Uranus

Uranus is the only giant planet whose equator is nearly at right angles to its orbit. A collision with an Earth-sized object may explain Uranus’ unique tilt. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint.

Uranus, discovered in 1781 by astronomer William Herschel, was the first planet found with the aid of a telescope. As the seventh planet from the sun, Uranus takes 84 Earth years to complete one orbit.

Like Venus, Uranus rotates east to west. Uranus’ rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side. This situation may be the result of a collision with a planet-sized body early in the planet’s history, which apparently radically changed Uranus’ rotation. Because of Uranus’ unusual orientation, the planet experiences extreme variations in sunlight during each 20-year-long season.

Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the sun because Uranus was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space.

Uranus reached equinox in December 2007, when it was fully illuminated as the sun passed over the planet’s equator. By 2028, the north pole will point directly at the sun, a reversal of the situation when Voyager flew by. Equinox also brings ring-plane crossing, when Uranus’ rings appear to move more and more edge-on as seen from Earth.

The Hubble Space Telescope and the Keck Observatory in Hawaii captured detailed images of Uranus as the planet approached equinox. While Voyager 2 saw only a few discrete clouds, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly evolving bright features and a new Great Dark Spot like those seen on Neptune.

Uranus is one of the two ice giants of the outer solar system (the other is Neptune). Uranus’ atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus’ cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting mostly of icy materials (water, methane and ammonia).

For nearly a quarter of the Uranian year, the sun shines directly over each pole, plunging the other half of the planet into a long, dark winter.

While magnetic fields are typically in alignment with a planet’s rotation, Uranus’ magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet’s axis of rotation, and is also offset from the center of the planet by one-third of the planet’s radius. The magnetic fields of both Uranus and Neptune are very irregular.

Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble observations and observations made at the Keck Observatory showed that the outer rings are brightly colored.

Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.

Infrared image of the dark side of the rings. NASA
Infrared image of the dark side of the rings. NASA
Discovered By
William Herschel
Date of Discovery
13 March 1781
Orbit Size Around Sun (semi-major axis)
Metric: 2,870,658,186 km
English: 1,783,744,300 miles
Scientific Notation: 2.8706582 x 109 km (1.9189165 x 101 A.U.)
By Comparison: 19.189 x Earth
Perihelion (closest)
Metric: 2,734,998,229 km
English: 1,699,449,110 miles
Scientific Notation: 2.73500 x 109 km (1.828 x 101 A.U.)
By Comparison: 18.593 x Earth
Aphelion (farthest)
Metric: 3,006,318,143 km
English: 1,868,039,489 miles
Scientific Notation: 3.00632 x 109 km (2.010 x 101 A.U.)
By Comparison: 19.766 x Earth
Sidereal Orbit Period (Length of Year)
84.016846 Earth years
30,687.15 Earth days
By Comparison: 84.017 x Earth
Orbit Circumference
Metric: 18,026,802,831 km
English: 11,201,335,967 miles
Scientific Notation: 1.803 x 1010 km
By Comparison: 19.180 x Earth
Average Orbit Velocity
Metric: 24,477 km/h
English: 15,209 mph
Scientific Notation: 6.7991 x 103 m/s
By Comparison: 0.228 x Earth
Orbit Eccentricity
0.04725744
By Comparison: 2.828 x Earth
Orbit Inclination
0.77 degrees
Equatorial Inclination to Orbit
97.8 degrees (retrograde rotation) (retrograde rotation)
By Comparison: 4.173 x Earth
Mean Radius
Metric: 25,362 km
English: 15,759.2 miles
Scientific Notation: 2.5362 x 104 km
By Comparison: 3.9809 x Earth
Equatorial Circumference
Metric: 159,354.1 km
English: 99,018.1 miles
Scientific Notation: 1.59354 x 105 km
By Comparison: 3.9809 x Earth
Volume
Metric: 68,334,355,695,584 km3
Scientific Notation: 6.83344 x 1013 km3
By Comparison: 63.085 x Earth
Mass
Metric: 86,810,300,000,000,000,000,000,000 kg
Scientific Notation: 8.6810 x 1025 kg
By Comparison: 14.536 x Earth’s
Density
Metric: 1.270 g/cm3
By Comparison: 0.230 x Earth
Surface Area
Metric: 8,083,079,690 km2
English: 3,120,894,516 square miles
Scientific Notation: 8.0831 x 109 km2
By Comparison: 15.847 x Earth
Surface Gravity
Metric: 8.87 m/s2
English: 29.1 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 91 pounds on Uranus.
Escape Velocity
Metric: 76,968 km/h
English: 47,826 mph
Scientific Notation: 2.138 x 104 m/s
By Comparison: 1.911 x Earth
Sidereal Rotation Period (Length of Day)
-0.718 Earth days (retrograde)
-17.23992 hours (retrograde)
By Comparison: 0.72 x Earth
Effective Temperature
Metric: -216 °C
English: -357 °F
Scientific Notation: 57 K
Atmospheric Constituents
Hydrogen, Helium, Methane
Scientific Notation: H2, He, CH4
By Comparison: Earth’s atmosphere consists mostly of N2 and O2.