Uranus << YUR uh nuhs or yu RAY nuhs >> is the farthest planet from the sun that can be seen without a telescope. Uranus’s average distance from the sun is about 1,784,860,000 miles (2,872,460,000 kilometers). At that distance, light from the sun takes about 2 hours and 40 minutes to reach Uranus.
Uranus was the first planet discovered since ancient times. The British astronomer William Herschel found it in 1781. Johann E. Bode, a German astronomer, named it Uranus after a sky god in Greek mythology. Most of our information about Uranus comes from the United States Voyager 2 space probe. In 1986, that craft flew within about 50,000 miles (80,000 kilometers) of the planet.
Uranus and Neptune have similar colors, interior structures, masses and densities. Astronomers sometimes refer to Uranus and Neptune as “twin planets.”
Orbit and rotation.
Uranus travels around the sun in an elliptical (oval-shaped) orbit. It completes an orbit in 30,685 Earth days, or just over 84 Earth years. As it orbits the sun, Uranus also rotates on its axis, an imaginary line through its center. The planet’s interior takes 17 hours and 14 minutes to spin around once on its axis. However, much of the atmosphere rotates more rapidly. The fastest winds on Uranus are measured about two-thirds of the way from the equator to the south pole. They blow at about 450 miles per hour (720 kilometers per hour). Thus, this area toward the south pole makes one complete rotation about every 14 hours. 
Uranus is tilted so far on its side that its axis lies nearly level with its path around the sun. Scientists measure the tilt of a planet relative to a line at a right angle to the orbital plane. The orbital plane is an imaginary surface touching all points of the orbit. Most planets’ axes tilt less than 30°. However, Uranus’s axis tilts 98°, so that the axis lies almost in the orbital plane. Many astronomers think that a collision with an Earth-sized planet knocked Uranus on its side early in the solar system’s formation.
Mass and density.
Uranus has a mass (amount of matter) 14 1/2 times that of Earth. However, the mass of Uranus is only about 1/20 that of the largest planet, Jupiter. Uranus has an average density of 1.27 grams per cubic centimeter. Density is the amount of mass in a substance divided by its volume. Uranus’s density is about 1 1/4 times that of water. The density of Uranus is 1/4 that of Earth. It is similar to that of Jupiter.
Structure and composition.
Many scientists believe Uranus is composed of three layers: (1) a core, (2) a mantle, and (3) an atmosphere. The planet has no crust or solid surface. Instead, most rocks or minerals are found deep within the planet.
The core
probably consists mainly of silicates. Silicates are minerals that also make up much of Earth’s crust (outer layer). Scientists suspect the core has a temperature of over 11,000 °F (6000 °C).
The mantle
has a composition that leads astronomers to call the planet an “ice giant.” It consists mainly of a mixture of solid and liquid water, methane, and ammonia, all under high pressure. The mantle’s high temperature—thousands of degrees Fahrenheit or Celsius—makes its ice significantly different from ice on Earth’s surface. Some scientists believe that the high temperature breaks some of the methane into individual atoms of carbon and hydrogen. Under the high pressure, the carbon atoms may crystallize to form diamonds.
The atmosphere
consists of about 83 percent hydrogen, 15 percent helium, 2 percent methane, and trace amounts of ammonia and ethane. The methane ice crystals in Uranus’s atmosphere give the planet its pale blue-green color. The visible surface of the atmosphere has a few small spots. These spots probably are violently swirling masses of gas resembling hurricanes.
In 2006, astronomers using the Hubble Space Telescope discovered a large, dark spot in Uranus’s northern hemisphere. It was made of whirling gases. The spot measured 1,100 miles (1,700 kilometers) by 1,900 miles (3,000 kilometers), almost as big as the United States or China. Scientists think it formed as spring brought weather changes to Uranus’s northern hemisphere.
Uranus has an average temperature of −323 °F (−197 °C), measured at an altitude where the atmosphere’s pressure equals the atmospheric pressure on Earth’s surface. Uranus seems to radiate as much heat into space as it gets from the sun. Because of Uranus’s extreme tilt, its poles receive more sunlight during a Uranian year than does its equator. However, Uranus’s winds seem to distribute the extra heat fairly evenly throughout the atmosphere.
Satellites.
Astronomers have identified at least 28 satellites of Uranus. But the planet probably has more that have yet to be discovered. Astronomers discovered the five largest satellites between 1787 and 1948. Photographs by Voyager 2 in 1985 and 1986 revealed 10 additional satellites. Astronomers later discovered more satellites using Earth-based telescopes. Nearly all of Uranus’s satellites are named for characters from the works of the English playwright William Shakespeare.
Miranda is the smallest of the five large satellites. It has certain surface features not seen anywhere else in the solar system. These are three oddly shaped regions called ovoids. Each ovoid is 120 to 190 miles (200 to 300 kilometers) across. The outer area of each ovoid resembles a racetrack, with parallel ridges and canyons wrapped about the center. In the center, however, ridges and canyons crisscross one another randomly. Miranda has some of the highest cliffs of any planet or moon in the solar system.
Rings.
Uranus has a number of rings around it. Ten of them are dark and narrow. They range in width from less than 3 miles (5 kilometers) to 60 miles (100 kilometers). They are no more than 33 feet (10 meters) thick. Two faint, dusty rings lie well outside the narrow rings. Uranus also has a broad, less distinct ring closer to the planet than the others. The exact composition of the rings is unknown. But they probably consist of small chunks and dust-sized grains of ice, coated in a layer of material that contains carbon. 
Magnetic field.
Uranus has a strong magnetic field. The field’s axis—an imaginary line connecting its north and south poles—is tilted 59° from the planet’s axis of rotation. The magnetic field has trapped high-energy, electrically charged particles—mostly electrons and protons—in radiation belts around the planet. As these particles travel back and forth between the magnetic poles, they send out radio waves. Voyager 2 detected the waves. But they are so weak that they cannot be detected on Earth. 
