Once considered one of the blander-looking planets, Uranus has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet is 1,783,939,400 miles (2,870,972,170 kilometers) from the sun—so distant that it takes 84 years to complete one orbit.
Why It's Unusual
Uranus, with no solid surface, is one of the gas giant planets. (The others are Jupiter, Saturn, and Neptune.) Its atmosphere is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus's cloud tops, which lie beneath a layer of methane gas. As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through and resulting in the blue-green color that we see.
The planet's atmospheric details are very difficult to see in visible light. 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.
An Off-Kilter Planet
Uranus's rotation axis is nearly horizontal, as though the planet has been knocked on its side. This unusual orientation may be the result of a collision with a planet-size body early in Uranus's history, which apparently radically changed the planet's rotation. Additionally, while magnetic fields are typically in alignment with a planet's rotation, Uranus's magnetic field is tipped over. One day on Uranus lasts for 17 hours and 15 minutes, and it is a retrograde planet, rotating in the opposite direction of the majority of the other planets in its system.
Even though Uranus is tipped on its side and experiences seasons that last over 20 years, the temperature differences on the summer and winter sides do not differ greatly because the planet is so far from the sun. Near the cloud tops, the temperature of Uranus is -357 degrees Fahrenheit (-216 degrees Celsius). The planet's diameter is 31,764 miles (51,119 kilometers).
Because of the planet's unusual orientation, Uranus's rings are perpendicular to its orbital path about the sun. The ten outer rings are dark, thin, and narrow, while the 11th ring is inside the others and is broad and diffuse.
Uranus has 27 known moons, named mostly for characters from the works of William Shakespeare and Alexander Pope. Miranda is the strangest-looking Uranian moon, appearing as though it were made of spare parts. 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 NASA/JPL