Uranus

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.

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

A

Interior of Uranus

Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Uranus, like Neptune, formed from the same material—principally frozen water and rock—that composes most of the planet’s moons. As the planet grew, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into a hot liquid.

Uranus probably has a relatively small rocky core (smaller in size than Earth’s core), with a radius no larger than 2,000 km (1,240 mi) and a temperature of about 6650°C (12,000°F). Uranus’s core may be small because most of the rock composing the planet remains mixed with the body of water that surrounds the core and extends upward to the planet’s atmosphere.

The vast body of liquid on Uranus accounts for most of the planet’s volume. This compressed, slushy liquid is sometimes described as an ocean or as ice. Scientists think this ocean consists mostly of water molecules, which are mixed with silicate, magnesium, nitrogen-bearing molecules such as ammonia, and hydrocarbons (molecules composed of carbon and hydrogen) such as methane. Uranus’s ocean is extremely hot (about 6650°C/about 12,000°F). Water at the surface of Earth evaporates, or boils, at 100°C (212°F). The ocean on Uranus remains liquid at such a high temperature, however, because the pressure deep in Uranus is about five million times stronger than the atmospheric pressure on Earth at sea level. Higher pressure holds molecules in liquids close together and prevents them from spreading out to form vapor.

B

Atmosphere

The atmosphere of Uranus, which contains hydrogen, helium, and trace amounts of methane, extends about 5,000 km (about 3,100 mi) above the planet’s ocean. At the time of the Voyager 2 flyby in 1986, the atmosphere was relatively calm and inactive, with few storms or clouds, but Hubble Space Telescope images showed more activity in 2001. Winds blow parallel to the equator of Uranus, moving in the same direction as the planet’s rotation at high latitudes, and opposite to the rotation at low latitudes. These winds layer Uranus’s clouds into bands. Light reflected from Uranus’s deep atmosphere is blue-green, because the atmospheric methane absorbs red and orange light. Unlike the other giant planets, Uranus radiates little heat into space from its deep interior.

Although Uranus is one of the giant planets, it is smaller and has a different chemical composition than Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Uranus captured a much smaller amount of these elements as the solar system formed. Instead, Uranus captured mostly water. Because water is more dense than hydrogen and helium, Uranus is more compact than Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi) while Uranus has a radius of 25,548 km (15,875 mi). If Uranus had the same mass it has now but consisted of the lighter elements hydrogen and helium, the planet would be larger but much less dense than Jupiter. Uranus is also slightly less massive, and thus less dense and less compact, than Neptune, which is otherwise very similar in composition. As a result, the radius of Uranus is slightly larger than the radius of denser Neptune.

C

Magnetic Field

Uranus, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Uranus’s deep oceans contain electrically charged particles called ions. Ocean currents on Uranus circulate these charged particles, which in turn creates a magnetic field. Scientists believe that ocean currents in the other Jovian planets—Neptune, Saturn, and Jupiter—are created by heat released from these planets’ cores. The core of Uranus releases less heat than the other three Jovian planets, however, and astronomers are unsure about what causes ocean currents in the planet’s fluid interior. Uranus’s magnetic field is similar in strength to Earth’s magnetic field. Uranus’s magnetic axis (the line joining the north and south poles of its magnetic field) is aligned with the planet’s strongly tilted rotational axis, although the magnetic field is offset from the center of the planet. The influence of Uranus’s magnetic field extends for several hundred thousand kilometers above the planet.

VI

RINGS AND MOONS

Astronomers have identified 13 rings of debris encircling Uranus’s equator. An inner set of extremely dark, narrow rings orbit the planet in the plane of its equator at distances from 38,000 km (24,000 mi) from the center of the planet. Many of these rings are made of ice and rock boulders about the size of large beach balls. Several observatories first detected five of the ten rings in 1977. Starting from the innermost ring, these five rings were called Alpha, Beta, Gamma, Delta, and Epsilon. In 1986 images taken by the Voyager 2 spacecraft helped scientists discover five more rings encircling Uranus. In 2005 astronomers using the Hubble Space Telescope reported the discovery of two new rings. These rings are so far from the planet that they make up a second ring system. The innermost of these more distant rings is about 67,000 km (41,632 mi) from the planet’s center and the outmost about 97,700 km (60,708 mi) from the center.

Astronomers have found at least 27 moons that orbit Uranus. Uranus’s moons are named for characters in the works of English playwright William Shakespeare and English poet Alexander Pope. The two largest and brightest moons, Titania and Oberon, were discovered by Sir William Herschel in 1787. British astronomer William Lassell detected the two next largest moons, Umbriel and Ariel. The surfaces of these four largest moons are old, heavily cratered, and geologically inactive. Astronomers believe that these four moons consist of half ice and half rock. American astronomer Gerard Peter Kuiper discovered a smaller fifth moon, Miranda, in 1948.

Voyager 2 helped scientists discover 11 of Uranus’s inner moons, each with a diameter of less than 100 km (60 mi). The tenth moon of the group, 1986U10, was discovered in 1999 from photos that Voyager 2 took in 1986. This moon was later named Perdita. In order of their distance from Uranus, these inner moons are Cordelia , Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Perdita, and Puck.

Two more distant moons were discovered in 1997 by Canadian astronomer Brett Gladman and collaborators using the 200-inch telescope and a special camera at the Palomar Observatory on Mount Palomar in California. These moons were subsequently named Caliban and Sycorax. In 1999 the same group reported the discovery of three additional small, distant moons: Prospero, Setebos, and Stephano. Prospero and Setebos are even more distant from Uranus than Sycorax, while Stephano’s average orbital distance lies between those of Caliban and Sycorax. Unlike the planet’s other moons, these five outer moons orbit Uranus in the direction opposite that in which the planet rotates and follow highly eccentric orbits that are inclined to the plane of Uranus’s equator. Astronomers believe that these oddball satellites are captured asteroids rather than satellites that formed from the same planetary nebula (cloud of dust and gases that condenses into planets) that formed Uranus (see Hale Observatories).

The Hubble Space Telescope enabled astronomers in 2005 to detect two more moons, named Mab and Cupid. Mab has a diameter of about 19 km (12 mi). Astronomers believe meteoroid impacts with Mab continually replenish dust in the newly discovered outer ring system.

Contributed By:
William B. Hubbard

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