Voyager 2 image of the blue-green planet Uranus
Hubble image of Uranus and its system of rings
False color Hubble view of Uranus showing cloud details

Voyager 2 image of the
blue-green planet Uranus

(NASA/JPL)

Hubble image of Uranus and its
system of delicate rings
(NASA, ESA, and M. Showalter)

False color Hubble view of Uranus showing cloud details (NASA and Erich Karkoschka, University of Arizona)

Wife of Cronus

The next planetary encounter on our tour brings us to the domain of the ice giants. These planets differ from the gas giants because they are composed primarily of frozen water, ammonia, and methane rather than hydrogen and helium. Here we find a large, blue planet with little or no surface features. It shines like a blue green gem in the heavens. This is the planet Uranus, and it is the seventh planet from the Sun. Uranus was named after the god of the heavens in Greek mythology. According to myth, Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans. The planet Uranus was the first planet to be discovered in modern times. The British astronomer Sir William Herschel found it with his telescope in 1781. Before this, it had been assumed to be a star. Herschel also discovered Uranus' two largest moons, Oberon and Titania. We now know of 27 moons in orbit around Uranus. They represent a wide range of sizes and shapes. Uranus has only been visited by one spacecraft. This was the extended Voyager 2 mission. That encounter took place on January 24, 1986. Nearly everything we know about Uranus was learned at this time.

Recent Hubble image of Uranus and it rings
Voyager 2 false color image of Uranus' showing details
Voyager 2 image of Uranus as seen from behind

Recent Hubble image of
Uranus and it rings

(Erich Karkoschka, University of
Arizona, and NASA)

Voyager 2 false color image
of Uranus' showing details

(NASA/JPL)

Voyager 2 image of Uranus as seen from behind
(NASA/JPL)

The Sideways Planet

Uranus is different from any other planet in the Solar System in that its axis of rotation is tilted 98 degrees. It is nearly parallel to the ecliptic. Uranus appears to be tipped on its side. This has some interesting effects on the planet. The Sun shines alternately on the poles and equatorial regions of the planet rather than constantly shining on its equatorial regions, as on Earth. Astronomers are not sure what accounts for this strange position. One theory suggests that a massive collision with another planet-sized body may have knocked Uranus on its side. This collision is believed to have taken place very early in the Solar System's history. Another strange fact about Uranus is that its magnetic field is tilted at a 60-degree angle to its axis of rotation. This angle is only 12 degrees on Earth. An interesting effect of Uranus' sideways tilt is that the tail of the planet's magnetic field is twisted like a corkscrew.

This false-color view of the rings of Uranus was made from images taken by Voyager 2 on Jan. 21, 1986, from a distance of 4.17 million kilometers (2.59 million miles). All nine known rings are visible in this image. (NASA/JPL)

Features of Uranus

When Voyager 2 arrived at Uranus, astronomers were expecting to find complex cloud structures similar to those on Jupiter and Saturn. But when the images finally arrived, nothing was visible but a featureless blue-green sphere. False-color images of the planet later showed that banded structures of clouds were present but were extremely faint. They are partially obscured by the overlying layer of methane. Uranus is the third of the gas giants from the Sun. It is believed to be composed of about 83% hydrogen, 15% helium, 2% methane, and trace amounts of acetylene and other hydrocarbons. It is the methane in the planet's upper atmosphere that gives it a blue-green color. This is because the methane absorbs red light. High winds in the middle latitudes of Uranus blow in the direction of the planet's rotation at up to 360 miles (580 km) per hour. Studies have shown somewhat slower winds blowing in the opposite direction at the planet's equator. Astronomers once thought that beneath these layers of clouds was a massive ocean of super-pressurized liquid ammonia and water. But data sent back from Voyager 2 seem to indicate that this is probably not the case. The internal structure of Uranus is probably very similar to that of Jupiter and Saturn, but without the rocky core and liquid metallic hydrogen center. Instead, there is probably a less-dense collection of rocky material at the core. In 1977, Earth-based astronomers discovered that Uranus, like all of the other gas giant planets, has a system of rings. A total of nine rings were identified from Earth. When Voyager 2 visited Uranus in 1986, two more rings were discovered. Two of Uranus' moons, Cordelia and Ophelia, act as shepherding moons for one of the rings. Voyager 2 also discovered that Uranus has a magnetic field. It is tilted 60 degrees from the planet's rotational axis, and is about the same intensity as the magnetic field on Earth. The magnetic field on Earth and the other terrestrial planets is probably caused by electrical currents produced in the planets' molten core. Since Uranus has no molten core, astronomers are not sure what is responsible for creating the magnetic field.

 

 

Statistics for Uranus

Discovered by

Year of Discovery

Diameter

Known Natural Satellites

Mean Distance from the Sun

Rotational Period

Orbital Period

Orbital Eccentricity

Orbital Inclination

Inclination of Axis

Mean Cloud Temperature

Main Atmospheric Component

Atmospheric Pressure

Apparent Magnitude

......

......

......

......

......

......

......

......

......

......

......

......

......

......

William Herschel

1781

31,755 miles (51,118 km)

27

1,783,487,000 miles (2,870,990,000 km)

17.3 hours

84 years

0.047

0.774 degrees

97.86 degrees

-315° F (-193° C)

Hydrogen

1.2 bars

5.52

 

The Moons of Uranus

Name

Pronunciation

Order

Diameter
(km) 

Orbital Period
(days)

Distance
(km)

Year
Discovered

Cordelia

Ophelia

Bianca

Cressida

Desdemona

Juliet

Portia

Rosalind

Cupid

Belinda

Perdita

Puck

Mab

Miranda

Ariel

Umbriel

Titania

Oberon

Francisco

Caliban

Stephano

Trinculo

Sycorax

Margaret

Prospero

Setebos

Ferdinand

kor-DEEL-ee-uh

oh-FEEL-ee-uh

bee-ANG-kuh

KRES-id-uh

dez-de-MOH-nuh

JOO-lee-et

POR-shuh

ROZ-uh-lind

KYU-pid

bel-LIN-duh

PUR-dih-tuh

PUK

MAB

mir-RAN-duh

AIR-ee-ul

UM-bree-ul

ti-TAH-nee-uh

OH-ber-on

fran-SISS-ko

KAL-i-ban

STEF-uh-noh

TRING-kyoo-loh

SIK-or-aks

MAR-gret

PROS-per-oh

SET-eh-bos

FER-dih-nand

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

40

43

51

80

64

93

135

72

18

90

30

162

25

472

1,158

1,169

1,578

1,522

22

72

32

18

150

20

50

48

20


0.34

0.38

0.43

0.46

0.47

0.49

0.51

0.56

0.62

0.62

0.64

0.76

0.92

1.41

2.52

4.14

8.7

13.46

−266.56

−579.73

−677.37

−749.24

−1288.28

1687.01

−1978.29

−2225.21

−2805.51

49,751

53,764

59,165

61,766

62,658

64,360

66,097

69,927

74,800

75,255

76,420

86,004

97,734

129,390

191,020

266,300

435,910

583,520

4,276,000

7,231,000

8,004,000

8,504,000

12,179,000

14,345,000

16,256,000

17,418,000

20,901,000

1986

1986

1986

1986

1986

1986

1986

1986

2003

1986

1999

1985

2003

1948

1851

1851

1787

1787

2003

1997

1999

2001

1997

2003

1999

1999

2003

 

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