, the important thing to designate it is to know when it was discovered: “the name of Georgium Sidus is presented to me as a name to provide information of the country and the time when and when the discovery was made”.
However, the name proposed by Herschel was not popular outside Britain and alternatives were quickly proposed. The French astronomer Jérôme Lalande suggested, for example, that the planet be named Herschel in honor of its discoverer. The Swedish astronomer Erik Prosperin proposed the name Neptune, which was then supported by other astronomers as it would also commemorate the victories of the Royal Navy fleet during the American Revolutionary War; similar proposals such as Neptune George III or Neptune Great-Britain are also put forward.
As early as 1781, Johann Bode proposed Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argues that the name should follow mythology so as not to stand out from those of the other planets, and that Uranus is an appropriate name as the father of the first generation of the Titans. He also notes the elegance of the name in that, just as Saturn was the father of Jupiter, the new planet should be named after Saturn’s father.
In 1789, Martin Klaproth, a compatriot and later colleague of Bode at the Royal Swedish Academy of Sciences, named the chemical element he had just discovered uranium to support this choice of name. Eventually, Bode’s suggestion became the most widely used and was universally recognized in 1850 when the HM Nautical Almanac Office, the last to ever use Georgium Sidus, abandoned the name proposed by Herschel for Uranus.
Uranus has a variety of translations into other languages. For example, in Chinese, Japanese, Korean, and Vietnamese, his name is literally translated as “star of the sky king” (天王星). In Hawaiian, his name is Heleʻekala, a loan for the discoverer Herschel.
After the discovery
Astronomy
In the nineteenth and twentieth centuries, it is very difficult to correctly observe the surface of Uranus because of its distance from the Earth. In 1937, scientists set by spectroscopy and photometry at 10 o’clock the rotation of the planet, which was then already seen as retrograde.
In 1948, Gerard Kuiper discovered Miranda, the smallest and last of the five major spherical satellites of Uranus, at the McDonald Observatory.
On March 10, 1977, the rings of Uranus were discovered, by chance, by astronomers James L. Elliot, Edward W. Dunham and Douglas J. Mink, embarked aboard the Kuiper Airborne Observatory. Astronomers want to use the occultation of the star SAO 158687 by Uranus to study the planet’s atmosphere.
However, the analysis of their observations shows that the star was briefly masked five times, before and after the occultation by Uranus; the three astronomers concluded that there was a system of narrow planetary rings. In their articles, they point to the five occultations observed by the first five letters of the Greek alphabet: α, β, γ, δ and ε; these designations are then reused to name the rings. Shortly thereafter, Elliot, Dunham and Mink discovered four more rings: one of them was located between the β and γ rings and the other three inside the α ring. The first is named η and the others 4, 5 and 6, according to the occultation numbering system adopted when drafting another article. The ring system of Uranus is the second discovered in the Solar System, after that of Saturn known since the seventeenth century.
Astrology
The astrological world needed some time to integrate Uranus into its symbolism (and again, according to traditional astrology, only the first seven stars visible to the naked eye are important). Thus, the prototypical formulation of the astrological meanings of the star dates from 33 years after its discovery: in The Urania in 1814, by J. Corfield. Indeed, as recalled by the specialist in the history of astrology Jacques Halbronn, this unexpected discovery shattered the planetary dignities inherited from Claudius Ptolemy. The system of planetary mastery over signs is central to astrology. Indeed, following Jean-Baptiste Morin de Villefranche, astrologers based their system of interpretation on “the articulation of astrological houses through masters”.
Ptolemy had assigned two masteries for Mercury, Venus, Mars, Jupiter, Saturn, and only one mastery for the Moon and the Sun, twelve masteries of astrological signs in total, as many as the signs. This corresponded to a traditional number of seven (hence the name Astrological Septenary) stars visible to the naked eye, including the two luminaires Sun and Moon. With the discovery of Uranus, all this clever device collapsed: whether Uranus was assigned two masteries or only one, there would be duplication. Some have claimed that being invisible, Uranus had no throne, a major exception to the theory.
Physical characteristics of Uranus
Mass and diameter
With a mass of 8.681 × 1025 kg, Uranus is an intermediate body between Earth and large gas giants such as Jupiter or Saturn. Indeed, the Uranian mass is 14.5 times the Earth’s mass but 1/22 of the Jovian mass.
By convention, the shape of the planet is defined by an ellipsoid model of revolution where the “surface” is defined as the place where atmospheric pressure is equal to 1 bar (100,000 Pa) and is used as a reference point for altitudes. Its equatorial radius is 25,559 km and its polar radius is 24,973 km, the latter being weaker due to the flattening caused by the rotation of the planet. Its gravity at 1 bar is 8.87 m/s2 or 90% of the surface gravity on Earth.
As Uranus is slightly less massive than Neptune (the latter having a mass of 1.024×1026kg), it is slightly larger due to gravitational compression (49,528 km in diameter for Neptune against 51,118 km for Uranus), with a radius of about four times the Earth’s radius.
In addition, Neptune and Uranus are often considered a subclass of the giant planets, called “ice giants”, because of their smaller size and higher concentration of volatile substances compared to Jupiter and Saturn. In the search for exoplanets, Uranus is sometimes used as a metonymy to describe discovered bodies with similar mass; however, the name “Neptunes” remains more common, for example, hot or cold Neptunes.
Internal structure
The density of Uranus is 1.27 g/cm3, making Uranus the second least dense planet, after Saturn. This value indicates that it is composed mainly of various ices, such as water, ammonia and methane, similar to Neptune. The total mass of ice inside Uranus is not precisely known, as the values differ depending on the model chosen. However, this value must be between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses, in proportions identical to those found within the Sun. The rest of the non-icy mass (0.5 to 3.7 Earth masses) is represented by rocky material.
The standard model of Uranus’ structure is divided into three layers: a rocky core (silicate, iron and nickel) in the center, an icy mantle in the middle and an outer shell of hydrogen and helium gas. The core is relatively small, with a mass of only 0.55 Earth masses and a radius of less than 20% of the planet, about the size of Earth. The mantle comprises most of its mass for 60% of the radius, and the upper atmosphere the remaining 20% for 0.5 Earth masses. With a density of the core of Uranus of about 4.42 g/cm3, the pressure at the center would be about 5.8Mbar (580GPa) — a little less than double that at the center of the Earth — and the temperature of the order of magnitude of 5,000 K (4,727 °C).
As is customary in planetology, the mantle is called icy even though it is a hot and dense fluid composed of water, ammonia and other volatile substances. This fluid, with high electrical conductivity, is sometimes called “water-ammonia ocean”. In 1981, theoretical studies and experiments carried out by laser compression led Marvin Ross, of Lawrence Livermore National Laboratory, to propose that this layer be totally ionized, and that methane be pyrolyzed into carbon in the form of metal or diamond. Methane decomposes into carbon and hydrocarbons due to the very high pressures and temperatures prevailing there. Then, carbon precipitation releases energy—gravitational potential energy converted into heat—causing convection currents that release hydrocarbons into the atmosphere. This model would explain the presence of various hydrocarbons in the atmosphere of Uranus.
In 2017, new experiments simulating the conditions presumed to prevail around 10,000 km below the surface of Uranus and Neptune reinforced this model by producing nano-cut diamonds. These conditions of high temperature and pressure cannot be maintained for more than a nanosecond on Earth but, in the atmospheres of Neptune or Uranus, nano-diamonds would have time to grow to give rains of diamonds. It is also hypothesized that this type of diamond shower occurs on Jupiter and Saturn. Also, the top of the mantle could be an ocean of liquid carbon where solid “diamonds” would float.
Some studies support the hypothesis that the mantle consists of a layer of ionic water in which water molecules decompose into hydrogen and oxygen ions, and more deeply into superionic water, in which oxygen crystallizes but hydrogen ions float freely in the oxygen network. However, other studies tend to establish that the presence of carbon (in the form of methane), would not allow the formation of superionic water (and more precisely oxygen crystals).
Although the model considered above is reasonably standard, it is not unique and other models are also being considered. For example, there could be substantial amounts of hydrogen and rock mixed in the ice mantle, causing the assumed total mass of ice to be greater than reality. The data currently available, coming almost exclusively from the flyby of Voyager 2, do not allow to have any certainty in this matter.
Internal heat
The internal heat of Uranus seems significantly lower than that of other giant planets, including Neptune, which has a similar mass and composition. Indeed, if Neptune irradiates 2.61 times more energy in space than it receives from the Sun, Uranus radiates practically no excess heat: the total power radiated by Uranus in the far infrared part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere. This difference in internal heat between the two icy planets explains the stronger climatic activity and faster winds present on Neptune. In fact, Uranus’ heat flux is only 0.042 ± 0.047 W/m², which is lower than Earth’s internal heat flux which is about 0.075 W/m2. The lowest temperature recorded in the tropopause of Uranus is 49K (−224°C), making Uranus the coldest planet in the Solar System.
One of the hypotheses to explain this discrepancy with Neptune is that Uranus would have been hit by an impactor; As a result, it would have expelled most of its primordial heat and eventually ended up with a lower core temperature. This impact hypothesis is also the one used in some attempts to explain the particular axial tilt of the planet. Another hypothesis is that there is a form of barrier in the upper layers of Uranus that would prevent heat from the core from reaching the surface. For example, convection may occur in a set of layers of different compositions, which could inhibit vertical heat conduction or cause double-diffusive convection to appear as a limiting factor.
It is, however, difficult to simultaneously explain Uranus’ lack of internal heat while observing its apparent similarity to Neptune. It is also possible that atmospheric activities on the two icy giants are more dependent on solar irradiation than on the amount of heat escaping from their interiors.
Atmosphere on Uranus
Although there is no definite solid surface inside Uranus, the outermost part of Uranus’ gaseous envelope is called its atmosphere. The Uranian atmosphere can be divided into three layers: the troposphere, between -300 and 50 km with pressures ranging from 100 to 0.1 bar, then the stratosphere, from 50 to 4,000 km and pressures ranging from 0.1 to 10−10 bar, then the thermosphere, extending from 4,000 km to 50,000 km from the surface — nearly two planetary radii from the surface to 1 bar.
composition
Uranus’ atmosphere, like Neptune’s, is different from those found on the two gas giants, Jupiter and Saturn. Although mainly composed of hydrogen and helium, it has a greater proportion of volatile substances such as water, ammonia and methane. Moreover, the latter having prominent absorption bands in the visible and near-infrared (IR), it is the cause of the aquamarine or cyan color of the planet. Traces of various hydrocarbons are found in the stratosphere of Uranus, which could be produced from methane by photolysis induced by ultraviolet solar radiation. Among them, and apart from methane, are ethane, acetylene, methylacetylene and diacetylene. Spectroscopy also reveals traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only come from external sources such as comets.
Troposphere
The troposphere is the lowest and densest part of the atmosphere, characterized by a decrease in temperature with altitude. The temperature drops from about 320 K (47 °C) at −300 km (base of the troposphere) to 53 K (−220 °C) at 50 km. Temperatures in the coldest upper region of the troposphere (the tropopause) range from 49 to 57K depending on the planetary latitude. The tropopause region is responsible for the vast majority of Uranus’ thermal far-infrared emissions, allowing its effective temperature of 59.1 K (−214 °C) to be determined.
The troposphere is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds, and seasonal changes.
Stratosphere
The middle layer of the Uranian atmosphere is the stratosphere, where the temperature generally increases with altitude from 53 K at the tropopause to between 800 and 850K (527 and 577 °C) at the base of the thermosphere. Stratospheric warming is caused by the absorption of solar UV and IR rays by methane and other hydrocarbons. Heat is also conducted from the hot thermosphere.
Hydrocarbons occupy a relatively narrow layer at altitudes between 100 and 300 km corresponding to a pressure range of 1000 to 10 Pa and temperatures between 75 and 170K (-198 and −103°C). Ethane and acetylene tend to condense in the cooler lower part of the stratosphere and in the tropopause (below 10 mbar) forming layers of haze, which may be partly responsible for Uranus’ dull appearance. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than that of the stratospheres of other giant planets.
Thermosphere
The outermost layer of the Uranian atmosphere is the thermosphere, which has a uniform temperature of about 800 and 850K (527 and 577°C). The heat sources needed to maintain such a high level are not fully explained, as neither solar ultraviolet radiation nor auroral activity can provide the energy needed to reach these temperatures — this activity being much lower than those of Jupiter or Saturn. The low cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mbar could, however, contribute.
In addition to molecular hydrogen, the thermosphere contains many free hydrogen atoms. Their low masses and high temperatures create a corona extending up to 50,000 km, or two Uranian radii from its surface. This extended corona is a unique feature of Uranus. Its effects induce drag on small particles orbiting Uranus, causing a general depletion of dust from Uranus’ rings. The thermosphere of Uranus with the upper part of the stratosphere corresponds to its ionosphere, extending from 2,000 to 10,000 km. The ionosphere of Uranus is denser than that of Saturn or Neptune, which may be a consequence of the low concentration of hydrocarbons in the stratosphere. The ionosphere is mainly maintained by solar UV radiation and its density depends on solar activity.
Climate
At ultraviolet and visible wavelengths, Uranus’ atmosphere appears dull compared to other giant planets. When Voyager 2 flew by Uranus in 1986, the spacecraft observed a small total of ten cloud features all over the planet. One explanation proposed for this shortage of characteristics is that the internal heat of Uranus is significantly lower than that of other giant planets, including Neptune, which otherwise resembles it. The lowest temperature recorded at the tropopause of Uranus is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.
Strip structure
In 1986, Voyager 2 discovered that the southern hemisphere visible from Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands. Their boundary is located at about a latitude of about -45°. A narrow band straddling the latitudinal range from -45 to -50° is the brightest feature on its visible surface: it is called the southern collar. It is assumed that the cap and collar are dense regions of methane clouds in the pressure range of 1.3 to 2 bar. In addition to the large-scale band structure, Voyager 2 observes ten small, bright clouds, most of which lie several degrees north of the collar. In all other respects, Uranus looks like a dynamically dead planet during this flyby.
Also, Voyager 2 arrives at the height of Uranus’ southern summer and therefore cannot observe the northern hemisphere. At the beginning of the twenty-first century, when the north polar region appeared, the Hubble Space Telescope and the Keck telescope initially observed neither a necklace nor a polar cap in the northern hemisphere: Uranus, therefore, appeared asymmetrical, luminous near the south pole and uniformly dark in the region north of the south collar. However, by 2007, when Uranus reached its equinox, the southern collar had almost disappeared and a slight north collar had emerged around 45° latitude.
Clouds
In the 1990s, the number of observed bright cloud features increased dramatically, thanks in part to new high-resolution imaging techniques. Most are found in the Northern Hemisphere as it began to become visible. There are differences between the clouds in each hemisphere: the clouds in the north are smaller, sharper and brighter. Also, they seem to be at a higher altitude.
The lifetime of clouds spans several orders of magnitude; while some small clouds live for a few hours, at least one cloud to the south seemed to have persisted since the flyby of Voyager 2 twenty years later. More recent observations also suggest that the clouds on Uranus are similar in some ways to those on Neptune. For example, the common dark spots on Neptune had never been observed on Uranus until 2006, when the first of its kind — called the Uranus Dark Spot — was imaged. It is speculated that Uranus would become more similar to Neptune when close to its equinoxes.
Monitoring cloud features can determine zonal winds blowing in the upper troposphere of Uranus. At the equator, the winds are retrograde, meaning they blow in the opposite direction of the planetary rotation. Their speeds range from -360 to −180 km/h. The wind speed increases with distance from the equator, reaching zero values near ± 20° latitude, where the minimum temperature of the troposphere is located. Closer to the poles, the winds move in a prograde direction. Wind speeds continued to increase to highs of 238m/s (856km/h) around ± 60° latitude before falling to zero at the poles.
Seasonal variations
For a short period from March to May 2004, large clouds appear in the Uranian atmosphere, giving it an appearance similar to that of Neptune. Observations included wind speeds of 229m/s (824km/h) and a persistent thunderstorm dubbed the “4th of July Fireworks”. In 2006, the first dark spot was observed. The reason why this sudden upsurge in activity occurred is not fully known, but it appears that the axial tilt of Uranus causes extreme seasonal variations in its climate.
It is difficult to determine the nature of this seasonal variation because precise data on the atmosphere of Uranus have existed for less than 84 years, or a full Uranian year. Photometry over the course of a Uranian half-year (from the 1950s) shows a steady variation in luminosity in two spectral bands, maxima occurring at the solstices and minima at the equinoxes. A similar periodic variation, with maxima at the solstices, is noted in microwave measurements of the deep troposphere begun in the 1960s. Stratospheric temperature measurements from the 1970s onwards also show maximum values close to the 1986 solstice. It is assumed that the majority of this variability occurs due to changes in visualization geometry.
There are some indications of physical seasonal changes occurring on Uranus. Indeed, if it is known to have a bright south polar region and a matte north pole, which would be incompatible with the pattern of seasonal change described above, the planet had nevertheless displayed high levels of luminosity during its previous solstice of the northern hemisphere around 1946. The north pole would not have always been so dark and the visible pole could thus brighten some time before the solstice and darken after the equinox. A detailed analysis of visible and microwave data reveals that periodic changes in brightness are not completely symmetrical around the solstices, which also indicates a change in meridian albedo patterns.
In the 1990s, as Uranus moved away from its solstice, Hubble and ground-based telescopes revealed that the south polar cap darkened noticeably (except for the southern collar, which remained bright), and then the northern hemisphere began in the early 2000s to experience increasing activity, such as cloud formations and stronger winds of up to 238m/s, reinforcing expectations that this hemisphere should soon become clearer. This actually happened in 2007 when the planet passed its equinox: a slight north polar collar rose and the south collar became almost invisible, although the zonal wind profile remained slightly asymmetrical, with the north winds being somewhat slower than those from the south.
Magnetosphere
Prior to the flyby of Voyager 2, no measurements of the Uranian magnetosphere had been made and its nature was therefore unknown. Before 1986, astronomers assumed that Uranus’ magnetic field was aligned with the solar wind, since it would then be aligned with the poles, which are on the plane of the ecliptic.
However, the observations of Voyager 2 reveal that the magnetic field of Uranus is particular, on the one hand because it does not originate from the geometric center of the planet but is offset by nearly 8,000 km from it (a third of the planetary radius), and on the other hand because it leans 59 ° with respect to the axis of rotation. This unusual geometry has the effect of inducing a highly asymmetric magnetosphere, the strength of the magnetic field on the surface of the south pole can be as low as 0.1 gauss (10 μT), while at the north pole, it can reach 1.1 gauss (110 μT). The average surface magnetic field is 0.23 gauss (23 μT).
In 2017, studies on Voyager 2 data suggested that this asymmetry caused Uranus’ magnetosphere to make a magnetic reconnection with the solar wind once a day Uranian, opening the planet to particles from the Sun. In comparison, the Earth’s magnetic field is about as strong at either pole, and its “magnetic equator” is roughly parallel to its geographic equator. The bipolar magnetic moment of Uranus is about 50 times that of Earth.
Neptune also has a similarly tilted and unbalanced magnetic field, suggesting that this may be a common feature of ice giants. One hypothesis is that, unlike the magnetic fields of terrestrial and gas giant planets, which are generated in their cores, the magnetic fields of ice giants would be generated by conductor movements at relatively shallow depths, for example, in the water-ammonia ocean. Another possible explanation for the particular alignment of the magnetosphere is that oceans of liquid diamond inside Uranus would affect the magnetic field.
Despite its strange alignment, the Uranian magnetosphere is, in many ways, similar to that of other planets: it has a shock arc with about 23 planetary radii in front of it, a magnetopause with 18 uranium radii, a well-developed magnetotail and radiation belts. Overall, the structure of Uranus’ magnetosphere is similar to that of Saturn. The tail of Uranus’ magnetosphere is also twisted due to its lateral rotation into a long, corkscrew shape extending millions of kilometers behind it.
The magnetosphere of Uranus contains charged particles, with mainly protons and electrons and a small amount of H2+ ions, but no heavier ions have been detected. Many of these particles are thought to have originated in the thermosphere. The particle population is strongly affected by the Uranian moons that sweep the magnetosphere, leaving significant gaps. The flux of these particles is high enough to cause spatial erosion of their surfaces on an astronomically fast time scale of 100,000 years. This could be the cause of the uniformly dark coloration of the satellites and rings of Uranus.
Uranus exhibits relatively developed auroras, which appear as luminous arcs around the two magnetic poles. Unlike Jupiter, the auroras of Uranus seem to be insignificant for the energy balance of the planetary thermosphere.
In March 2020, NASA astronomers report the detection of a large atmospheric magnetic bubble, also known as a plasmoid. It was reportedly released into space by the planet Uranus during the flyby of the planet in 1986, this discovery having been made after re-evaluating old data recorded by the Voyager 2 space probe.
Orbital characteristics
Orbit of Uranus
The period of revolution of Uranus around the Sun is about 84 Earth years (30,685 Earth days), the second largest of the planets in the Solar System after Neptune. The intensity of the solar flux on Uranus is about 1/400 of that received by Earth.
The semi-major axis of Uranus is 19,218 astronomical units, or about 2,871 million kilometers. Its orbital eccentricity of 0.046,381 implies that the difference between its distance from the Sun at aphelion and perihelion is 1.8 AU—the largest of all planets in the Solar System.
Calculation of its orbit
In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. However, over time, discrepancies began to appear between the predicted and observed orbits and the French astronomer, noting these unexplained gravitational perturbations, conjecturing that an eighth planet, more distant, could be the cause. The British astronomers John Couch Adams in 1843 and the French astronomer Urbain Le Verrier in 1846 independently calculated the predicted position of this hypothetical planet. Thanks to the latter’s calculations, it is finally observed for the first time on September 23, 1846, by the Prussian astronomer Johann Gottfried Galle, at one degree from the predicted position.
Rotation of Uranus
The rotation period of the inner layers of Uranus is 17 hours and 14 minutes. However, like all giant planets, the upper atmosphere of Uranus experiences very strong winds in the direction of rotation. The wind on the surface of Uranus can reach speeds of the order of 700 or 800km/h around +60° latitude and, as a result, visible parts of its atmosphere move much faster and complete a complete rotation in about 14 hours.
Its equatorial radius is 25,559 km and its polar radius is 24,973 km, the latter being weaker due to the flattening caused by the rotation of the planet.
Axis tilt
Unlike all other planets in the Solar System, Uranus has a very strong inclination of its axis relative to the normal — perpendicular — of the ecliptic. Thus, with an axial inclination of 97.77° — by comparison the inclination of the Earth’s axis is about 23° — this axis is almost parallel to the orbital plane. The planet ” rolls” so to speak in its orbit and alternately presents to the Sun its north pole, then its south pole.
This creates seasonal changes completely different from those of other planets. Near the solstice, one pole faces the Sun continuously and the other faces outwards. Each pole thus obtains about 42 years of continuous sunshine followed by as many years of darkness. Only a narrow band around the equator experiences a fast day-night cycle, but with the sun very low on the horizon. On the other side of Uranus’ orbit, the orientation of the poles towards the Sun is reversed. One result of this axis orientation is that, on average over a Uranian year, Uranus’ polar regions receive more solar energy than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles; The mechanism behind this counterintuitive result is unknown but may be due to a process of heat distribution by climate.
Near the equinox, the Sun faces the equator of Uranus, giving it for a time a period of day-night cycles close to those observed on most other planets. Uranus reaches its most recent equinox on December 7, 2007.
| Years of the Uranian solstices and equinoxes | ||
|---|---|---|
| Year | Northern Hemisphere | Southern Hemisphere |
| 1901, 1985 | Winter Solstice | Summer Solstice |
| 1923, 2007 | Spring equinox | Fall equinox |
| 1946, 2030 | Summer Solstice | Winter Solstice |
| 1966, 2050 | Fall equinox | Spring equinox |
Several hypotheses can explain this particular configuration of the planet’s axis of rotation. One of them describes the presence of a satellite that gradually caused Uranus to tilt by a resonance phenomenon before being ejected from its orbit. Another thesis is that the tipping is due to at least two impacts with impactors that occurred before the satellites of Uranus were formed. In support of this argument, in 2018, more than fifty impact simulations performed with supercomputers conclude a major collision between a young protoplanet and Uranus, at the North Pole and at a speed of 20 km/s. The protoplanet of rock and ice would have tipped Uranus before disintegrating and forming a layer of ice on the mantle. This collision would have released some of the internal heat of the planet, explaining why it is the coldest in the Solar System.
During the flyby of the planet by Voyager 2 in 1986, the south pole of Uranus was oriented almost directly toward the Sun. We can say that Uranus has an inclination slightly greater than 90 ° or that its axis has an inclination slightly less than 90 ° and that it then rotates on itself in the retrograde direction. The labeling of this pole as “south” uses the definition currently approved by the International Astronomical Union, namely that the north pole of a planet or satellite is the pole that points above the invariable plane of the Solar System, regardless of the direction in which the planet rotates. Thus, by convention, Uranus has an inclination greater than 90° and therefore has a retrograde rotation, like Venus.
Training and migration
The formation of the ice giants, Uranus and Neptune, is difficult to model accurately. Current models suggest that the density of matter in the outer regions of the Solar System is too low to account for the formation of such large bodies from the traditionally accepted method of core accretion, also known as the core accretion model. Thus, various hypotheses are put forward to explain their appearance.
The first hypothesis is that the ice giants were not formed by accretion of the core, but from instabilities in the original protoplanetary disk that then saw their atmosphere blown by radiation from a massive OB association nearby.
Another hypothesis is that they formed closer to the Sun, where the density of matter was higher, and then made a planetary migration to their current orbits after the withdrawal of the gaseous protoplanetary disk. This post-formation migration hypothesis is now preferred because of its ability to better explain the occupation of populations of small objects observed in the trans-Neptunian region. The most widely accepted stream of explanations of the details of this hypothesis is known as the Nice model, which explores the effect of a migration of Uranus and the other giant planets on the structure of the Kuiper belt.
Procession of Uranus
Moons
Uranus has 27 known natural satellites. Their combined mass — as well as that of the rings, negligible — represents less than 0.02% of the mass of the planet. The names of these satellites are chosen from characters in the works of Shakespeare and Alexander Pope.
William Herschel discovered the first two moons, Titania and Oberon, in 1787 — six years after the discovery of the planet. They were named 65 years later by his son John Herschel. In addition, William Herschel believes to have discovered four others in the following years but their correspondence with existing moons is not verified. These observations are then of great importance because they make it possible to estimate the mass and volume of the planet.
William Lassell officially announced the discovery of Ariel and Umbriel in 1851, the result of joint work with William Dawes. Almost a century later (in 1948), Gerard Kuiper discovered Miranda. The remaining twenty moons were discovered after 1985, some during the flyby of Voyager 2 and others with telescopes on the ground.
The satellites of Uranus are divided into three groups: thirteen inland satellites, five major satellites, and nine irregular satellites.
Inner satellites are small, dark bodies with characteristics and origins in common with the planet’s rings. Their orbit is located inside that of Miranda and they are strongly related to the rings of Uranus, some moons probably having caused some rings by fragmentation. Puck is the largest inner satellite of Uranus, with a diameter of 162 km, and the only one for which photos taken by Voyager 2 show details. Other interior satellites include in order of distance to the planet Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita and Mab.
The five major satellites—Miranda, Ariel, Umbriel, Titania, and Oberon—have sufficient mass to be in hydrostatic equilibrium. All but Umbriel show signs of internal activity on the surface, such as canyon formation or volcanism. The largest satellite of Uranus, Titania, is the eighth largest in the Solar System, with a diameter of 1,578 km, or just under half the Moon and a mass twenty times less. The combined mass of the five major satellites is less than half that of Triton (Neptune’s largest natural satellite) alone. They have relatively low geometric albedos, ranging from 0.21 for Umbriel to 0.39 for Ariel — which also have the oldest and youngest surface of the major satellites, respectively.
They are conglomerates of ice and rock composed of about 50% ice (ammonia and carbon dioxide) and 50% rock, similar to Saturn’s icy satellites. Only Miranda appears to be composed primarily of ice and has canyons 20 km deep, plateaus, and chaotic variations in its surface features unique to the Solar System. Miranda’s past geological activity is thought to have been driven by tidal warming at a time when its orbit was more eccentric than it is now, probably due to an ancient 3:1 orbital resonance with Umbriel.
The irregular satellites of Uranus have elliptical and strongly inclined (mostly retrograde) orbits, and orbit at great distances from the planet. Their orbit lies beyond that of Oberon, the farthest large moon from Uranus. They were probably all captured by Uranus shortly after its formation. Their diameter is between 18 km for Trinculo and 150 km for Sycorax. Margaret is the only known irregular satellite of Uranus with a prograde orbit. It is also one of the satellites of the Solar System with the most eccentric orbit at 0.661, although Nereid, a moon of Neptune, has a higher average eccentricity at 0.751. Other irregular satellites are Francisco, Caliban, Stephano, Prospero, Setebos and Ferdinand.
Planetary rings
Uranus has a system of thirteen known planetary rings, the ring system of Uranus being less complex than that of Saturn, but more elaborate than those of Jupiter or Neptune.
William Herschel describes the possible presence of rings around Uranus in 1787 and 1789. This observation is generally considered doubtful, as the rings are dark and tenuous, and in the following two centuries, none were noted by other observers. Yet Herschel makes an accurate description of the size of the epsilon ring, its angle to Earth, its red color, and its apparent changes as Uranus orbited the Sun. The ring system was explicitly discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham and Jessica Mink using the Kuiper Airborne Observatory.
The discovery is fortuitous because they planned to use the occultation of the star SAO 158687 by Uranus to study its atmosphere. When analyzing their observations, they discovered that the star had briefly disappeared five times before and after its disappearance behind Uranus, leading them to conclude that there was a ring system around Uranus. This is the second system of planetary rings discovered after that of Saturn. Two other rings were discovered by Voyager 2 between 1985 and 1986 by direct observation.
| Rings of Uranus | ||
|---|---|---|
| Name | Distance (km) | Width (km) |
| Ζ | 39 600 | 3 500 |
| 6 | 41 840 | 1 to 3 |
| 5 | 42 230 | 2 to 3 |
| 4 | 42 580 | 2 to 3 |
| To | 44 720 | 7 to 12 |
| Β | 45 670 | 7 to 12 |
| Η | 47 190 | 0 to 2 |
| Γ | 47 630 | 1 to 4 |
| Δ | 48 290 | 3 to 9 |
| Λ | 50 024 | 2 to 3 |
| Ε | 51 140 | 20 to 100 |
| ν | 67 300 | 3 800 |
| Μ | 97 700 | 17 800 |
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 away from Uranus that they are called the ” outer” ring system. Hubble also spotted two small satellites, one of which, Mab, shared its orbit with the outermost newly discovered ring. In April 2006, images of the new rings by the Keck Observatory revealed their colors: the outermost is blue and the other red. One hypothesis regarding the blue color of the outer ring is that it is composed of tiny particles of water ice from Mab’s surface that are small enough to scatter blue light.
Their distances to the center of Uranus range from 39,600 km for the ζ ring to about 98,000 km for the μ ring. If the first ten rings of Uranus are thin and circular, the eleventh, the ε ring, is brighter, eccentric and wider, extending from 20 km at the nearest point on the planet to 98 km at the furthest point. It is framed by two moons “shepherdesses”, ensuring its stability, Cordelia and Desdemona. The last two rings are much further away, with the μ ring being twice as far away as the ε ring. There are probably faint bands of dust and incomplete arcs between the main rings.
These rings are very dark: the Bond albedo of the particles composing them does not exceed 2%, which makes them very little visible. They are probably composed of ice and organic elements blackened by radiation from the magnetosphere. In view of the age of the solar system, the rings of Uranus would be quite young: their duration of existence would not exceed 600 million years and they did not form with Uranus. The material forming the rings was probably once part of a moon — or moons — that would have been broken by high-speed impacts. Of the many pieces of debris formed as a result of these shocks, only a few particles survived, in stable areas corresponding to the locations of the current rings.
Other entourage of Uranus
A Trojan asteroid of Uranus is an asteroid located around one of the two stable Lagrangian points (L4 or L5) of the Sun-Uranus system, that is to say located 60° ahead or behind the orbit of Uranus. The Minor Planet Center (MPC) lists only one Trojan of Uranus: 2011 QF99, located around point L4. 2014 YX49 is proposed as the second Trojan of Uranus but is still not approved by the CPM.
Also, other objects are co-orbiters of Uranus without being classified as Trojan. Thus, 83982 Crantor is a minor planet with a horseshoe orbit vis-à-vis Uranus. Other examples of potential coorbiters such as (472651) 2015 DB 216 or 2010 EU65 have also been discovered.
Studies show that it would be possible for a theoretical quasi-satellite of Uranus or Neptune to remain so for the lifetime of the Solar System, under certain conditions of eccentricity and inclination. However, such objects have not yet been discovered.
Observation
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. This range of brightness being close to the limit of the naked eye located at +6, it is thus possible with a perfectly dark sky – with eyes accustomed to darkness – and clear to see it as a very faint star, especially when it is in opposition.
This variability is largely explained by how much planetary latitude of Uranus is simultaneously illuminated by the Sun and seen from Earth. Its apparent size is between 3.3 and 4.1 arcseconds, depending on whether its distance from Earth varies from 3.16 to 2.58 billion kilometers, and it is thus easily distinguishable with binoculars. With a telescope with an objective diameter between 15 and 23 cm, Uranus appears as a pale cyan disk with center-edge darkening. With a telescope with a larger lens, it becomes possible to distinguish its clouds as well as some of its larger satellites, such as Titania and Oberon.
Since 1997, nine outdoor irregular satellites have been identified using ground-based telescopes. Two additional interior moons, Cupid and Mab, were discovered by the Hubble Space Telescope in 2003. The Margaret satellite is the latest discovered with its discovery published in October 2003. The Hubble Space Telescope can also take correct pictures of Uranus from Earth, even though they are in lower relative resolution than the images of Voyager 2. Between 2003 and 2005, thanks to the observations thus made, a new pair of rings was discovered, later named the outer ring system, which brings the number of rings of Uranus to 13.
In 2007, Uranus approached its equinox and cloud activity developed there. Most of this activity cannot be perceived other than with the Hubble Space Telescope or large telescopes with adaptive optics.
Exploration
Voyager 2 flyby
The planet was visited and studied at short range by only one space probe: Voyager 2 (NASA) in 1986, which is, therefore, the source of most of the known information on the planet. The main objective of the Voyager mission being the study of the systems of Jupiter and Saturn, the flyby of Uranus is made possible only because they have gone perfectly before.
Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, 81,500 km from the top of the planet’s clouds before continuing its journey to Neptune. The probe studies the structure and chemical composition of Uranus’ atmosphere, including its unique climate, caused by its axial tilt of 97.77°. She made the first detailed surveys of her five largest moons and discovered 10 new ones. It examines the nine known rings of the system, discovers two others and establishes that their appearance is relatively recent. Finally, it studied its magnetic field, irregular structure, inclination, and unique corkscrew magnetotail caused by its orientation.
Voyager 1 could not visit Uranus because the investigation of Saturn’s moon, Titan, was considered a priority. This trajectory then caused the probe to leave the plane of the ecliptic, ending its planetology mission.
Voyager 2
The possibility of sending Saturn’s Cassini-Huygens orbiter to Uranus was evaluated during a mission extension planning phase in 2009 but was ultimately rejected in favor of its destruction in the Saturnian atmosphere because it would have taken about twenty years to get to the Uranian system after leaving Saturn. In addition, New Horizons 2 — which was later abandoned — could also have made a close flyby of the Uranian system.
An orbiter named Uranus Orbiter and Probe is recommended by the Planetary Science Decadal Survey 2013-2022 as part of the New Frontiers program published in 2011. This proposal envisaged a launch in 2020-2023 and a 13-year cruise to Uranus. The probe could be inspired by the Pioneer Venus Multiprobe and descend into the Uranian atmosphere.
The European Space Agency is evaluating a “middle-class” mission called Uranus Pathfinder. Other missions such as OCEANUS, ODINUS or MUSE are being studied.
Uranus in culture
Historical references
The chemical element uranium was discovered in 1789 by the German chemist Martin Heinrich Klaproth, named after Uranus which had just been discovered eight years earlier. It was then isolated by the French chemist Eugène-Melchior Péligot in 1841 and remained the heaviest element known until 1940, when the first transuranic element was discovered: the neptunium, named after the planet Neptune.
Operation Uranus is the name given to the successful military operation of World War II by the Red Army to retake Stalingrad. It leads to Operation Saturn. The same war will then know as Operation Neptune, the code name given to the landing in Normandy of the allied troops in June 1944.
Music and poetry
“Uranus, the Magician” is the 6th movement of the work for the large orchestra Les Planètes, composed and written by Gustav Holst between 1914 and 1916. In addition, the moons of Uranus Oberon, Miranda and Titania are mentioned in the song Astronomy Domine by Pink Floyd.
In John Keats’ poem On First Looking into Chapman’s Homer, the two lines “Then felt I like some watcher of the skies / When a new planet swims into his ken” are a reference to William Herschel’s discovery of Uranus.
Literature and film
Since its discovery, Uranus has appeared in many works of science fiction. For example, it was the setting for the Doctor Who episode The Daleks’ Master Plan or certain levels in the Mass Effect video game series, and the subject of Ben Bova’s fictional novel Uranus.
However, it has not only inspired science fiction works. Thus, Uranus is a novel by Marcel Aymé published in 1948 and adapted for the screen by Claude Berri in 1990. The title of the novel comes from an anecdote told by a character, Professor Watrin: a bombing killed his wife one evening in August 1944 while reading in an astronomy book the chapter devoted to Uranus and the name of the planet thus reminds him of this memory.
Pun
In popular culture in the English language, many puns are derived from the common pronunciation of the name Uranus with the expression “your anus” in French: “ton/votre anus” and are notably used as a headline in press articles about the planet, and this since the late nineteenth century. This pun consequently influenced the recommended pronunciation of the planet to avoid homonymy.
This has also been used in works of fiction, for example in the animated series Futurama where the planet was renamed to “end this stupid joke once and for all” in “Urectum”.
Symbolism
Uranus has two astronomical symbols. The first to be proposed ♅ was suggested by Jérôme Lalande in 1784. In a letter to William Herschel, the discoverer of the planet, Lalande described it as “a globe surmounted by the first letter of your name”. A later proposition is a hybrid of the symbols of Mars and the Sun because Uranus represents the sky in Greek mythology, ⛢ which was believed to be dominated by the combined powers of the Sun and Mars. In modern times, it is still used as an astronomical symbol of the planet, although its use is discouraged in favor of the initial “U” by the International Astronomical Union.
References (sources)
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