Uranus: Definition and Much More from Answers.com

83±3%	Hydrogen (H₂)
15±3%	Helium
2.3%	Methane
0.009% (0.007-0.015%)	Hydrogen deuteride (HD)^[14]
Ices:
	Ammonia
	water
	ammonium hydrosulfide (NH₄SH)
	methane (CH₄)

Uranus (IPA: /ˈjʊərənəs, jʊˈreɪnəs/),^[15] the seventh planet from the Sun, is the third largest and fourth most massive planet in the solar system. It is named after the ancient Greek deity of the sky (Uranus, Οὐρανός), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planets, it was never recognised as a planet by ancient observers due to its dimness.^[16] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.

Uranus and Neptune have different internal and atmospheric compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus' atmosphere, while still composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K, and has a complex layered cloud structure, in which water is thought to make up the lowest clouds, while methane makes up the uppermost layer of clouds.^[10]

Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun; its north and south poles lie where most other planets have their equators.^[17] Seen from Earth, Uranus' rings appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.^[17] However, ground-based observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approaches its equinox. The wind speeds on Uranus can reach 250 m/s.^[18]

Discovery

Uranus had been observed on many occasions prior to its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued Uranus as 34 Tauri and observed it at least six times. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,^[19] including on four consecutive nights.

Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),^[20] but initially reported it (on 26 April 1781) as a "comet".^[21] Herschel "engaged in a series of observations on the parallax of the fixed stars",^[22] using a telescope of his own design.

He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".^[23] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".^[24] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:^[25]

“

The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.

”

Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".^[26]

While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.^[27] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".^[28] Bode concluded that its near-circular orbit was more like a planet than a comet.^[29]

The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."^[30] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so the Royal Family could have a chance to look through his telescopes.^[31]

Naming

Maskelyne asked Herschel to "do the astronomical world the faver (sic) to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."^[32] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.^[33] He explained this decision in a letter to Joseph Banks:^[30]

William Herschel, discoverer of Uranus

“

In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, 'In the reign of King George the Third.'

”

Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer.^[34] Bode, however, opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.^[31]^[35]^[36] The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death.^[37]^[38] The name Georgium Sidus or "the Georgian" was still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.^[35]

Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is "Uranian". The element uranium, discovered in 1789, was named in its honour by its discoverer, Martin Klaproth.^[39] The stressed syllable in the name Uranus is properly the first, because the penultimate vowel a is short (ūrănŭs) and in an open syllable. Such syllables are never stressed in Latin.^[40] The historically correct pronunciation of the name by English speakers is therefore [ˈjʊ.rə.nəs]. The historically incorrect pronunciation, [jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūrānŭs) has become very common. Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.^[41] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name").^[34] In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).^[42]^[43]

Orbit and rotation

HST image of Uranus showing cloud bands, rings, and moons

Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km. The intensity of sunlight on Uranus is about 1/400 that of Earth.^[44] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.^[27] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus' orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.^[45]

The rotational period of the interior of Uranus is 17 hours, 14 minutes. However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about 2/3 of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.^[46]

Axial tilt

Uranus axis of rotation lies on its side with respect to the plane of the solar system, with an axial tilt of 98 degrees. This gives it a completely different exchange of seasons to the other major planets. Other planets can be visualized to rotate like tilted spinning tops relative to the plane of the solar system, while Uranus rotates more like a tilted rolling ball. Near the time of Uranian solstices, one pole faces the Sun continually while the other pole faces away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon like in the Earth's polar regions. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness.^[47] Near 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 will reach its next equinox on December 7, 2007.^[48]^[49]

Northern hemisphere	Year	Southern hemisphere
Winter solstice	1902, 1986	Summer solstice
Vernal equinox	1923, 2007	Autumnal equinox
Summer solstice	1944, 2028	Winter solstice
Autumnal equinox	1965, 2049	Vernal equinox

One result of this axis orientation is that, on average during the year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism which causes this is unknown. The reason for Uranus' unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing the skewed orientation.^[50] Uranus' south pole was pointed almost directly at the Sun at the time of Voyager 2's flyby in 1986. The labeling of this pole as "south" uses the definition currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite shall be the pole which points above the invariable plane of the solar system (regardless of the direction the planet is spinning).^[51]^[52] However, a different convention is sometimes used, where a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.^[53] In terms of this latter coordinate system it was Uranus' north pole which was in sunlight in 1986. Astronomer Patrick Moore, commenting on the issue, summed it up by saying "Take your pick!"^[54]

Visibility

From 1995 to 2006, Uranus' apparent magnitude fluctuated between +5.6 and +5.9, placing it just above the limit of naked eye visibility at +6.0.^[8] Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter.^[8] At opposition, Uranus is visible to the naked eye in dark, un-light polluted skies, and becomes an easy target even in urban conditions with binoculars.^[6] In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25 cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.^[55]

Physical characteristics

Internal structure

Size comparison of Earth and Uranus

Uranus' mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.29g/cm³ makes it the second least dense planet after Saturn.^[7] Though of a similar diameter to Neptune (roughly four times Earth's), it is less massive.^[4] These values indicate that it is made primarily of various ices, such as water, ammonia, and methane.^[9] The total mass of ice in Uranus' interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses.^[9]^[56] Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses.^[9] The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.^[9]

The standard model of Uranus' structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.^[9]^[57] The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus'; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus' radius.^[9]^[57] Uranus' core density is around 9 g/cm³, with a pressure at the core/mantle boundary of 8 million bar and a temperature of about 5000 K.^[56]^[57] 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.^[9]^[57] This fluid, which has a high electrical conductivity, is sometimes called water–ammonia ocean.^[58] The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification, ice giants.

While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow us to determine which model is correct.^[56] The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.^[9] However for the sake of convenience an oblate spheroid of revolution, where pressure equals 1 bar, is designated conditionally as a ‘surface’. It has equatorial and polar radii of 25,559 ± 4 and 24,973 ± 20 km, respectively.^[4] This surface will be used throughout this article as a zero point for altitudes.

Internal heat

Uranus' internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.^[59]^[18] Why Uranus' internal temperature is so low is still not understood. Neptune, which is Uranus' near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.^[18] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.^[60]^[10] In fact, Uranus' heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m².^[60] The lowest temperature recorded in Uranus' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.^[60]^[10]

Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface.^[9] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.^[10]^[60]

Atmosphere

Main article: Atmosphere of Uranus

Although there is no well-defined solid surface within Uranus' interior, the outermost part of Uranus' gaseous envelope that is accessible to remote sensing, is called its atmosphere.^[10] Remote sensing capability extends down to roughly 300 km below the 1 bar level, with a corresponding pressure around 100 bar and temperature of 320 K.^[61] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.^[62] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10^–10 bar; and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.^[10] There is no mesosphere.

Composition

The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.^[10] The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium is 0.15 ± 0.03^[12] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.^[10]^[60] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,^[63] indicating that helium has not settled in the center of the planet as it has in the gas giants.^[10] The third most abundant constituent of the Uranian atmosphere is methane (CH₄).^[10] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.^[10] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar; about 20 to 30 times that found in the Sun.^[10]^[11]^[64] The mixing ratio^[65] is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.^[66] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.^[10]^[67] In addition to methane trace amounts of various hydrocarbons was found in the upper atmosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.^[68] They include ethane (C₂H₆), acetylene (C₂H₂), methylacetylene (CH₃C₂H), diacetylene (C₂HC₂H).^[66]^[69]^[70] Spectroscopy also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.^[70]^[69]^[71]

Troposphere

Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated.

The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.^[10] The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.^[61]^[64] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.^[10]^[59] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.^[59]^[60]

The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar, ammonium hydrosulfide clouds in the range of 20 and 40 bar, ammonia or hydrogen sulfide clouds at between 3 and 10 bar and finally directly detected thin methane clouds at 1 to 2 bar.^[10]^[61]^[72]^[11] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.^[18]

Upper atmosphere

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.^[62] The heating of the stratosphere is caused by 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.^[66]^[68] Heating from the hot thermosphere may also be significant.^[73]^[74] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 280 km corresponding to a pressure range of 10 to 0.1 mbar and temperatures of between 75 and 170 K.^[66] The most abundant hydrocarbons are acetylene and ethane with mixing ratios of around ×10⁻⁷ relative to hydrogen, which is similar to the mixing ratios of methane and carbon monoxide at these altitudes.^[66]^[69]^[71] Heavier hydrocarbons, carbon dioxide and water vapor have mixing ratios three orders of magnitude lower.^[69] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming haze layers,^[68] which may be partly responsible for the bland appearance of Uranus. However, the concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.^[66]^[73]

The outmost layer of the Uranian atmosphere is the thermosphere or corona, which has a uniform temperature around 800 to 850 K.^[10]^[73] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy, although weak cooling efficiency due to the lack of hydrocarbons in the upper part of the stratosphere may also contribute.^[62]^[73] In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of free hydrogen atoms. Their small molecular mass together with the high temperatures may help to explain why the corona extends as far as 50,000 km or two Uranian radii from the planet.^[62]^[73] This extended corona is a unique feature of Uranus.^[73] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.^[62] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.^[64] Observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km.^[64] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.^[73]^[75] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.^[76] Auroral activity is not as significant as at Jupiter and Saturn.^[73]^[77]

Planetary rings

Main article: Rings of Uranus

Uranus' inner rings. The bright outer ring is the epsilon ring, eight other rings are present

An enhanced colour schematic of the inner rings

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter.^[17] It was the next ring system to be discovered in the Solar System after Saturn's.^[78] 13 distinct rings are presently known, the brightest being the epsilon ring. Uranus’ rings are probably quite young; gaps in their circumference as well as differences in their opacity suggest that they did not form with Uranus. The matter in the rings may once have been part of a moon which was shattered by a high-speed impact or tidal forces.^[78]^[79]

William Herschel claimed to have seen rings at Uranus in 1789 (see below), however this is doubtful as in the two following centuries no rings were noted by other observers. The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet.^[80] The rings were directly imaged when Voyager 2 passed Uranus in 1986.^[17] Voyager 2 also discovered two additional faint rings bringing the total number to eleven.^[17]

In December 2005, the Hubble Space Telescope detected a pair of previously unknown bluish rings. The largest is located at twice the distance from the planet than the previously known rings. These new rings are so far from the planet that they are being called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. These two rings bring the total number of Uranus rings to 13.^[81] In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: one was blue and the other red.^[82]^[83] One hypothesis concerning the outer rings' blue colour is that it is composed of minute particles of water ice taken from the surface of Mab that are small enough to scatter blue light.^[82]^[84] The planet's inner rings appear grey.^[82]

Regarding William Herschel's observations in the 18th century, the first mention of a Uranian ring system comes from his notes detailing his observations of Uranus, which include the following passage: "February 22, 1789: A ring was suspected".^[85] Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case.^[82] Herschel's notes were published in a Royal Society journal in 1797. However, in the two centuries between 1797 and 1977 the rings are rarely mentioned, if at all. This casts serious doubt whether Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing. Still, it has been claimed by some that Herschel actually gave accurate descriptions of the ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour.^[86]

Magnetic field

The magnetic field of Uranus as seen by Voyager 2 in 1986. S and N are magnetic south and north poles.

Prior to the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.^[87]

Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.^[87]^[88] In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by