Uranus

Uranus

The Magician

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.

        orbit:    2,870,990,000 km (19.218 AU) from Sun
        diameter: 51,118 km (equatorial)
        mass:     8.683e25 kg

Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus" , not "your anus" or "urine us".

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures (right). Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.

Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 13 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.

Uranus' Satellites

Uranus has 27 named moons:
  • Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope.
  • They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones (right), and the newly discovered much more distant ones.
  • Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical.

           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Cordelia         50      13    ?      Voyager 2    1986
Ophelia          54      16    ?      Voyager 2    1986
Bianca           59      22    ?      Voyager 2    1986
Cressida         62      33    ?      Voyager 2    1986
Desdemona        63      29    ?      Voyager 2    1986
Juliet           64      42    ?      Voyager 2    1986
Portia           66      55    ?      Voyager 2    1986
Rosalind         70      27    ?      Voyager 2    1986
Cupid            75       6    ?      Showalter    2003
Belinda          75      34    ?      Voyager 2    1986
Perdita          76      40    ?      Voyager 2    1986
Puck             86      77    ?      Voyager 2    1985
Mab              98       8    ?      Showalter    2003
Miranda         130     236  6.30e19  Kuiper       1948
Ariel           191     579  1.27e21  Lassell      1851
Umbriel         266     585  1.27e21  Lassell      1851
Titania         436     789  3.49e21  Herschel     1787
Oberon          583     761  3.03e21  Herschel     1787
Francisco      4281       6    ?      Sheppard     2003
Caliban        7169      40    ?      Gladman      1997
Stephano       7948      15    ?      Gladman      1999
Trinculo       8578       5    ?      Holman       2001
Sycorax       12213      80    ?      Nicholson    1997
Margaret      14689       6    ?      Sheppard     2003
Prospero      16568      20    ?      Holman       1999
Setebos       17681      20    ?      Kavelaars    1999
Ferdinand     21000       6    ?      Sheppard     2003

Uranus' Rings

         Distance   	Width
Ring       (km)     	 (km)
-------  --------   	-----
Zeta 	39,600		3,500 (formally 1986U2R)
6 	41,840		1-3
5 	42,230 		2-3
4 	42,580 		2-3
Alpha 	44,720 		7-12
Beta 	45,670 		7-12
Eta 	47,190 		0-2
Gamma 	47,630 		1-4
Delta 	48,290 		3-9
Lambda 	50,024 		1-2 (formally 1986 U1R)
Epsilon 51,140 		20-100
Nu 	67,300 		3,800 (formally R/2003 U2)
Mu 	97,700 		17,000 (formally R/2003 U1)

(distance is from Uranus' center to the ring's inner edge)

More about Uranus and its satellites

Open Issues

  • Why doesn't Uranus radiate more heat than it receives from the Sun as the other gas planets do? Is its interior cold?
  • Why is its axis so unusually tilted? Was it due to a massive collision?
  • Why do Uranus and Neptune have so much less hydrogen and helium than Jupiter and Saturn? Is it simply because they are smaller? or because they're farther from the Sun?
  • What will happen to Uranus's weather as it progresses through its seasons?

Page update August 14th 2014


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