Uranus

## General Information

Uranus is a gaseous blue ice giant located seven planets away from the Sun. The majority of the planet is composed of supercritical phases of matter, known as "ice" or volatiles in astronomy, which are composed of water, ammonia, and methane. Out of all the planets in the Solar System, the planet's atmosphere has the lowest minimum temperature, 49 K and a complicated layered cloud structure. It rotates retrogradely at a velocity of 17 hours and has a noticeable axial tilt of 97.8°. This indicates that over the Sun's 84-year orbit, the poles experience around 42 years of constant sunshine and another 42 years of continuous darkness.

Among the planets in the Solar System, Uranus has the fourth biggest mass and the third largest diameter. According to current theories, the planet's rocky core is located inside its volatile mantle layer, and a thick atmosphere of hydrogen and helium surrounds it. There have been trace quantities of carbon dioxide (which is assumed to have originated from comets) and hydrocarbons (which are thought to have created by hydrolysis) found in the high atmosphere. Uranus's atmosphere exhibits several inexplicable climatic phenomena, including fluctuations in its polar cap, unpredictable cloud production, and a maximum wind speed of 900 km/h. It is still unknown why the planet has so little internal heat when compared to other big planets.

Uranus features a ring system, circling natural satellites, and a magnetosphere, just like the other major planets. The known 13 inner moons are located within its ring system, which is very dark, reflecting barely 2% of the incoming light. Distant from Uranus are the known nine irregular moons, and the planet's bigger five main moons, Miranda, Ariel, Umbriel, Titania, and Oberon. The planet's rings and moons may have darkened due to the extremely asymmetric and charged particle-rich magnetosphere of the planet.

Although Uranus can be seen with the unaided eye, it is extremely faint and was not recognized as a planet until William Herschel made his first observations of it in 1781. It was decided to name the planet after one of the Greek primal deities, Uranus (Ouranos), some seven decades after it was discovered. As of 2023, the planet had only received a close encounter during a 1986 flyby by the Voyager 2 spacecraft. Despite the fact that telescopes can now resolve and study it, there is a strong desire to return to the planet, as evidenced by the Planetary Science Decadal Survey's decision to give the planned Uranus Orbiter and Probe mission top priority in the survey scheduled for 2023–2022.

### History

Even though Uranus can be seen with the naked eye, its sluggish orbit and lack of brightness prevented ancient observers from ever classifying it as a planet. On March 13, 1781, Sir William Herschel made the first observation of Uranus, which resulted in the planet's discovery. This event expanded the Solar System's recognized bounds and made Uranus the very first planet to be identified as such using a telescope.

Before Uranus was officially recognized as a planet, it was often spotted, but it was thought to be a star. Hipparchos may have made the first known sighting of it in 128 BC when he added it to his star list, which was eventually included in Ptolemy's Almagest. John Flamsteed saw it at least six times in 1690, the first confirmed observation, and he recorded it as 34 Tauri. Pierre Charles Le Monnier, a French astronomer, made at least twelve observations of Uranus between 1750 and 1769, including four nights in a row.

On March 13, 1781, Sir William Herschel noticed Uranus from the garden of his Bath, Somerset, England, home at 19 New King Street. This home is now home to the Herschel Museum of Astronomy. On April 26, 1781, Herschel first described Uranus as a comet. Using a self-made 6.2-inch reflecting telescope, Herschel conducted several studies on the fixed stars' parallax.

#### Naming

The Latin name Uranus refers to the ancient Greek deity of the sky Uranus, also called Caelus in Roman mythology. He was the grandfather of Zeus (Jupiter) and the father of Cronus (Saturn).

After the planet was discovered, about 70 years passed before a consensus was reached on the name.

Bode offered Uranus, the Latinized name for Ouranos, the Greek deity of the sky, in a paper dated March 1782. Bode contended that Uranus was a fitting name for the father of the initial generation of Titans, and that the name should be derived from the mythology to avoid making the planet stand out from the others. Bode's recommendation ultimately gained the most traction and was adopted by everybody when the last remaining holdout, HM Nautical Almanac Office, began adopting Uranus instead of Georgium Sidus in 1850.

### Properties

Uranus's apparent magnitude ranges from 5.38 to 6.03, with a standard deviation of 0.17 and a mean of 5.68. This brightness range is almost as bright as what the unaided eye can see. The degree to which the planetary latitudes are lighted by the Sun and seen from Earth determines a large portion of the variability. Its angular diameter is 3.4–3.7 arcseconds, while Jupiter's is 32–45 arcseconds and Saturn's is 16–20 arcseconds. When Uranus opposes, it is visible to the unaided eye in dimly lit sky.

#### Structure

Uranus is the least massive of the large planets, with a mass of around 14.5 times that of Earth. At around four times the diameter of Earth, it is somewhat bigger than Neptune. With a density of 1.27 g/cm3, Uranus is the second-least dense planet in the Solar System after Saturn. This number suggests that the main constituents are ices, including water, ammonia, and methane. The exact amount of ice in Uranus's interior is unknown since it varies based on the model used; nonetheless, it has to be between 9.3 and 13.5 Earth masses. Hydrogen and Helium make up a small fraction of the overall mass, ranging from 0.5 to 1.5 Earth masses.

Rocks make up the remaining portion of the non-ice bulk. According to the traditional hypothesis, Uranus is made up of three layers: an ice mantle in the middle, an outer layer of gaseous hydrogen and helium, and a rocky (silicate/iron-nickel) core in the center. Less than 20% of Uranus' radius is made up of the upper atmosphere, which is relatively insubstantial, weighing only 0.5 Earth masses and making up the bulk of Uranus. The bulk of Uranus is made up of the mantle, which has a mass of approximately 13.4 Earth masses. The core of Uranus has a density of around 9 g/cm3, a central pressure of 8 million bars (800 GPa), and a temperature of roughly 5000 K.

The methane molecules may break apart due to the intense pressure and heat found deep beneath Uranus, causing the carbon atoms to condense as diamond crystals that fall through the mantle like hailstones. Diamond raindrops, which scientists believe to occur on Jupiter, Saturn, and Neptune, are comparable to this occurrence. According to very-high-pressure experiments conducted at Lawrence Livermore National Laboratory, there might be a metallic liquid carbon ocean at the base of the mantle, possibly including solid "diamond-bergs" floating over it.

Astronomically speaking, Uranus has a low thermal flux, meaning that its interior heat seems to be far less than that of the other large planets. Scientists still don't know why Uranus's interior temperature is so low. Uranus emits very little surplus heat into space compared to Neptune, its near twin in terms of size and composition, which radiates 2.61 times as much energy as Uranus gets from the Sun. Uranus is the coldest planet in the Solar System because of the total power it radiates in the far infrared section of the spectrum, which is 1.06±0.08 times the solar energy absorbed in its atmosphere.

#### Atmosphere

Uranus's atmosphere is the outermost portion of its gaseous envelope that may be observed by remote sensing, despite the fact that Uranus's interior lacks a clearly defined solid surface. The capacity of remote sensing descends to around 300 kilometers beneath the 1 bar level, when the temperature is 320 Kelvin and the pressure is approximately 100 bar.

The atmosphere of Uranus is made mostly of molecular hydrogen and helium, which is distinct from the planet's mass composition. The upper troposphere has a helium molar percentage of 0.15±0.03, which is equivalent to a mass fraction of 0.26±0.05. This suggests that helium has not settled in the center of the atmosphere like it does in gas giants. Methane makes up the third most prevalent part of the atmosphere of Uranus. At 1.3 bar of pressure, methane molecules make up 2.3% of the atmosphere by molar fraction under the methane cloud deck. Methane exhibits noticeable absorption bands in the visible and near-infrared spectrum. This is around 20–30 times the amount of carbon contained in the Sun.

Comparing Uranus's atmosphere to those of the other major planets—including Neptune, which it generally resembles—at ultraviolet and visible wavelengths, it is bland. In 1986, Voyager 2 made a circumnavigation of Uranus and recorded ten distinct cloud characteristics. Since Uranus is the coldest planet in the Solar System, it has been suggested that this lack of characteristics is due to the planet's far lower interior heat than the other major planets.

Voyager 2 discovered in 1986 that Uranus's visible southern hemisphere is separated into two areas: a dark equatorial band and a dazzling polar cap. Their border lies at around -45° latitude. The brightest big feature on its visible surface is a thin band that spans the latitudinal range of −45 to −50°. A southern "collar" is what it's called. It is believed that the cap and collar are a thick area of methane clouds situated between 1.3 and 2 bar of pressure. Ten tiny, luminous clouds were seen by Voyager 2, the most of which were located several degrees north of the collar, in addition to the extensive banded structure. In 1986, Uranus seemed to be a dynamically lifeless planet in every other way.

Arriving at the height of the southern summer on Uranus, Voyager 2 was unable to view the northern hemisphere. When the northern polar area emerged in the early 21st century, neither a collar nor a polar cap could originally be seen in the northern hemisphere, according to observations made by the Hubble Space Telescope and Keck telescope. Hence, Uranus gave the impression of being asymmetrical, with uniform darkness north of the southern collar and brightness towards the south pole. When Uranus crossed its equinox in 2007, the southern collar mostly vanished and a weak northern collar arose about 45° latitude. A polar vortex was found in 2023 when a team using the Very Large Array noticed a black collar near 80° latitude.

A significant increase in the quantity of brilliant cloud features was seen in the 1990s, in part due to the development of new high-resolution imaging methods. When it first became visible, the majority were located in the northern hemisphere. It was proven that an early explanation—that brilliant clouds are easier to spot in its dark section, whereas the bright collar covers them in the southern hemisphere—was untrue. However, the clouds in each hemisphere differ from one another. The clouds in the north are brighter, sharper, and smaller. They seem to be located at a greater elevation.

The identification of zonal winds flowing in Uranus' upper troposphere was made possible by the tracking of many cloud characteristics. Winds at the equator blow retrogradely, or in the opposite direction of the planetary rotation. They go between −360 and −180 km/h (−220 and −110 mph). As one gets farther from the equator, wind speeds rise and eventually zero out at around ±20° latitude, which is the troposphere's temperature minimum.

There are hints that Uranus is experiencing physical seasonal variations. Uranus's north pole is rather dark, despite the planet's recognized brilliant south polar area, which contradicts the previously described paradigm of seasonal variation. Uranus showed high light levels on its previous northern solstice in 1944, indicating that the north pole was not always so faint. This data suggests that the visible pole has a gradual brightening prior to the solstice and a darkening following the equinox.

When Uranus moved away from its solstice in the 1990s, Hubble and ground-based telescopes showed that the south polar cap darkened significantly, while increased activity in the northern hemisphere—such as cloud formations and stronger winds—bolstered expectations that Uranus should brighten soon.

#### Magnetosphere

According to Voyager's studies, Uranus has a strange magnetic field that is 59° inclined from the axis of rotation and does not emanate from its geometric center. In actuality, there is a movement of up to one-third of the planetary radius in the magnetic dipole, from the center of Uranus to the south rotating pole.

According to 2017 studies using data from Voyager 2, this imbalance opens Uranus to solar wind interactions once each Uranian day, allowing particles from the Sun to enter the planet's magnetosphere. In contrast, Earth's magnetic field has almost the same strength at both poles, and the "magnetic equator" of the planet is roughly parallel to the geographical equator. Uranus has a dipole moment fifty times greater than Earth's.

Charged particles make up Uranus's magnetosphere, primarily protons and electrons with a trace quantity of H2+ ions. A large number of these particles most likely originate in the thermosphere. Up to 4 and 1.2 megaelectronvolts, respectively, may be reached by the ion and electron energies. In the inner magnetosphere, low-energy ion density is around 2 cm^−3.

### Moons

* Umbriel
* Titania
* Oberon
* Miranda
* Ariel
* Puck
* Cressida
* Desdemona
* Perdita
* Setebos
* Trinculo
* Stephano
* Sycorax
* Ophelia
* Cordelia
* Mab
* Caliban
* Prospero
* Cupid
* Rosalind
* Portia
* Bianca
* Juliet
* Belinda
* Margaret
* Francisco
* Ferdinand