Uranus planet Facts 

Uranus is named for the personifi cation of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 bil-lion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant plan-ets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour.

Uranus has been visited by a space-

craft only once—by the U.S. Voyager 2

probe in 1986. Before then, astronomers

had known little about the planet, since its

distance from Earth makes the study of its

 visible surface difficult even with the most

powerful telescopes available. Earth-

based attempts to measure a property as

basic as the planetary rotation period had

produced widely diff ering values, ranging

from 24 to 13 hours, until Voyager 2 fi nally

established a 17.24-hour rotation period

for the Uranian interior. Since Voyager’s

encounter, advances in Earth-based obser-

vational technology have added to

knowledge of the Uranian system.

Uranus and its neighbour Neptune,

the next planet outward from the Sun,

are nearly twins in size. Measured at the

level of the atmosphere at which the

pressure is one bar (equivalent to Earth’s

sea-level pressure), Uranus’s equatorial

radius of 25,559 km (15,882 miles) is 3.2

percent greater than that of Neptune.

But Uranus has only 85 percent the mass

of Neptune and thus is significantly less

dense. The difference in their bulk den-

sities—1.285 and 1.64 grams per cubic

cm, respectively—reveals a fundamental

difference in composition and internal

structure. Although Uranus and Neptune

are significantly larger than the terres-

trial planets, their radii are less than

half those of the largest planets, Jupiter

and Saturn.

Because Uranus’s spin axis is not per-

fectly parallel to the ecliptic, one of its

poles is directed above the ecliptic and

the other below it. (The terms above and

below refer to the same sides of the eclip-

tic as Earth’s North and South poles,

respectively.) According to international

convention, the north pole of a planet is

defined as the pole that is above the eclip-

tic regardless of the direction in which

the planet is spinning. In terms of this

definition, Uranus spins clockwise, or in a

retrograde fashion, about its north pole,

which is opposite to the prograde spin of

Earth and most of the other planets.

When Voyager 2 flew by Uranus in 1986,

the north pole was in darkness, and the

Sun was almost directly overhead at the

south pole. In 42 years, or one-half the

Uranian year, the Sun will have moved to

a position nearly overhead at the north

pole. The prevailing theory is that the

severe tilt arose during the final stages of

planetary accretion when bodies compa-

rable in size to the present planets

collided in a series of violent events that

knocked Uranus on its side. An alternate

theory is that a Mars-sized moon, orbit-

ing Uranus in a direction opposite to the

planet’s spin, eventually crashed into the

planet and knocked it on its side.

Uranus’s rotation period of 17.24

hours was inferred when Voyager 2

detected radio wave emissions with that

period coming from charged particles

trapped in the planet’s magnetic field.

Subsequent direct measurements of the

field showed that it is tilted at an angle of 58.6° relative to the rotation axis and

that it turns with the same 17.24-hour

period. Because the field is thought to

be generated in the electrically conduct-

ing interior of the planet, the 17.24-hour

period is assumed to be that of the inte-

rior. The relatively fast rotation causes an

oblateness, or flattening of the planet’s 

poles, such that the polar radius is about  

2.3 percent smaller than the equatorial

radius. Winds in the atmosphere cause

cloud markings on the visible surface to

rotate around the planet with periods

ranging from 18 hours near the equator

to slightly more than 14 hours at higher

latitudes.

The atmosphere

Molecular hydrogen and atomic helium

are the two main constituents of the

Uranian atmosphere. Hydrogen is detect-

able from Earth in the spectrum of

sunlight scattered by the planet’s clouds.

The ratio of helium to hydrogen was

determined from the refraction (bend-

ing) of Voyager 2’s radio signal by the

atmosphere as the spacecraft passed

behind the planet. Helium was found to

make up 15 percent of the total number of

hydrogen molecules and helium atoms, a

proportion that corresponds to 26 per-

cent by mass of the total amount of

hydrogen and helium. These values are

consistent with the values inferred for

the Sun and are greater than those

inferred for the atmospheres of Jupiter

and Saturn. It is assumed that all four

giant planets received the same propor-

tions of hydrogen and helium as the Sun

during their formation but that, in the

cases of Jupiter and Saturn, some of the

helium has settled toward their centres.

The processes that cause this settling

have been shown in theoretical studies

not to operate on less-massive planets

like Uranus and Neptune.

Methane absorbs strongly at near-

infrared wavelengths, and it dominates

that part of the spectrum of reflected light

even though the number of methane mol-

ecules is only 2.3 percent of the total.

Astronomers determined this estimate of

methane abundance using Voyager 2’s

radio signals that probed to atmospheric

depths at which the methane-to-hydro-

gen ratio is likely to be constant. If this

constancy is characteristic of the planet

as a whole, the carbon-to-hydrogen ratio

of Uranus is 24 times that of the Sun.

(Methane [CH4

] comprises one atom of

carbon and four of hydrogen.) The large

value of the carbon-to-hydrogen ratio

suggests that the elements oxygen, nitro-

gen, and sulfur also are enriched relative

to solar values. These elements, however,

are tied up in molecules of water, ammo-

nia, and hydrogen sulfide, which are

thought to condense into clouds at levels

below the part of the atmosphere that can

be seen. Earth-based radio observations

reveal a curious depletion of ammonia

molecules in the atmosphere, perhaps

because hydrogen sulfide is more abun-

dant and combines with all the ammonia

to form cloud particles of ammonium

hydrosulfide. Voyager’s ultraviolet spec-

trometer detected traces of acetylene and

ethane in very low abundances. These

gases are by-products of methane, which

dissociates when ultraviolet light from

the Sun strikes the upper atmosphere.

On average, Uranus radiates the same

amount of energy as an ideal, perfectly

absorbing surface at a temperature of

59.1 kelvins (K; −353 °F, −214 °C). This

radiation temperature is equal to the

physical temperature of the atmosphere

at a pressure of about 0.4 bar. Temperature

decreases with decreasing pressure—i.e.,

with increasing altitude—throughout this

portion of the atmosphere to the 70-mil-

libar level, where it is about 52 K (−366 °F,−221 °C), the coldest temperature in

Uranus’s atmosphere. From this point

upward the temperature rises again until

it reaches 750 K (890 °F, 480 °C) in the

exosphere—the top of the atmosphere at

a distance of 1.1 Uranian radii from the

planetary centre—where pressures are on

the order of a trillionth of a bar. The cause

of the high exospheric temperatures

remains to be determined, but it may

involve a combination of ultraviolet

absorption, electron bombardment, and

inability of the gas to radiate at infrared

wavelengths.

Voyager 2 measured the horizontal

variation of atmospheric temperature in

two broad altitude ranges, at 60–200 mil-

libars and 500–1,000 millibars. In both

ranges the pole-to-pole variation was

found to be small—less than 1 K (1.8 °F, 1

°C)—despite the fact that one pole was

facing the Sun at the time of the flyby.

This lack of global variation is thought to

be related to the efficient horizontal heat

transfer and the large heat-storage capac-

ity of the deep atmosphere.

Although Uranus appears nearly

featureless to the eye, extreme-contrast-

enhanced images from Voyager 2 and

more-recent observations from Earth

reveal faint cloud bands oriented paral-

lel to the equator. The same kind of zonal

flow dominates the atmospheric circula-

tion of Jupiter and Saturn, whose

rotational axes are much less tilted than

Uranus’s axis and thus whose seasonal

changes in solar illumination are much

different. Apparently, rotation of the

planet itself and not the distribution of absorbed sunlight controls the cloud

patterns. Rotation manifests itself

through the Coriolis force, an effect that

causes material moving on a rotating

planet to appear to be deflected to either

the right or the left depending on the

hemisphere—northern or southern—

being considered. In terms of cloud

patterns, therefore, Uranus looks like a

tipped-over version of Jupiter or Saturn.

The wind is the motion of the atmo-

sphere relative to the rotating planet. At

high latitudes on Uranus, this relative

motion is in the direction of the planet’s

rotation. At equatorial latitudes the rela-

tive motion is in the opposite direction.

Uranus is like Earth in this regard. On

Earth these directions are called east and

west, respectively, but the more general

terms are prograde and retrograde. The

winds that exist on Uranus are several

times stronger than on Earth. The wind is

720 km (450 miles) per hour prograde at a

latitude of 55° S and 400 km (250 miles)

per hour retrograde at the equator.

Neptune’s equatorial winds are also ret-

rograde, but those of Jupiter and Saturn

are prograde. No satisfactory theory

exists to explain these differences.

Uranus has no large spots like

Jupiter’s long-lived Great Red Spot or

the Great Dark Spot observed on

Neptune by Voyager 2 in 1989. Voyager’s

measurements of the wind profile on

Uranus came from just four small spots

whose visual contrast was no more than

2 or 3 percent relative to the surround-

ing atmosphere. Because the giant

planets have no solid surfaces, the spots 

must represent atmospheric storms. For

reasons that are not clear, Uranus seems

to have the smallest number of storms of

any of the giant planets.

Uranus’s

moons and rings

Uranus’s 27 known moons are accompa-

nied by at least 10 narrow rings. Each of

the countless particles that make up the

rings can be considered a tiny moon in

its own orbit. In general, the rings are

located closest to the planet, some small

moons orbit just outside the rings, the

largest moons orbit beyond them, and

other small moons orbit much farther out.

The orbits of the outermost group of

moons are eccentric (elongated) and

highly inclined to Uranus’s equatorial

plane. The other moons and the rings are

essentially coplanar with the equator.

Moons

Uranus’s five largest moons range from

about 240 to 800 km (150 to 500 miles) in

radius. All were discovered telescopically

from Earth, four of them before the 20th

century. Ten small inner moons were

found by Voyager 2 in 1985–86. They are

estimated to be between about 10 and 80

km (6 and 50 miles) in radius, and they

orbit the planet at distances between

49,800 and 86,000 km (31,000 and 53,500

miles). The innermost moon, Cordelia,

orbits just inside the outermost rings,

Lambda and Epsilon. An 11th tiny inner

moon, Perdita, photographed by Voyager

near the orbit of Belinda remained unno-

ticed in the images until 1999 and was

not confi rmed until 2003. Two additional

inner moons, one near Belinda’s orbit,

Cupid, and the other near Puck’s, Mab,

were discovered in observations from

Earth in 2003. All 18 of the above are reg-

ular, having prograde, low-inclination,

and low-eccentricity orbits with respect

to the planet.

 Nine small outer moons in roughly

the same size range as the Voyager fi nds

were discovered from Earth beginning in

1997. These are irregular satellites, hav-

ing highly elliptical orbits that are

inclined at large angles to the planet’s

equator; all but one also orbit in the retro-

grade direction. Their mean distances

from the planet lie between 4 million and

21 million km (2.5 million and 13 million

miles), which is 7–36 times the distance of

the outermost known regular moon,

Oberon. The irregular moons likely were

captured into orbits around Uranus after

the planet formed. The regular moons

probably formed in their equatorial orbits

at the same time that the planet formed.

 The four largest moons— Titania ,

 Oberon , Umbriel , and Ariel , in order of

decreasing size—have densities of 1.4–1.7

grams per cubic cm. This range is only

slightly greater than the density of a

hypothetical object that would be

obtained by cooling a mixture of solar

composition and removing all the gas-

eous components. The object that

remained would be 60 percent ice and 40 

percent rock. In contrast to these four

is Miranda, the fifth largest Uranian

moon, but only half the size of Ariel or

Umbriel. Like the smaller moons of

Saturn, Miranda has a density (1.2 g/cm3

[0.7 oz/in3 ]) that is slightly below the  

solar composition value, which indicates

a higher ice-to-rock ratio.

The discovery of Uranus

Uranus was discovered by the English

astronomer William Herschel, who had

undertaken a survey of all stars down to

eighth magnitude—i.e., those about five

times fainter than stars visible to the naked

eye. On March 13, 1781, he found “a curious

either nebulous star or perhaps a comet,”

distinguished from the stars by its clearly

visible disk. Its lack of any trace of a tail

and its slow motion led within months to

the conclusion that the object was a planet,

rather than a comet or an asteroid, moving

in a nearly circular orbit well beyond

Saturn. Observations of the new planet

during the next 65 years revealed discrep-

ancies in its orbital motion—evidence of

gravitational forces on Uranus that were

not due to any other known planet, which

ultimately led to the discovery of yet more

distant Neptune in 1846.

Herschel suggested naming his

new discovery Georgium Sidus (Latin:

“Georgian Planet”) after his patron, King

George III of England, while the French

favoured the name Herschel. The planet

was eventually named according to the

tradition of naming planets for the gods

of Greek and Roman mythology; Uranus

is the father of Saturn, who is in turn the

father of Jupiter.

Umbriel

 Umbriel is the third nearest of the fi ve

major moons of Uranus and the one hav-

ing the darkest and oldest surface of the

group. Its discovery is attributed to the

English astronomer William Lassell in

1851, although the English astronomer

William Herschel, who discovered Uranus

and its two largest moons, may have

glimpsed it more than a half century ear-

lier. Umbriel was named by Herschel’s

son, John , for a character in Alexander

Pope’s poem The Rape of the Lock .

 It orbits Uranus once every 4.144 days

at a mean distance of 265,970 km (165,270

miles). Umbriel has a diameter of 1,170

km (727 miles) and a density of about 1.4

g/cm3 (0.81 oz/in3 ).

The only images of Umbriel’s surface

have come from the U.S. Voyager 2 space-

craft’s fl yby encounter with the Uranian

system in 1986. These show that Umbriel

is distinct from the other major moons of

Uranus in having no evidence of past tec-

tonic activity, as shown by the presence

of many large impact craters. The most

notable feature of the hemisphere imaged

by Voyager is a bright ring, dubbed

Wunda, that appears to line the fl oor of a

crater 40 km (25 miles) across.

 Titania

Titania is the largest of the moons of

 Uranus . It was fi rst detected telescopi-

cally in 1787 by William Herschel , who

had discovered Uranus itself six years earlier. Titania was named by William’s

son, John Herschel, for a character in

William Shakespeare’s play A Midsummer

Night’s Dream.

Titania orbits at a mean distance of

435,840 km (270,820 miles) from the cen-

tre of Uranus, which makes it the second

outermost of the planet’s major moons.

Its orbital period is 8.706 days, as is its

rotational period. It is thus in synchro-

nous rotation, keeping the same face

toward the planet and the same face for-

ward in its orbit. Its diameter is 1,578 km

(980 miles), and it has a density of about

1.71 g/cm3

 (1 oz/in3

).

Titania was observed close up on only

one occasion, when the U.S. Voyager 2

spacecraft swiftly flew through the

Uranian system in 1986. Spacecraft images

show its surface to have many bright

impact craters up to 50 km (30 miles) in

diameter, but few large ones, along with

trenches and a deep fault line extending

roughly 1,600 km (1,000 miles). These and

other related features strongly suggest

the occurrence of internal geologic pro-

cesses in the moon’s ancient past.

Oberon

The outermost of the five major moons of

Uranus and the second largest is Oberon,

which was discovered in 1787 by William

Herschel. As with Titania, it was also

named by William’s son for a character in

William Shakespeare’s play A Midsummer

Night’s Dream.

The mean distance of Oberon from

the centre of Uranus is about 582,600 km

(362,000 miles), and its orbital period is

13.46 days. Like all of Uranus’s large moons,

Oberon rotates synchronously with its

orbital period, keeping the same hemi-

sphere toward the planet and the same

hemisphere forward in its orbit. The moon

has a diameter of 1,522 km (946 miles) and

a density of 1.63 g/cm3 (0.94 oz/in3 ) .

Photographic images transmitted by

the U.S. Voyager 2 spacecraft when it flew

past the Uranian system in 1986 revealed

that Oberon’s surface is old and that a few

of the numerous bright craters appear to

have been flooded by some kind of dark

material that upwelled from the moon’s

interior.

Spacecraft exploration

Although the missions of the twin Voyager

1 and 2 spacecraft originally called for fly-

bys of only Jupiter and Saturn, the timing

of Voyager 2’s launch allowed for a change

to its trajectory so that it could be retar-

geted to Uranus and Neptune for an

extended mission, which ultimately was

carried out. After more than eight years

in space, Voyager 2 sped through the

Uranian system on Jan. 24, 1986. Its

instruments provided an accurate deter-

mination of the masses and radii of the

planet and its major moons, detected

Uranus’s magnetic field and determined

its strength and orientation, and mea-

sured the planet’s interior rotation rate.

Images of the Uranian system, which

totaled more than 8,000, revealed for the

first time the weather patterns in the

planet’s atmosphere and the surface char-

acteristics of the moons. In addition to

Voyager’s discoveries of new moons, a

ring, and dust bands between the rings, it

provided details of ring structure at scales

not achievable from Earth.

Yet, despite these achievements,

Voyager left many unanswered questions

that only another spacecraft mission or a

major advance in Earth-based observa-

tional technology would be able to

address. As of 2009, no missions to

Uranus are planned.

The interior

Although Uranus has a somewhat lower

density than Jupiter, it has a higher pro-

portion of elements heavier than hydrogen

and helium. Jupiter’s greater mass (by a

factor of 22) leads to a greater gravitational

force and thus greater self-compression

than for Uranus. This additional compres-

sion adds to Jupiter’s bulk density. If

Uranus were made of the same propor-

tions of material as Jupiter, it would be

considerably less dense than it is.

Different models proposed for the

Uranian interior assume different ratios of

rock (silicates and metals), ices (water,

methane, and ammonia), and gases (essen-

tially hydrogen and helium). At the high

temperatures and pressures within the

giant planets, the “ices” will in fact be liq-

uids. To be consistent with the bulk density

data, the mass of rock plus ice must consti-

tute roughly 80 percent of the total mass

of Uranus, compared with 10 percent for

Jupiter and 2 percent for a mixture of the

Sun’s composition. In all models Uranus is a fluid planet, with the gaseous higher

atmosphere gradually merging with the

liquid interior. Pressure at the centre of the

planet is about five megabars.

Scientists have obtained more infor-

mation about the interior by comparing a

given model’s response to centrifugal

forces, which arise from the planet’s rota-

tion, with the response of the actual

planet measured by Voyager 2. This

response is expressed in terms of the

planet’s oblateness. By measuring the

degree of flattening at the poles and relat-

ing it to the speed of rotation, scientists

can infer the density distribution inside

the planet. For two planets with the same

mass and bulk density, the planet with

more of its mass concentrated close to

the centre would be less flattened by rota-

tion. Before the Voyager mission, it was

difficult to choose between models in

which the three components—rock, ice,

and gas—were separated into distinct lay-

ers and those in which the ice and gas

were well mixed. From the combination

of large oblateness and comparatively

slow rotation for Uranus measured by

Voyager, it appears that the ice and gas

are well mixed and a rocky core is small

or nonexistent.