Uranus (planet)

I

Introduction

Uranus (planet), seventh planet in distance from the Sun, third largest planet in diameter, and fourth largest in mass in the solar system. Unlike other major planets, Uranus is tipped sideways on its axis of rotation. It experiences extreme seasons, and its 13 rings and 27 known moons revolve around its equator nearly vertically to the plane of its orbit around the Sun.

Because of its great size and mass, scientists classify Uranus as one of the giant or Jovian (Jupiter-like) planets—along with Jupiter, Saturn, and Neptune. Like more distant Neptune, Uranus is also classified as an ice giant planet, mainly made of the ice-forming molecules water, ammonia, and methane as a liquid mixture above what is thought to be a rocky core. Its atmosphere is mainly hydrogen and helium, along with methane gas that gives the planet a blue-green color.

Uranus looks like a star to the naked eye, but appears as a blue-green disk through a large telescope—Uranus was the first planet discovered by using a telescope. A flyby by the Voyager 2 space probe in 1986 provided most of the information we have about the planet, its rings, and its moons. Uranus is named after the god of the heavens in Greek and Roman mythology.

Uranus orbits the Sun at an average distance of 2,860 million km (1,780 million mi) in a period of 84 Earth years. The planet only receives about 1/400th of the sunlight that Earth does. The diameter of Uranus at its equator is 51,118 km (31,763 mi). The planet’s mass is 14.54 times greater than the mass of Earth, and its volume is 67 times greater than that of Earth. The force of gravity at the surface of Uranus is 1.17 times the force of gravity on Earth.

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II

Discovery of Uranus

Sir William Herschel, a German-born British musician and astronomer, discovered the planet in 1781 with a telescope he built himself. Herschel accidentally discovered the new planet while measuring shifts in the positions of stars in the constellation Gemini. He observed that Uranus is a moving object, so he first reported his discovery to the British Royal Society as a comet. However, people had observed and plotted Uranus on star charts dating back to 1690 (believing it was a star). Uranus is so faint that people did not consider it important enough to include among the stars outlining the familiar constellations. Astronomers used these earlier observations to identify the object as a planet and to establish its orbit. Herschel originally named the planet Georgium Sidus (Star of George) in honor of King George III of Great Britain. Later, astronomers named the planet after Uranus, a figure who embodied the heavens and was the father of Saturn and the grandfather of Jupiter in Greek and Roman mythology.

III

Observation from Earth and Space

Because Uranus is so far from Earth (2,840 million km/1,760 million mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, which was launched on August 20, 1977, was able to pass by Jupiter (in 1979), Saturn (in 1981), Uranus (in 1986), and Neptune (in 1989). Scientists launched Voyager 2 with just enough energy to pass Jupiter. However, the strong gravitational pull of Jupiter accelerated the spacecraft as it passed by the planet so that Voyager 2 had enough energy to reach Saturn. As Voyager 2 successively passed each of the four giant planets, the gravitational pull of each planet accelerated the spacecraft enough to help it reach the next planet.

As Voyager 2 passed by Uranus, the spacecraft recorded and transmitted images of the planet, its rings, and some of its moons. Astronomers studying these images discovered five previously undetected rings and ten previously undiscovered moons. In addition to discovering these inner moons, Voyager 2 passed close to Miranda, the 11th satellite from Uranus, and mapped the moon’s surface in detail. Surface features of Miranda include craters, canyons, and geologically young systems of ridges and grooves. Because the other large satellites were more distant from the spacecraft’s path, Voyager 2 was unable to make detailed images of their surfaces.

The Hubble Space Telescope has also observed Uranus in different wavelengths, including infrared radiation. Discoveries include two additional moons and two additional rings, and changes in the planet’s atmosphere.

IV

Motion of Uranus

Uranus’s orbit varies from 2,740 million km (1,700 million mi ) to 3,000 million km (1,860 million mi) in distance from the Sun, with an average distance of 2,860 million km (1,780 million mi), or 19.10 astronomical units (AU). An AU is equal to the average distance between Earth and the Sun, or about 150 million km (93 million mi). The orbit of Uranus traces out a flat region of space called the planet’s orbital plane. The orbital plane of Uranus lies close to Earth’s orbital plane. As a result, Uranus always crosses the same region of Earth’s sky. Uranus takes 84 years to complete a single revolution around the Sun, so a year on Uranus is 84 times longer than a year on Earth.

Uranus spins in place around its axis (an imaginary line that runs down the middle of the planet) once every 17.25 hours (0.718 of an Earth day), just as Earth spins once every 24 hours. The ends of the axis mark the north and south poles of Uranus, just as Earth’s axis marks the North Pole and the South Pole on Earth. Uranus rotates about an axis (the way a plastic globe spins on a rod) that tilts 98° into its orbital plane (the plane created by Uranus’s orbit around the Sun). Another method is sometimes used to describe its rotation and its axis. If the North Pole is considered the pole that projects above the plane of its orbit, Uranus can be described as rotating in a retrograde (clockwise) direction in -0.718 Earth days tilted at an angle of 82.2° to the plane of its orbit.

Scientists do not know why Uranus’s axis of rotation is so strongly tilted. One theory is that the planet was struck by another large body early in the history of the solar system, tipping its axis from a more upright position. This cataclysmic event must have happened before its moons and rings formed since these objects orbit in the plane of the planet’s equator and in the same direction as the planet turns. Another theory suggests that gravitational interactions with the planet Saturn may have shifted Uranus’s axis. The giant planets may have formed nearer to the Sun and moved outward to their current orbits, affecting the orbits of other bodies in the solar system.

Because of this tilt, one pole of Uranus points almost directly toward the Sun during half of Uranus’s 84-year orbit, and the other pole points toward the Sun during the second half. This pattern creates 42-year-long seasons of lightness and darkness, alternately, on each end of Uranus. Despite these long seasons, the difference in temperature between the two poles is not great (the planet’s average temperature in its upper atmosphere is about -212°C/-350°F). This uniform temperature indicates that heat is conducted efficiently, or travels easily, throughout the planet.

As Uranus spins about its axis, material near the planet’s equator must travel farther to make one rotation than material near the poles must travel. This equatorial material must then move faster than material at the poles. All material has inertia (the tendency of a moving mass to continue moving in a straight line), and this property makes the fast-moving material near the equator want to fly off from the planet in a straight line. The rest of the planet’s mass gravitationally attracts the material and keeps it glued to the planet, but the material’s inertia makes the planet bulge out at the equator. The bulge around the equator of Uranus is about 2 percent of the radius, or about 500 km (about 300 mi).

V

Composition and Structure