Uranus is the seventh planet discovered in the Solar System that also led to the discovery of the last planet, Neptune they are both referred to as ice giants. Officially recognized in 1781 after many observations in the past, it is the third-largest planet of the Solar System.
Key Facts & Summary
Since ancient times it was not recognized due to its dimness and slow orbit. However, in 1781 Sir William Herschel announced its discovery being the first planet discovered with the help of a telescope. It was given the name Uranus, after the Greek god of the sky Ouranos. It is the only planet whose name is derived directly from a figure of Greek mythology.
Uranus is the seventh planet from the Sun, around 1.8 billion miles or 2.9 billion kilometers away. It has the third-largest planetary radius and fourth-largest mass in the Solar System. It has a radius of 25.362 kilometers or 15.759 miles, and has about 14.5 times the mass of Earth and four times its diameter. It is on an average distance of 19.2 Astronomical Units (AU) away from the Sun and currently 18.8 AU away from Earth. Its actual distance can be checked online as Uranus is constantly tracked. Its volume is about 63 times greater than Earth’s, which means that 63 Earths can fit inside it. It orbits the sun quite unusually, being the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees. Because of this, it rotates in the opposite direction than most planets, from East to West. Venus also does this but Uranus is the only known planet to rotate on its side. It takes Uranus 84 years to complete an orbit of the Sun, the longest from all the planets in the solar system. It also has the shortest day. One rotation on Uranus takes about 17 hours. Uranus has the coldest planetary atmosphere in the solar system, -224 degrees Celsius;-371 degrees Fahrenheit. Uranus and Neptune share similar composition, because of this Uranus and Neptune are classified as ice giants rather than gas giants. Uranus has a similar atmosphere to Jupiter and Saturn in its primary composition of hydrogen and helium yet, it contains more “ices” such as water, ammonia, methane and traces of other hydrocarbons.
Though it appears featureless, without the cloud bands or storms associated with the other giant planets, it does have complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. Through analysis, the interior of Uranus has been stated to be made up primarily of ices and rock. Uranus has 13 known rings around it. The innermost rings are narrow and dark, and its outermost rings are brightly colored. Like the other gas giants, it also has many moons. As of 2020, 27 moons have been discovered orbiting Uranus. They were named after characters from the works of William Shakespeare and Alexander Pope. Uranus has been visited only once by a spacecraft: Voyager 2. Though it is visible to the naked eye, Uranus escaped clear classification for decades.
Generally mistaken for a star, it is speculated that Hipparchos might have recorded it first in 128 BC in his star catalogue. However, the earliest definite sighting was in 1690. John Flamsteed observed it six times, cataloguing it as 34 Tauri. Later it was observed for about fourteen-times by Charles Le Monnier between 1750 and 1769. The man who finally solved this elusive object was William Herschel who observed it on March 13, 1781 with a telescope. Although he first classified it as a comet, soon after reporting his discovery to other famous astronomers, it was concluded that in fact it was a planet. By 1783, Herschel acknowledged this and later was rewarded by the then king of England George III, on the condition that he moved to Windsor, in order for the royal family to also see it.
In 1782 Johann Bode proposed the name Uranus, the Latinized version of the Greek god of the sky, Ouranos. His argument was that the new planet would stand out from the others if it wasn’t following the mythology. Just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn. In 1789, an acquaintance of Bode, Martin Klaproth named his newly discovered element uranium in support of Bode’s choice. The name became universal in 1850.
In other languages such as Chinese, Japanese, Korean and Vietnamese, Uranus is translated as “sky king star.” Its official name in Thai is “Dao Yurenat”, in Mongolian, “Tengeriin Van”, meaning “King of the Sky” but in Hawaiian its name is “Hele’ekala”, a loanword for the discoverer of Uranus, Herschel.
Orbit and Rotation
It takes Uranus about 7 years to pass through each zodiac constellation, and a total of 84 years to make a complete trip around the sun. Its interesting orbing caught the attention of astronomers because the planet escaped its predicted location, and thus it led to the discovery of the last planet in 1846 that would later be called Neptune.
The interior rotation of Uranus is completed in about 17 hours, 14 minutes, and as in the case of all the gas giants its upper atmosphere experiences strong winds in the direction of rotation thus making a full rotation faster, in about 14 hours.
The axis of rotation is approximately parallel with the plane of the Solar System, with a tilt of 97.77 degrees. This feature gives Uranus completely different seasonal changes unlike those of other planets.
Near the solstice, one pole faces the sun continuously while the other is covered in complete darkness. At the other side of Uranus’s orbit the orientation of the poles towards the Sun is reversed with each pole getting around 42 years of continuous light, and the other of darkness. Neat the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets.
Uranus is hotter at its equator than at its poles. We do not know why this is so, nor do we know exactly why Uranus has such an unusual axial tilt. However, speculations suggest that Uranus suffered about 3 to 4 billion years ago, a collision with an Earth-sized protoplanet while the solar system was forming.
The standard model structure of Uranus consists of three layers: a rocky silicate/iron-nickel core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.
The core’s mass is estimated to be about 0.55 percent Earth masses with a radius less than 20% of the whole of Uranus. The mantle comprises its bulk, with around 13.4 Earth masses. The upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and expanding for the last 20 percent of Uranus’s radius. The core density is around 9 g/cm3, with a pressure in the middle of about 8 million bars, and a temperature of about 5.000 K. What is referred to as the ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles.
It is believed that the extreme temperatures and pressure condense carbon atoms into crystals of diamond. It is thought that these rainfalls of solid diamonds also occur on Jupiter, Saturn and Neptune. The prime factor that justifies the difference between Uranus and Neptune to Saturn and Jupiter is the fact that ice dominates over gases, hence the separate classification as “ice giants”. Ice giants are believed to have liquid oceans and gas giants about 85 percent just gas.
Uranus has no solid surface because of its fluid interior structure. The atmosphere comprised of gases gradually transitions into the internal liquid layers. But for easier understanding, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar is conditionally designated as a “surface”.
Though it is the coldest planet in the solar system, it is not know why. Something prevents the heat of Uranus’s core from reaching the surface. It is believed that a form of barrier exists in the planets upper layers that stop the heat. Another theory suggests that after a massive collision, Uranus expelled most of its primordial heat, leaving its core temperature depleted.
Uranus lacks a well-defined solid surface in its interior. However, the outermost part of the planet enveloped by gasses that is accessible to remote sensing is called its atmosphere. Remote-sensing capability extends down to roughly 300 kilometers or 186 miles below the 1 bar level, with a corresponding pressure of 100 bars and a temperature of 320 K (47 C; 116 F). Its atmosphere is mostly consistent in molecular hydrogen, helium, methane, ammonia, water, hydrogen sulfide, various hydrocarbons and water vapor, carbon monoxide and carbon dioxide possibly due to an external source such as in falling dust and comets.
Uranus has no mesosphere, but its atmosphere can be divided into three layers:
* Troposphere, between altitudes of -300 and 50 km (-186 and 31 mi), with pressures from 100 to 0.1 bar. It is the lowest and densest part of the atmosphere, the temperature decreases with altitude. It falls from approximately 320 K (47 C; 116 F) at the base of the nominal troposphere at −300 km or 186 miles to 53 K (−220 C; −364 F) at 50 km or 31 miles. The troposphere is thought to have highly complex cloud structures and is a dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes.
* Stratosphere, spanning altitudes between 50 and 4,000 km (31 and 2,485 mi), with pressures between 0.1 and 10−10 bar. Temperatures gradually increase with altitude from 53 K (−220 C; −364 F) in the boundary of the troposphere to between 800 and 850 K (527 and 577 C; 980 and 1,070 F) at the base of the thermosphere. The heating is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons.
* Thermosphere, extending from 4,000 km to 50,000 km (2,485 and 31,068 mi). It’s the outermost layer of the atmosphere and corona, with a uniform temperature around 800 to 850 K.
Uranus’s atmosphere appears quite bland in comparison to the other giant planets, especially to Neptune whom it otherwise closely resembles.
A total of 10 cloud features were observed on the entire planet by the Voyager 2 spacecraft when it flew by Uranus in 1986. One explanation for this great lack of features is that the internal heat appears markedly lower than that of the other giant planets.
But all of this changed when the Hubble Space Telescope observed the ice giant. Analysis revealed that the planet had a dynamic weather, characterized by storms half as big as the United States or even twice in size.
In 2006 a dark cloud was spotted on the planet. This storm was gigantic, two thirds the size of the United States. The average temperatures of the clouds were -315 degrees Fahrenheit. The wind speeds can reach up to 560 miles or 900 kilometers per hour blowing in the retrograde at the equator, in the reverse direction of the planet’s rotation.