Uranus - The 7th Planet

Facts & Figures

Once considered one of the blander-looking planets, URANUS (pronounced YOOR un nus) has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and a fragile ring system that wobbles like an unbalanced wagon wheel.

Uranus gets its blue-green color from methane gas above the deeper cloud layers (methane absorbs red light and reflects blue light). Uranus was discovered in 1781 by astronomer William Herschel, who at first believed it to be a comet. This seventh planet from the Sun is so distant that it takes 84 years to complete an orbit. 

The third largest planet in our solar system, Uranus is classified as a “gas giant” planet because it has no solid surface. The atmosphere of Uranus is hydrogen and helium, with a small amount of methane and traces of water and ammonia. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of “icy” materials (water, methane, and ammonia), with higher density material at depth. 

In 1986, Voyager 2 observed faint cloud markings in the southern latitudes blowing westward between 100 and 600 km/hr. In 1998, the Hubble Space Telescope observed as many as 20 bright clouds at various altitudes in Uranus’s atmosphere. The bright clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the atmosphere of Uranus. 

Uranus currently moves around the Sun with its rotation axis nearly horizontal with respect to the ecliptic plane. This unusual orientation may be the result of a collision with a planet-sized body early in the planet’s history, which apparently changed Uranus’s rotation radically. 

Uranus's magnetic field is unusual in that the magnetic axis is tilted 60 degrees from the planet’s axis of rotation and is offset from the center of the planet by one-third of the planet’s radius. 

Uranus is so far from the Sun that, even though tipped on its side and experiencing seasons that last over twenty years, the temperature differences on the summer and winter sides of the planet do not differ that greatly. Near the cloudtops, the temperature of Uranus is near -215 °C. 

Six of Uranus’s rings were discovered in 1977 by scientists aboard NASA’s Kuiper Airborne Observatory who were watching a star pass behind Uranus. They noticed the starlight winking on and off as the star first appeared to move toward the planet, and then again as the star moved away from the planet. Perth Observatory found three more rings that same day, and Voyager 2 found two more rings in 1986, bringing the count to 11. The rings are in the planet’s equatorial plane, perpendicular to its orbit about the Sun. The 10 outer rings are dark, thin, and narrow, while the 11th ring is inside the other ten and is broad and diffuse. The rings of Uranus are very different from those surrounding Uranus and Saturn. When viewed with the Sun behind the rings, fine dust can be seen scattered throughout all of the rings. 

Uranus has at least 21 moons, named mostly for characters from the works of Shakespeare and Alexander Pope. Miranda is the strangest Uranian moon. The high cliffs and winding valleys of the moon may indicate partial melting of the interior, with icy material occasionally drifting to the surface.

Text courtesy of NASA