Uranus (planet)

Uranus
Voyager 2 captured Uranus on January 24, 1986
Properties of the orbit
Major semi-axis	19,201  AE (2,872.4 million km)
Perihelion - aphelion	18.324-20.078 AU
eccentricity	0.0472
Inclination of the plane of the orbit	0.7699 °
Sidereal orbital period	84,011 a
Synodic period	369.66 d
Average orbital velocity	6.81 km / s
Smallest distance to earth	17,259 AU
Largest distance from the earth	21.105 AU
Physical Properties
Equatorial diameter ∗	51,118 km
Pole diameter ∗	49,946 km
Dimensions	≈14.5 earth masses 8.681 · 10 25  kg
Medium density	1.271 g / cm 3
Main components (proportion of fabric in the upper layers) Hydrogen : 82.5 ± 3.3% Helium : 15.2 ± 3.3% Methane : 2.3%
Gravitational acceleration ∗	8.87 m / s 2
Escape speed	21.3 km / s
Rotation period	17 h 14 min 24 s
Inclination of the axis of rotation	97.77 °
Geometric albedo	0.488
Max. Apparent brightness	+5.38 m
Temperature ∗ Min. - Average - Max.	76 K  (–197 ° C )
∗ related to the zero level of the planet
Others
Moons	27 + ring system
Size comparison between Earth (left) and Uranus

The Uranus ( Latinization of ancient Greek οὐρανός Uranus , German , sky ' ) is from the sun from the sun with an average distance of 2.9 billion kilometers, the seventh planet in the solar system and the ice giants expected. It was discovered by Wilhelm Herschel on March 13, 1781 and is named after the Greek god Uranos . It is the only planet named after a god of the Greek world of gods.

The diameter of this giant planet is over 51,000 kilometers, about four times the diameter of the earth , the volume is about 65 times as large as that of the earth. Physically, Uranus with Neptune comparable and takes him around 14 Earth masses in the mass Ranking in the solar system with the planet fourth one. In terms of diameter, it is just ahead of Neptune in third place - after Jupiter and Saturn .

The astronomical symbol of Uranus is similar to the Martian symbol . In contrast to this, the circle has a central point and the arrow on the circle is vertical. Another symbol used mainly in astrology is Uranus .

Uranus is only under very favorable circumstances freiäugig visible, but to see all right in small binoculars. Its pale green disc is about 3.5 ″ when viewed from Earth . As of 2020, Uranus is in the constellation Aries and can therefore be easily observed in the autumn and winter skies . It runs on the ecliptic by a good 4 ° per year to the east, in 2024 it will enter the constellation Taurus .

Orbit and rotation

Orbit

Uranus moves in an approximately circular orbit with an eccentricity of 0.0457 between Saturn and Neptune around the sun. Its point closest to the Sun, the perihelion , is 18.324  AU and its point furthest from the Sun , the aphelion , is 20.078 AU. With a distance of almost 3 billion km, it is about twice the distance to the sun as the closest planet Saturn . This orbital radius fits exactly to the Titius-Bode series formulated in 1766 , so that the discovery of Uranus was considered a confirmation of the view of "world harmony" established by Kepler at that time.

The plane of the orbit is only slightly inclined to the plane of the earth's orbit at 0.772 ° and thus has the lowest inclination in comparison with the other planets . It takes Uranus about 84 years to make one orbit around the sun. At an average orbit speed of 6.81 km / s, it takes around two hours to cover its own diameter (the earth takes around seven minutes).

rotation

Uranus rotates around its axis once every 17 hours, 14 minutes and 24 seconds. As with all giant planets, strong winds blow in the direction of rotation in the high atmosphere. In southern latitudes (around 60 °) the visible atmosphere moves much faster and the period of rotation there is correspondingly shorter at 14 hours.

As a special feature, the planet's axis of rotation is approximately in its orbital plane, it “rolls” on this, as it were, when the axis points towards the sun. The axis inclination against the orbital plane is 97.77 °, so that Uranus rotates in reverse . As a result of this inclination, the northern hemisphere and the southern hemisphere face the sun after every half orbit. With the exception of a narrow equatorial region, it is then continuously light or dark on the respective hemispheres for at least one full revolution (comparable to polar day and polar night on Earth).

When Voyager 2 passed Uranus on January 24, 1986, the sun was approximately over its south pole. In 2007, at the equinox, it was briefly in its equatorial plane .

The cause of the strong axis inclination is unknown. The most common hypothesis assumes a collision with a large protoplanet during the formation phase. According to computer simulations, during its formation and the formation of its lunar and ring system, Uranus would have to have been hit by two or more celestial bodies or by a protoplanet twice the size of the earth to explain the inclination of the entire Uranus system to the orbit of the planet.

Physical Properties

Uranus has a density of 1.27 g / cm³, which is typical for giant planets. The equatorial diameter is 51,118 km, which is four times the diameter of the earth. Due to the rapid rotation, reinforced by the low density, Uranus with a pole diameter of 49,946 km shows a clear flattening of 1:44. It is the third largest planet in the solar system after Jupiter and Saturn , but less massive than Neptune due to its low density.

In the structural model, Uranus is viewed as a liquid planet with a gaseous upper layer or atmosphere that is not clearly delimited below. Since the pressure increases with increasing depth above the critical point , the gas envelope changes from a gaseous to a liquid state without a phase transition . The surface area was defined as the area where the pressure is 1 bar . The gravity makes on the one-bar level around 90% of the earth's gravity from.

Although Uranus has proportionally more heavier elements (heavier than hydrogen and helium) than Jupiter, its density is lower than that of Jupiter due to its lower mass and lower pressures inside (800 GPa instead of 3000… 4500 GPa).

Upper layers

Uranus with clouds, rings and moons in the near infrared ; a false color image from the Hubble Space Telescope from 1998

The main components of the upper layers of the gas envelope are molecular hydrogen with 82.5 ± 3.3 vol%, atomic helium with 15.2 ± 3.3 vol% and about 2.3 vol% methane . The helium: hydrogen mass ratio of 0.26 is very close to the original mass ratio in the sun of 0.27. Deuterium follows as a secondary component with a volume fraction of around 148  ppm . As aerosols are ammonia ice , water ice , ammonium hydrosulfide and methane discussed. Hydrogen can be detected (from Earth) in the spectrum of sunlight that is scattered by the planetary clouds. The hydrogen to helium ratio was determined by the refraction of Voyager 2's radio signals through the atmosphere as the probe passed through the planet's radio shadow.

The sunlight is reflected from the upper layers of the cloud. These are located under a layer of methane gas. When the reflected light passes through this layer, the reddish part of the light is absorbed by the methane gas , while the blue part can pass unhindered. This makes Uranus appear blue-green.

Three layers can be distinguished in the structure of the atmosphere : The troposphere at altitudes between −300 and 50 km and pressures of 100 to 0.1 bar. The stratosphere is located at heights between 50 and 4000 km and the pressures are 0.1 to 10 ⁻¹⁰  bar. The thermosphere (corona) extends from 4000 km to 50,000 km above the surface. There is no mesosphere.

Troposphere

Uranus' southern hemisphere (Voyager 2)
left: in the visible (orange, green, blue); right: in short-wave spectral ranges (orange, violet, UV). The latter show Uranus' subtle cloud bands and an atmospheric "hood".

The troposphere is the lowest and densest part of the atmosphere. Their temperature drops with increasing altitude. At the lower end of the troposphere, which is about 300 km below the one-bar level, the temperature is about 320 K. Up to the upper part of the troposphere, which is 50 km above sea level, the temperature drops to about 53 K. It contains almost the entire mass of the atmosphere and is also responsible for most of the planetary heat radiation (far infrared radiation ).

The clouds appear to consist of particles of frozen methane, which rose as a hot gas from deeper layers and condensed in the outer layers. It is believed that water forms the lower clouds, while the upper clouds are more like methane. The wind speeds are up to 200 m / s or around 700 km / h. The temperature is around 76 K (−197 ° C) at 1 bar and 53 K (−220 ° C) at 0.1 bar.

Its effective temperature is only 58.1 K (−214 ° C), little more than the more distant Neptune. This radiation temperature is the temperature which the Uranus atmosphere has in the range of 0.4 bar. The lowest temperature in the atmosphere is measured at 70 mbar with 52 K (−221 ° C).

stratosphere

In the stratosphere, the middle layer of the Uranus atmosphere, the temperature generally increases with altitude. At the lower limit at 50 km (at the tropopause ) it is still 53 K, while the temperature at an altitude of 4000 km (at the limit to the thermosphere) is already 800 to 850 K. The cause of the heating of the stratosphere is the absorption of solar UV and IR radiation by methane and other hydrocarbons that form in this part of the atmosphere as a result of methane photolysis . The heat transport from the hot thermosphere could also help. The hydrocarbons occupy a relatively narrow area at altitudes of 100 to 280 km. The pressure is about 10 to 0.1  mbar and the temperatures are between 75 and 170 K.

Ethane and ethyne ( acetylene ) tend to form foggy layers in the colder lower part of the stratosphere and in the tropopause. They could be partly responsible for the detailed appearance of Uranus. The concentration of hydrocarbons in Uranus' stratosphere above these nebulae is much lower than in the stratospheres of the other giant planets of the solar system. This and the weak vertical mixing above the nebula make the stratosphere of Uranus more transparent and, as a result, colder than that of the other giant planets.

Thermosphere and corona

The outermost layer of Uranus' atmosphere is the thermosphere and corona . It has a uniform temperature of 800 to 850 K. This is much higher than the 420 K in Saturn's thermosphere. The heat sources for this are not known. Neither solar ultraviolet light nor aurora activity can provide enough energy. Reduced heat radiation due to the lack of hydrocarbons in the upper stratosphere could help maintain the high temperature. In addition to molecular hydrogen, the thermosphere and corona contain a large proportion of free hydrogen atoms. Their low molecular mass, together with the high temperatures, could explain why the corona expands so far (50,000 km or two Uranus radii) away from the planet. This enlarged corona is a unique feature of Uranus. The corona slows down the small particles orbiting Uranus. As a result, the rings of Uranus are very low in dust.

ionosphere

The ionosphere of Uranus corresponds to its thermosphere along with the upper part of the stratosphere. Mainly is known about the ions through measurements from Voyager 2 decision, and the H by infrared emissions ₃ ⁺ - ions that were detected by ground-based telescopes. The observations show that the ionosphere occupies altitudes between 2,000 and 10,000 km. It is mainly sustained by the sun's UV radiation and its density depends on solar activity. The activity of the aurora ( polar lights ) is not as noticeable as in Jupiter and Saturn. The upper ionosphere (the region of the thermosphere) is the source of Uranus' UV emission known as the "day glow" or "electro glow". Like the IR radiation of the H ₃ ⁺ ions, this only emanates from the sunlit side of the planet. This puzzling phenomenon, which occurs in the thermospheres of all giant planets, is now interpreted as UV fluorescence from atomic and molecular hydrogen, which is excited by sun rays with the possible participation of photoelectrons .

internal structure

internal structure

Under the dense, gaseous hydrogen-methane shell, Uranus consists of partially liquefied gases, ice and possibly a small rock core. The gas envelope is compressed into a “crust” of hydrogen and helium, which makes up about 30% of the planet's radius. The mass of this upper layer is about 0.5 to 1.5 times the mass of the earth.

The somewhat thicker mantle of water , methane and ammonia probably has the consistency of ice and contains most of the mass of Uranus. This dense liquid, which is electrically very conductive, is sometimes also called the water-ammonia ocean.

This jacket encloses a small, possibly liquid core made of silicon and iron with a mass comparable to that of the earth.

This structure is comparable to that of Neptune , but differs significantly from the giant planets Jupiter and Saturn . These have proportionally more hydrogen and less helium (similar to the sun ), and their coats are largely made of metallic hydrogen . The nuclei of Uranus and Neptune are similar to those of Jupiter and Saturn, but the highly compressed shell of hydrogen is missing. In the center of Uranus there is likely to be a pressure of around eight million bar at a temperature of around 5000 ° C.

It is believed that the matter of Uranus is relatively evenly distributed. In terms of internal heat sources, it is an exception among the outer planets. For reasons that have not yet been explained, there is no longer any heat reserve from the original contraction and separation of substances. One possible explanation for the lack of the internal heat source is that most of the original internal heat was lost as a result of the impact, which tilted its axis of rotation. Another theory suggests that there are some barriers in the upper layers that prevent heat from moving from the interior. Its energy source is only absorbed solar radiation , because, unlike other giant planets, it does not radiate more heat than it receives from the sun.

weather

Uranus: Rings, the southern "collar" and a bright cloud in the northern hemisphere are visible. ( HST , 2005)

Voyager 2 images in 1986 showed virtually no surface detail in the visible spectrum. You hardly ever saw bands of clouds or storms that you can otherwise see on other giant planets. The cloud bands, which were blowing rapidly in the direction of the rotation, were only very weakly pronounced. A possible explanation for this comparatively calm weather and the inconspicuous cloud formations could lie in the weak internal heat source of Uranus.

The first dark spot observed on Uranus.
The picture was taken in 2006 on the HST by the
“ Advanced Camera for Surveys ” (ACS).

During the flyby of Voyager 2, the sun was over the South Pole. Still, for unknown reasons, Uranus was warmer at the equator than at the sunny pole. From this, the scientists had calculated that even the dark pole is slightly warmer than the one exposed by the sun. The temperatures in the atmosphere are surprisingly balanced due to this very slow cooling - and on the other hand very slow warming.

The southern hemisphere can be divided into two regions: a light polar cap and darker equatorial bands. The limit is around 45 ° south latitude. A narrow band spanning the planet between 45th and 50th latitude south is the brightest large feature on the planet's surface. It is called the southern "collar". The polar cap and collar are possibly a dense region of methane clouds. However, at the beginning of the 21st century, when the region of the polar cap came into view, the Hubble Space Telescope and the Keck Telescope in Hawaii could neither see a collar nor a polar cap in the northern hemisphere. This is why Uranus appears asymmetrical: light near the south pole and uniformly dark in the region north of the southern collar.

In recent years, Uranus is approaching its equinox, and with it the northern hemisphere is increasingly illuminated. As a result of this increased solar exposure, recent Hubble Space Telescope images show much more developed ligaments and increased weather activity in the northern hemisphere. According to this, there are distinct seasons in the atmosphere of the giant planet, despite its great distance from the sun. It receives only a four-hundredth of the heat from the sun that the earth receives. From there, the sun appears only as a tiny disk. Yet it still shines 1100 times brighter than the full moon appears from Earth.

Pictures from the Keck Observatory in 2004 showed that cyclones sometimes last for many months. In the northern hemisphere , the researchers discovered a 29,000 km long cloud formation. This was the largest cloud structure observed so far. However, it broke up a month later. A major storm in the southern hemisphere that had been moving up and down across five degrees of latitude for unknown reasons for several years was more durable.

In 2014, the storms could even be observed by amateurs with the telescope, they had become so noticeable.

Magnetic field

Uranus' magnetic field as seen by Voyager 2 in 1986: N and S are the north and south magnetic poles.

An aurora on Uranus at the level of the rings.

The magnetic field of Uranus is unusual and has the shape of a quadrupole with 2 north and 2 south poles. A pair of poles is inclined by almost 60 ° in relation to the axis of rotation and does not originate in the center of the planet, but is offset by a third of the planet's radius to the south. Presumably it is created by movement at not too great a depth, possibly by ionized water. Neptune has a similarly shaped and shifted magnetic field, which suggests that the large deviation has nothing to do with the size of the axis tilt. The magnetosphere of Uranus is swirled like a corkscrew over its night side by the rotation.

The unusual geometry results in a highly asymmetrical magnetosphere, in which the strength of the magnetic field in the southern hemisphere is as low as 0.1  Gauss (10 µT ) and up to 1.1 Gauss (110 µT) in the northern hemisphere can. The average field on the surface is 0.23 Gauss (23 µT). In comparison, the earth's magnetic field is roughly equally strong at both poles, and its “magnetic equator” is roughly parallel with its physical equator. The dipole moment of Uranus is 50 times stronger than that of Earth.

The magnetosphere contains charged particles: protons and electrons and a small amount of H2 + ions. The particle flux is high enough to cause darkening or erosion of the moon's surfaces in an astronomically short period of 100,000 years. This could be the reason for the uniformly dark coloring of the moons and the rings.

Uranus had relatively well-developed auroras when Voyager 2 passed by , which are seen as bright arcs around the magnetic poles. The Hubble Space Telescope was able to observe small round auroras on the Uranus day side in 2011 that lasted only a few minutes. This means that Uranus auroras have changed significantly since Voyager 2 flew by, which is probably due to the fact that the planetary axis and thus the magnetic field to the sun are oriented differently than when Voyager 2 flew by.

Ring system

→ Main article : Rings of Uranus

Ring system of Uranus

Image of the Uranus rings by Voyager 2 from 1986 (as a false color image ), the Epsilon ring on the right

Image of the Uranus rings from Voyager 2

Like all giant planets in the solar system, Uranus is surrounded by a multitude of very small bodies and particles, which orbit the planet in the direction of its rotation and, with their orbits of different densities, form a system of concentric rings. These are mostly located in the equatorial plane of the planet and mainly within the Roche boundary .

The ring system of Uranus was discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham and Douglas J. Mink with the Kuiper Airborne Observatory . The discovery was a stroke of luck. They planned to observe the occlusion of the star SAO 158687 by Uranus to study its atmosphere and diameter. When analyzing their observations, they found that the star briefly disappeared five times shortly before and after the actual eclipse. From this they concluded that there must be a ring system around the planet. The rings were imaged directly by Voyager 2 when the probe passed Uranus in 1986. It was the second after Saturn's ring system to be discovered in the solar system.

ring	Distance of the inner edge from ... (km)		Width (km)
	center	equator
Zeta (1986 U2R)	38,000	12,440	3,500
6th	41,840	16,280	1-3
5	42,230	16,670	2-3
4th	42,580	17,020	2-3
alpha	44,720	19,160	7-12
beta	45,670	20,110	7-12
Eta	47,190	21,630	0-2
gamma	47,630	22,070	1-4
delta	48,290	22,730	3-9
Lambda (1986 U1R)	50.020	24,460	1-2
epsilon	51,140	25,580	20-96
Ny (R / 2003 U 2)	65,400	39,840	3,800
My (R / 2003 U 1)	86,000	60,440	17,000

Like Jupiter, Uranus has a very fine and dark ring system. As far as the size of the particles is concerned, as with Saturn, it consists of both coarse particles and chunks with a diameter of up to 10 meters, as well as fine, but proportionally much smaller dust . On average, the particles are larger than those of Saturn's rings, but in total they are much fewer. Voyager 2 found that the total mass of the Uranus rings is less than the particle mass in the Cassini division of Saturn's rings. The most striking difference to the structures of the other giant planets is that they are mostly narrow, but sharply delimited and separated from one another by large apparent empty spaces. Not all of them are circular or lie in the equatorial plane of Uranus. The brightest of them - from Uranus from the eleventh - is denoted by the Greek letter epsilon (ε). In its area closest to the planet it is 20 km wide and almost opaque, but in its section furthest from Uranus it is five times wider and five times as transparent at 96 km. The innermost moons Cordelia and Ophelia , as shepherd moons, hold the dense epsilon ring together from inside and outside through their gravitational effect.

The two rings last discovered by the Hubble telescope in December 2005 are far outside the previously known eleven and are much wider. Because of their great distance from Uranus, they are called the outer ring system. The larger ring is twice as far from the planet as the previously known rings. This means that 13 rings are known. In April 2006, pictures from the Keck Observatory showed the colors of the new rings: one was blue, the other red.

Hubble had also spotted two small moons in 2003, one of which, Mab , shares its orbit with the outermost newly discovered ring. This ring My (μ) shows its highest density at a distance of 97,700 km from the center of Uranus and its ring particles could come from the moon Mab.

The inner edge of this ring is 86,000 km from the planet's center, on the orbit of the moon Puck . A special feature of the ring is a blue glow that was previously only known from the E-ring of Saturn. That ring of Saturn apparently consists of very fine ice crystals which reflect sunlight and which have their source in geysers on Saturn's moon Enceladus . This supports the assumption that the less than 0.0001 millimeter large ice particles of the Uranus ring, which are small enough to scatter blue light, come from the very ice-rich Uranus moon Mab and were transported into space by meteorite impacts.

The inner ring Ny (ν) is reddish in color and probably consists of both small and larger components than the blue ring. Its highest density is at a distance of 67,300 km from the center of Uranus. No moon has yet been discovered at its density maximum. The inner rings of the planet, on the other hand, appear gray.

The rings of Uranus are apparently not exactly centered around the planet, but rather swing a little around it. The astronomers suspect the causes of this are the gravitational effects of its moons and its flattening.

The rings of Uranus are likely to be relatively young. Fissures around them as well as differences in their opacity suggest that they did not arise with Uranus. The particles of matter in the rings could once have been parts of a moon that was shattered by a high-speed impact or by tidal forces.

Moons

See also : List of Uranus moons

Uranus with seven of its moons in the infrared ( Paranal Observatory , 2002)

There are 27 known moons of Uranus. Their diameters are between 10 and 1600 km. Four of them are so large that due to their mass they are in hydrostatic equilibrium and therefore have the shape of an ellipsoid of revolution. A fifth ( Miranda ) probably does too.

The first two were discovered by Wilhelm Herschel in 1787 and named by his son John Herschel after characters from Shakespeare's Midsummer Night 's Dream Titania and Oberon . Two other moons discovered by William Lassell in 1851 were named Ariel and Umbriel , and Gerard Kuiper discovered the moon Miranda in 1948 . All other moons of Uranus were also named after characters from Shakespeare or Alexander Pope . Ten other moons were discovered when the Voyager 2 spacecraft passed by in January 1986. The Perdita satellite was later identified on images from Voyager 2. Two other small inner moons were discovered with the Hubble telescope. Until 1997, Uranus was the only giant planet without known "irregular moons". Since then, nine distant irregular moons have been found using terrestrial telescopes.

The most recent discoveries date back to 2003, when the Hubble telescope discovered two more rings and two more moons. One of these two moons, which has been named Mab , is believed to be slowly crumbling under the constant bombardment of micrometeorites , thereby forming one of the two newly discovered rings. On this occasion it was also discovered that especially the orbits of the densely packed inner moons between Miranda and the main rings are not stable Kepler orbits , but that the moons exchange energy and angular momentum in a chaotic way . According to calculations published in Science , in a few million years some of the moons, which could interfere with one another by means of intersecting orbits, could go on a collision course.

The satellites of Uranus form three different groups: a group close to the planet with small diameters and circular orbits, a middle group of the five large satellites, and an outer group of small satellites with very wide, pronounced eccentric and very steeply inclined or mostly retrograde orbits. Among the large moons of Uranus there is none of the size of the Galilean moons of Jupiter or the Saturn moon Titan , or even the largest Neptune moon Triton .

When the equatorial regions pointed in the direction of the sun during the opposition in August 2006, a passage of one of its moons (Ariel) and the casting of shadows could be observed for the first time with the Hubble space telescope .

Main moons

The five main moons are Miranda , Ariel , Umbriel , Titania, and Oberon . Uranus' satellite system is the least massive of the giant planets. The total mass of the five largest moons together is less than half of the Neptune moon Triton and corresponds to about 13% of that of the Earth's moon. The largest satellite, Titania, has a radius of only 788.9 km. That's less than half that of Triton, but a little more than Rhea , Saturn's second largest moon . This makes Titania the eighth largest moon in the solar system. The moons have a relatively low albedo . This ranges from 0.20 for Umbriel to 0.35 for Ariel . The moons are a collection of around 50% ice and 50% rocks. The ice could contain ammonia and carbon dioxide . The thermal inertia of their surfaces is similar to that of dwarf planets such as Pluto or Haumea . This means that they differ in their composition and their surface properties from the irregular moons.

Among the moons, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel appears to be the oldest. Miranda has canyons 20 kilometers deep with faults, terraced layers, and a chaotic variation in age and surface features. According to one hypothesis, Miranda could have been completely blown apart a long time ago by a massive impact and then randomly reassembled. Miranda's recent geological activity was believed to have been influenced by the heat generated by tidal forces. Back then, at 3: 1 resonance with Umbriel, orbit was more eccentric than it is now. Rift fractures, combined with ascending diapirs , are apparently the reasons for the oval shape of the moon. Ariel, too, had likely formed a 4-1 track response with Titania.

Irregular moons

→ Main article : Irregular satellite

Irregular moons are captured satellites at a great distance from the planet; they have a high orbit inclination and are often in decline.

The diagram illustrates the orbits of the irregular moons that have been discovered so far. The eccentricity of the orbits is represented by yellow segments (which cover the area from the pericenter to the apocenter ) and the inclination by the Y-axis. The satellites above the X-axis move progressively (clockwise), the satellites below it move retrograde (retrograde). The X-axis is labeled Gm (million km) and the relevant fraction of the Hill sphere . The gravitational influence, within which an orbit around the planet is possible, reaches about 70 million km into space with Uranus.

Irregular moons of Uranus

In contrast to Jupiter's irregular satellites, no connection between orbit radius and inclination can be found in the known Uranus population. Instead, the retrograde moons can be divided into two groups based on the ratio of the semi-major axis and eccentricity . The inner group includes moons closer to Uranus (a <0.15 r _H ) and are moderately eccentric (~ 0.2), namely: Francisco, Caliban, Stephano and Trinculo. The outer group (a> 0.15 r _H ) includes satellites with high eccentricity (~ 0.5): Sycorax, Prospero, Setebos and Ferdinand.

Emergence

When the ice giants were formed, they never reached the critical point of Jupiter and Saturn to attract even more matter with their little mass of accumulated matter from gas nebulae. Current theories about the formation and formation of the solar system struggle to explain the existence of Uranus and Neptune so far beyond the orbits of Jupiter and Saturn. They are too big to have formed from matter that would be expected in the early solar system at this distance. Rather, some scientists suspect that Uranus and Neptune formed much closer to the sun and were thrown out by the gravitational influence of Jupiter. However, other simulations made in the late 1990s that took planetary migration into account showed the possibility that Neptune and Uranus could form near their present positions.

A simulation based on the Nice model showing the outer planets and the Kuiper belt:
a) before the Jupiter / Saturn 2: 1 resonance, b) dispersion of the objects of the Kuiper belt into the solar system after the orbit of Neptune had shifted, c) after the ejection of objects of the Kuiper Belt by Jupiter

According to another theory from 2007 based on the Nice model , Uranus and Neptune should not only have formed closer to the sun, but Uranus would also have formed further away from the sun than Neptune, since it is lighter than Neptune. Later, the two planets would have swapped places as they entered their current orbits.

observation

→ Main article : Uranus positions until 2092

The Uranus (bottom center) in the constellation Aries (Aries) compared to some striking celestial objects: Above left the planet Mars in the golden gate of the ecliptic in the constellation Taurus (Taurus) between the red giant Aldebaran (α Tauri) in the open star cluster of the Hyades and the open star cluster of the Pleiades . In the neighborhood clockwise the bright stars, Algol (β Persei), Alamak (γ Andromedae), Mirach (β Andromedae), Hamal (α Arietis), Menkar (α Ceti).

The apparent brightness fluctuated from 1995 to 2006 between +5.6 ^m and +5.9 ^m . It was barely visible to the naked eye (the limit of visibility is +6.0 ^m ). Its angular diameter is between 3.4 " and 3.7" (in comparison: Saturn between 16 "and 20", Jupiter between 32 "and 45"). During its opposition , Uranus can theoretically be seen with the naked eye on a clear, dark night under favorable conditions. It's always easy to find with binoculars. In larger amateur telescopes with an objective diameter between about 15 and 23 cm, Uranus appears as a pale, cyan-colored disk with a distinct darkening of the edges . With a more powerful telescope of 25 cm or more, cloud structures and some of the larger moons like Titania and Oberon could also be seen.

discovery

Uranus is freely visible under very good conditions , but its brightness only corresponds to a barely recognizable star 6th mag . By contrast, among all of the Sun and the Earth nearer planets - from Mercury to Saturn - with a brightness of at least 1. Size of the most striking objects in the sky and have since ancient times and the ancient well-known as planets. Due to its slow orbital movement, Uranus remained undetected as a planet for a long time after the invention of the telescope and was mistaken for a fixed star in isolated observations . So did John Flamsteed , who first cataloged it in 1690 as "34 Tauri ", or Tobias Mayer in 1756.

The musician and amateur astronomer Wilhelm Herschel became an ennobled professional astronomer through his discovery of Uranus.

Sir Friedrich Wilhelm Herschel accidentally discovered the planet on March 13, 1781 between ten and eleven o'clock in the evening with a self-made 6-inch reflector telescope when he was carrying out a sky survey from his garden in the English city of Bath , in order to test it with a higher magnification To measure fixed star parallax . However, he initially considered the celestial body moving on the border between the constellations of Taurus and Gemini to be a comet , as hardly anyone had thought that there could be more than the six well-known planets. Uranus was the first not known in ancient times.

Within three months of its discovery, science finally recognized Herschel's find as a new planet. The first precise orbit was determined by the Benedictine Placidus Fixlmillner (1721–1791) at the Kremsmünster observatory . The mathematicians and astronomers Anders Johan Lexell and Pierre-Simon Laplace were soon able to show with their calculations that it must be a planet that moves 19 times the distance from the earth to the sun. Uranus thus fitted itself exactly into the Titius-Bode series of orbital radii published by the German astronomer Johann Elert Bode , which seemed to confirm impressively the "harmony of the sky" postulated since Johannes Kepler . The expansion of the known solar system had thus doubled.

Six years after Uranus, Wilhelm Herschel also discovered its two largest and most striking moons, Titania and Oberon . The great inclination of the orbital planes of these and all other Uranus moons led by analogy from the great known Saturn and Jupiter moons for a long time to the well-founded, now confirmed assumption that the axis of rotation of Uranus must also be very strongly inclined according to the lunar orbits.

The increasing deviations of the observed positions of Uranus from the calculated ones suggested orbital disturbances due to another still unknown celestial body and led to the targeted search for an even more distant planet, which was successful with the discovery of Neptune in 1846 .

designation

With the discovery of a new planet, a debate about its name that had lasted over sixty years began. Herschel himself named it in honor of the English King George III. Georgium Sidus - Georg's star. The Jesuit and astronomer Maximilian Hell had Urania , the name of the muse of astronomy proposed. In France , astronomers referred to him as Herschel until Bode suggested that it be named after the Greek god Uranos . However, the name did not gain acceptance until around 1850 and was adapted to the Latin spelling in accordance with the Roman names of the other planets. In Roman mythology , Uranus is the father of Saturn, who in turn is the father of Jupiter.

This made it the only planet in the solar system that was not directly named after a Roman deity or, in most languages, bears the name of a Roman deity. The more distant Neptune and Pluto were again named according to the previous scheme. The planets known up to then were equated with gods in antiquity. The Romans took over the names of the Greeks , but used their own gods, corresponding to the Greek ones.

symbol

The borrowed platinum symbol as a sign of the planet Uranus		The designed symbol for the planet by H schel

Like the seven classic planets, the new one should also be assigned a planetary metal, for which platinum was considered. From him, Bode had borrowed the symbol as an astronomical symbol for Uranus after a suggestion by Johann Gottfried Köhler .

In this more modern time, however, in 1790 the chemist Martin Heinrich Klaproth named the element " uranium " (today's name: uranium ), which he had discovered the year before, after the new planet.

In France and also in England a Uranus symbol was used, which was specially designed and indicates Herschel with the initial H. In a letter to Herschel in 1784 , the French astronomer Jérôme Lalande placed the symbol with the words

"Un globe surmonté par la première lettre de votre nom" ("A ball with the first letter of your name above it")

before. Today the symbol is mainly used in astrology , although it goes back to these two astronomers in the 18th century .

exploration

Composite Hubble recordings from 2003 and 2005, with the two most recently discovered, outermost rings.

Retrospective image with the night side of Uranus, which is not visible from Earth, from the Voyager 2 space probe on January 25, 1986 after the passage

Wilhelm Herschel had already described a ring around Uranus in his manuscripts from 1797, but this observation was taken as an illusion. Even after the discovery in 1977, no one trusted the historical records, as the rings showed themselves to be far too faint for him to have been able to see them with his own means. Until the British researcher Stuart Eves examined the notes and found matches with the size, position and color of the epsilon ring. In April 2007 he presented a thesis that the Uranus rings could have been brighter 200 years ago. He justifies this with similar changes in the rings of Saturn , which are becoming more diffuse and darker.

The only spacecraft that Uranus visited so far was Voyager 2 . It started on August 20, 1977. On its grand tour to all four giant planets, it performed a swing-by to Saturn on Jupiter in 1979 , on which it took further swing in 1981 towards Uranus. It happened on January 24, 1986 and transmitted most of the images and data known to him today. Your signals from Uranus - as well as its reflected light - only reached Earth after two hours and 45 minutes.

During the approach, the probe discovered two more rings and ten new moons in addition to the nine known rings and five moons. The 16th satellite was discovered 13 years later in your photographs and was confirmed after another four years with the Hubble Space Telescope. As Uranus turned its south pole region towards the Sun during the passage, Voyager 2 flew between the concentric orbits of its moons as if through the circles of an upright target, and because it had to swing-by in the direction of Neptune, it was unable to use several Uranus moons approach one after the other. It only provided high-resolution photos of Miranda , which turned out to be the geologically most interesting of the five larger moons. The magnetic field , the irregular structure, the inclination and its unique corkscrew-like vortex (magnetic tail), caused by the sideways movement of Uranus, were investigated. Another space probe to Uranus is currently not planned.

In 2007, Uranus found itself in one of its rare “edge positions” - an event that only occurs every 42 years. This gave the researchers special opportunities for measurements, even with earth-based telescopes. On the one hand, attempts were made to find changes in its atmosphere caused by the changing of the seasons on Uranus. On the other hand, the special geometry resulted in mutual coverings and eclipses of the Uranus moons. If these events were measured, the orbital parameters of the moons could be determined much more precisely than was previously the case - this is the assumption of the researchers.

There are several suggestions for future missions to Uranus. From the NASA a study of Uranus probe was presented that the planet and its when launched an atmosphere capsule Monde is to explore for several rounds. Another study as part of the New Frontiers program describes an orbiter for exploring the Uranus magnetosphere . The ESA is planning a mission to explore the moons, rings and atmosphere in the Cosmic Vision program. It is currently unclear whether these studies will ultimately be carried out in a specific mission (status 2021).

See also : List of discoveries of planets and their moons

See also

• Planetology
• List of the planets of the solar system
• List of the most visited bodies in the solar system
• Uranus positions

literature

• Patrick Moore , G. Hunt: Atlas of the Solar System. In collaboration with the Royal Astronomical Society , Herder-Verlag, Freiburg 1986, ISBN 3-451-19613-1 .

Web links

• NASA: Planetary Science - Uranus (English)
• Solar System Exploration: Uranus. In: NASA.gov. (English). 
• Eugen Reichel: No lobby for Uranus , in Astra's Spacelog on scilogs.spektrum.de.

media

Commons : Uranus  - album with pictures, videos and audio files

Wikibooks: Uranus  - learning and teaching materials

• Why is Uranus crooked? from the alpha-Centauri television series(approx. 15 minutes). First broadcast on Apr 15, 2001.
• Hubble video on the dynamics of the rings, moons and atmosphere (English)

Individual evidence

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