Uranus is the seventh planet
from the Sun. Its name is a reference to the Greek god of the sky,
Uranus (Caelus), who, according to Greek mythology, was the
great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and
father of Cronus (Saturn). It has the third-largest planetary radius
and fourth-largest planetary mass in the Solar System. Uranus is
similar in composition to Neptune, and both have bulk chemical
compositions which differ from that of the larger gas giants Jupiter
and Saturn. For this reason, scientists often classify Uranus and
Neptune as "ice giants" to distinguish them from the
other giant planets.
As with gas giants, ice giants also
lack a well defined "solid surface." Uranus's
atmosphere is similar to Jupiter's and Saturn's in its primary
composition of hydrogen and helium, but it contains more "ices"
such as water, ammonia, and methane, along with traces of other
hydrocarbons. It has the coldest planetary atmosphere in the Solar
System, with a minimum temperature of 49 K (−224 °C; −371 °F),
and has a complex, layered cloud structure with water thought to make
up the lowest clouds and methane the uppermost layer of clouds. The
interior of Uranus is mainly composed of ices and rock.
Like the other giant planets, Uranus
has a ring system, a magnetosphere, and numerous moons. The Uranian
system has a unique configuration because its axis of rotation is
tilted sideways, nearly into the plane of its solar orbit. Its north
and south poles, therefore, lie where most other planets have their
equators. In 1986, images from Voyager 2 showed Uranus as an almost
featureless planet in visible light, without the cloud bands or
storms associated with the other giant planets. Voyager 2 remains the
only spacecraft to visit the planet. Observations from Earth have
shown seasonal change and increased weather activity as Uranus
approached its equinox in 2007. Wind speeds can reach 250 metres per
second (900 km/h; 560 mph).
History
Like the classical planets, Uranus is
visible to the naked eye, but it was never recognized as a planet by
ancient observers because of its dimness and slow orbit. Sir William
Herschel first observed Uranus on 13 March 1781, leading to its
discovery as a planet, expanding the known boundaries of the Solar
System for the first time in history and making Uranus the first
planet classified as such with the aid of a telescope.
Discovery
Uranus had been observed on many
occasions before its recognition as a planet, but it was generally
mistaken for a star. Possibly the earliest known observation was by
Hipparchos, who in 128 BC might have recorded it as a star for his
star catalogue that was later incorporated into Ptolemy's Almagest.
The earliest definite sighting was in 1690, when John Flamsteed
observed it at least six times, cataloging it as 34 Tauri. The French
astronomer Pierre Charles Le Monnier observed Uranus at least twelve
times between 1750 and 1769, including on four consecutive nights.
Sir William Herschel observed Uranus on
13 March 1781 from the garden of his house at 19 New King Street in
Bath, Somerset, England (now the Herschel Museum of Astronomy), and
initially reported it (on 26 April 1781) as a comet. With a homemade
6.2-inch reflecting telescope, Herschel "engaged in a series
of observations on the parallax of the fixed stars."
Herschel recorded in his journal: "In
the quartile near ζ Tauri ... either Nebulous star or perhaps a
comet." On 17 March he noted: "I looked for the
Comet or Nebulous Star and found that it is a Comet, for it has
changed its place." When he presented his discovery to the
Royal Society, he continued to assert that he had found a comet, but
also implicitly compared it to a planet:
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
luster 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 23 April 1781: "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."
Although Herschel continued to describe
his new object as a comet, other astronomers had already begun to
suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell,
working in Russia, was the first to compute the orbit of the new
object. Its nearly circular orbit led him to a conclusion that it was
a planet rather than a comet. 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". Bode concluded that its near-circular
orbit was more like a planet's than a comet's.
The object was soon universally
accepted as a new planet. By 1783, Herschel acknowledged this 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." In recognition of his
achievement, King George III gave Herschel an annual stipend of £200
on condition that he move to Windsor so that the Royal Family could
look through his telescopes (equivalent to £26,000 in 2021).
Name
The name of Uranus references the
ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός),
known as Caelus in Roman mythology, the father of Cronus (Saturn) and
grandfather of Zeus (Jupiter), which was rendered as Ūranus in Latin
(IPA: [ˈuːranʊs]). It is the only planet whose English name is
derived directly from a figure of Greek mythology. The adjectival
form of Uranus is "Uranian". The pronunciation of the name
Uranus preferred among astronomers is /ˈjʊərənəs/ YOOR-ə-nəs,
with stress on the first syllable as in Latin Ūranus, in contrast to
/jʊˈreɪnəs/ yoo-RAY-nəs, with stress on the second syllable and
a long a, though both are considered acceptable.
Consensus on the name was not reached
until almost 70 years after the planet's discovery. During the
original discussions following discovery, Maskelyne asked Herschel to
"do the astronomical world the faver [sic] to give a name to
your planet, which is entirely your own, [and] which we are so much
obliged to you for the discovery of". In response to Maskelyne's
request, Herschel decided to name the object Georgium Sidus (George's
Star), or the "Georgian Planet" in honor of his new
patron, King George III. He explained this decision in a letter to
Joseph Banks:
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'.
Herschel's proposed name was not
popular outside of Britain and Hanover, and alternatives were soon
proposed. Astronomer Jérôme Lalande proposed that it be named
Herschel in honour of its discoverer. Swedish astronomer Erik
Prosperin proposed the name Neptune, which was supported by other
astronomers who liked the idea to commemorate the victories of the
British Royal Naval fleet in the course of the American Revolutionary
War by calling the new planet even Neptune George III or Neptune
Great Britain.
In a March 1782 treatise, Bode proposed
Uranus, the Latinized version of the Greek god of the sky, Ouranos.
Bode argued that the name should follow the mythology so as not to
stand out as different from the other planets, and that Uranus was an
appropriate name as the father of the first generation of the Titans.
He also noted that elegance of the name in that just as Saturn was
the father of Jupiter, the new planet should be named after the
father of Saturn. Bode was however apparently unaware that Uranus was
only the Latinized form of the titular deity, and his Roman
equivalent was Caelus. In 1789, Bode's Royal Academy colleague Martin
Klaproth named his newly discovered element uranium in support of
Bode's choice. Ultimately, Bode's suggestion became the most widely
used, and became universal in 1850 when HM Nautical Almanac Office,
the final holdout, switched from using Georgium Sidus to Uranus.
Uranus has two astronomical symbols.
The first to be proposed, ⛢, was proposed by Johann Gottfried
Köhler at Bode's request in 1782. Köhler suggested that the new
planet be given the symbol for platinum, which had been described
scientifically only 30 years before. As there was no alchemical
symbol for platinum, he suggested ⛢ or ⛢, a combination of the
planetary-metal symbols ☉ (gold) and ♂ (iron), as platinum (or
'white gold') is found mixed with iron. Bode thought that an upright
orientation, ⛢, fit better with the symbols for the other planets
while remaining distinct. This is the symbol that's preferred for
modern astronomical use, the little that any symbol is. The second
symbol, ♅, was 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 surname"). The second symbol is nearly
universal in astrology.
Uranus is called by a variety of names
in other languages. In Chinese, Japanese, Korean, and Vietnamese, its
name is literally translated as the "sky king star"
(天王星). In Thai, its
official name is Dao Yurenat (ดาวยูเรนัส),
as in English. Its other name in Thai is Dao Maritayu (ดาวมฤตยู,
Star of Mṛtyu), after the Sanskrit word for 'death', Mrtyu
(मृत्यु). In Mongolian, its
name is Tengeriin Van (Тэнгэрийн ван), translated as
'King of the Sky', reflecting its namesake god's role as the
ruler of the heavens. In Hawaiian, its name is Heleʻekala, a
loanword for the discoverer Herschel. In Māori, its name is
Whērangi.
Orbit and rotation
Uranus orbits the Sun once every 84
years, taking an average of seven years to pass through each of the
dozen constellations of the zodiac. In 2033, the planet will have
made its third complete orbit around the Sun since being discovered
in 1781. The planet has returned to the point of its discovery
northeast of Zeta Tauri twice since then, on 25 March 1865 and 29
March 1949. Uranus will return to this location again on 3 April
2033. Its average distance from the Sun is roughly 20 AU (3 billion
km; 2 billion mi). The difference between its minimum and maximum
distance from the Sun is 1.8 AU, larger than that of any other
planet, though not as large as that of dwarf planet Pluto. The
intensity of sunlight varies inversely with the square of distance,
and so on Uranus (at about 20 times the distance from the Sun
compared to Earth) it is about 1/400 the intensity of light on Earth.
The orbital elements of Uranus were
first calculated in 1783 by Pierre-Simon Laplace. 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's orbit. On 23 September 1846, Johann Gottfried Galle
located a new planet, later named Neptune, at nearly the position
predicted by Le Verrier.
The rotational period of the interior
of Uranus is 17 hours, 14 minutes. As on all the giant planets, its
upper atmosphere experiences strong winds in the direction of
rotation. At some latitudes, such as about 60 degrees south, visible
features of the atmosphere move much faster, making a full rotation
in as little as 14 hours.
Axial tilt
The Uranian axis of rotation is
approximately parallel with the plane of the Solar System, with an
axial tilt of 97.77° (as defined by pro-grade rotation). This gives
it seasonal changes completely unlike those of the other planets.
Near the solstice, one pole faces the Sun continuously and the other
faces away, with only a narrow strip around the equator experiencing
a rapid day–night cycle, with the Sun low over the horizon. At the
other side of Uranus's 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. 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.
One result of this axis orientation is
that, averaged over the Uranian year, the near-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 that causes this is
unknown. The reason for Uranus's 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. Research by Jacob
Kegerreis of Durham University suggests that the tilt resulted from a
rock larger than the Earth crashing into the planet 3 to 4 billion
years ago. Uranus's 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 is the pole that points above the invariable
plane of the Solar System, regardless of the direction the planet is
spinning. A different convention is sometimes used, in which a body's
north and south poles are defined according to the right-hand rule in
relation to the direction of rotation.
List of solstices and equinoxes
Winter solstice 1902, 1986, 2069
Summer solstice
Vernal equinox 1923, 2007, 2092
Autumnal equinox
Summer solstice 1944, 2030 Winter
solstice
Autumnal equinox 1965, 2050 Vernal
equinox
Visibility
The mean apparent magnitude of Uranus
is 5.68 with a standard deviation of 0.17, while the extremes are
5.38 and 6.03. This range of brightness is near the limit of naked
eye visibility. Much of the variability is dependent upon the
planetary latitudes being illuminated from the Sun and viewed from
the Earth. Its angular diameter is between 3.4 and 3.7 arc-seconds,
compared with 16 to 20 arc-seconds for Saturn and 32 to 45
arc-seconds for Jupiter. At opposition, Uranus is visible to the
naked eye in dark skies, and becomes an easy target even in urban
conditions with binoculars. In larger amateur telescopes with an
objective diameter of between 15 and 23 cm, Uranus 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.
Physical characteristics
Internal structure
Uranus's mass is roughly 14.5 times
that of Earth, making it the least massive of the giant planets. Its
diameter is slightly larger than Neptune's at roughly four times that
of Earth. A resulting density of 1.27 g/cm3 makes Uranus the second
least dense planet, after Saturn. This value indicates that it is
made primarily of various ices, such as water, ammonia, and methane.
The total mass of ice in Uranus's interior is not precisely known,
because different figures emerge depending on the model chosen; it
must be between 9.3 and 13.5 Earth masses. Hydrogen and helium
constitute only a small part of the total, with between 0.5 and 1.5
Earth masses. The remainder of the non-ice mass (0.5 to 3.7 Earth
masses) is accounted for by rocky material.
The standard model of Uranus's
structure is that it consists of three layers: a rocky
(silicate/iron–nickel) core in the centre, an icy mantle in the
middle and an outer gaseous hydrogen/helium envelope. The core is
relatively small, with a mass of only 0.55 Earth masses and a radius
less than 20% of Uranus'; the mantle comprises its bulk, with around
13.4 Earth masses, and the upper atmosphere is relatively
insubstantial, weighing about 0.5 Earth masses and extending for the
last 20% of Uranus's radius. Uranus's core density is around 9 g/cm3,
with a pressure in the centre of 8 million bars (800 GPa) and a
temperature of about 5000 K. 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. This fluid, which
has a high electrical conductivity, is sometimes called a
water–ammonia ocean.
The extreme pressure and temperature
deep within Uranus may break up the methane molecules, with the
carbon atoms condensing into crystals of diamond that rain down
through the mantle like hailstones. Very-high-pressure experiments at
the Lawrence Livermore National Laboratory suggest that the base of
the mantle may comprise an ocean of metallic liquid carbon, perhaps
with floating solid 'diamond-bergs'. Scientists also believe
that rainfalls of solid diamonds occur on Uranus, as well as on
Jupiter, Saturn, and Neptune.
The bulk compositions of Uranus and
Neptune are different from those of Jupiter and Saturn, with ice
dominating over gases, hence justifying their separate classification
as ice giants. There may be a layer of ionic water where the water
molecules break down into a soup of hydrogen and oxygen ions, and
deeper down super-ionic water in which the oxygen crystallizes but
the hydrogen ions move freely within the oxygen lattice.
Although the model considered above is
reasonably 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 a scientific determination of which model is correct. The fluid
interior structure of Uranus means that it has no solid surface. The
gaseous atmosphere gradually transitions into the internal liquid
layers. For the sake of convenience, a revolving oblate spheroid set
at the point at which atmospheric pressure equals 1 bar (100 kPa) is
conditionally designated as a "surface". It has
equatorial and polar radii of 25,559 ± 4 km (15,881.6 ± 2.5 mi) and
24,973 ± 20 km (15,518 ± 12 mi), respectively. This surface is used
throughout this article as a zero point for altitudes.
Internal heat
Uranus's internal heat appears markedly
lower than that of the other giant planets; in astronomical terms, it
has a low thermal flux. Why Uranus's internal temperature is so low
is still not understood. Neptune, which is Uranus's near twin in size
and composition, radiates 2.61 times as much energy into space as it
receives from the Sun, but Uranus 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. Uranus's 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. The lowest temperature recorded in Uranus's tropopause is
49 K (−224.2 °C; −371.5 °F), making Uranus the coldest planet
in the Solar System.
One of the hypotheses for this
discrepancy suggests that when Uranus was hit by a supermassive
impactor, which caused it to expel most of its primordial heat, it
was left with a depleted core temperature. This impact hypothesis is
also used in some attempts to explain the planet's axial tilt.
Another hypothesis is that some form of barrier exists in Uranus's
upper layers that prevents the core's heat from reaching the surface.
For example, convection may take place in a set of compositionally
different layers, which may inhibit the upward heat transport;
perhaps double diffusive convection is a limiting factor.
In a recent study, the ice giants'
interior conditions were mimicked by compressing water containing
minerals like olivine and ferropericlase. It showed that much
magnesium could be dissolved in the liquid interiors of Uranus and
Neptune. A thermal insulation layer made of dissolved magnesium in
Uranus due to a larger quantity in Uranus than Neptune was proposed
as a possible explanation of Uranus's low temperature.
Atmosphere
Although there is no well-defined solid
surface within Uranus's interior, the outermost part of Uranus's
gaseous envelope that is accessible to remote sensing is called its
atmosphere. Remote-sensing capability extends down to roughly 300 km
below the 1 bar (100 kPa) level, with a corresponding pressure around
100 bar (10 MPa) and temperature of 320 K (47 °C; 116 °F). The
tenuous thermosphere extends over two planetary radii from the
nominal surface, which is defined to lie at a pressure of 1 bar. The
Uranian atmosphere can be divided into three layers: the troposphere,
between altitudes of −300 and 50 km (−186 and 31 mi) and
pressures from 100 to 0.1 bar (10 MPa to 10 kPa); the stratosphere,
spanning altitudes between 50 and 4,000 km (31 and 2,485 mi) and
pressures of between 0.1 and 10−10 bar (10 kPa to 10 µPa); and the
thermosphere extending from 4,000 km to as high as 50,000 km from the
surface. There is no mesosphere.
Composition
The composition of Uranus's atmosphere
is different from its bulk, consisting mainly of molecular hydrogen
and helium. The helium molar fraction, i.e. the number of helium
atoms per molecule of gas, is 0.15±0.03 in the upper troposphere,
which corresponds to a mass fraction 0.26±0.05. This value is close
to the protosolar helium mass fraction of 0.275±0.01, indicating
that helium has not settled in its centre as it has in the gas
giants. The third-most-abundant component of Uranus's atmosphere is
methane (CH4). Methane has prominent absorption bands in the visible
and near-infrared (IR), making Uranus aquamarine or cyan in color.
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 (130 kPa); this represents about 20 to 30 times the carbon
abundance found in the Sun. The mixing ratio 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. The
abundances of less volatile compounds such as ammonia, water, and
hydrogen sulfide in the deep atmosphere are poorly known. They are
probably also higher than solar values. Along with methane, trace
amounts of various hydrocarbons are found in the stratosphere of
Uranus, which are thought to be produced from methane by photolysis
induced by the solar ultraviolet (UV) radiation. They include ethane
(C2H6), acetylene (C2H2), methyl-acetylene (CH3C2H), and diacetylene
(C2HC2H). Spectroscopy has 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.
Troposphere
The troposphere is the lowest and
densest part of the atmosphere and is characterized by a decrease in
temperature with altitude. The temperature falls from about 320 K (47
°C; 116 °F) at the base of the nominal troposphere at −300 km to
53 K (−220 °C; −364 °F) at 50 km. The temperatures in the
coldest upper region of the troposphere (the tropopause) actually
vary in the range between 49 and 57 K (−224 and −216 °C; −371
and −357 °F) depending on planetary latitude. The tropopause
region is responsible for the vast majority of Uranus's thermal far
infrared emissions, thus determining its effective temperature of
59.1 ± 0.3 K (−214.1 ± 0.3 °C; −353.3 ± 0.5 °F).
The troposphere is thought to have a
highly complex cloud structure; water clouds are hypothesized to lie
in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium
hydro-sulfide clouds in the range of 20 to 40 bar (2 to 4 MPa),
ammonia or hydrogen sulfide clouds at between 3 and 10 bar (0.3 and 1
MPa) and finally directly detected thin methane clouds at 1 to 2 bar
(0.1 to 0.2 MPa). The troposphere is a dynamic part of the
atmosphere, exhibiting strong winds, bright clouds and seasonal
changes.
