I INTRODUCTION
Uranus (planet), major planet in the solar system, seventh planet from the
Sun. Uranus revolves outside the orbit of Saturn and inside the orbit of
Neptune (see Solar System). The average distance from Uranus to the Sun is
2.87 billion km (1.78 billion mi). Uranus has an inner rocky core that is
surrounded by a vast ocean of water mixed with rocky material. From the
core, this ocean extends upward until it meets an atmosphere of hydrogen,
helium, and methane. Uranus has 10 known rings, 18 confirmed moons, and 3
other bodies that may be moons, which were first sighted in 1999. The mass
of Uranus is 14.5 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. Because of its size and
mass, scientists classify Uranus as one of the giant or Jovian (like
Jupiter) planets—along with Jupiter, Saturn, and Neptune.
Uranus was the first planet that people discovered by using a telescope. Sir
William Herschel, a German-born British musician and astronomer, discovered
the planet in 1781. Herschel accidentally discovered it 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).
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.
II OBSERVATION FROM EARTH AND SPACE
Uranus orbits the Sun, varying from 2.74 x 109 km (1.70 x 109 mi) to 3.00 x
109 km (1.86 x 109 mi) in distance from the Sun. 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, which appears to be a
star to the naked eye, is so faint that people did not consider it important
enough to include among the stars outlining the familiar constellations.
Through a large telescope, the planet appears as a blue-green disk with a
diameter of about 3.5 arc seconds. Arc seconds describe the size of objects
in the night sky by giving the size of the angle that the objects block out
in the sky (a quarter held at arm’s length is approximately 7,000 arc
seconds).
Because Uranus is so far from Earth (2.84 × 109 km/1.76 × 109 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.
III MOTION OF URANUS
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, 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). 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).
IV COMPOSITION AND STRUCTURE
Uranus contains mostly rock and water, with hydrogen and helium (and trace
amounts of methane) in its dense atmosphere. Astronomers believe that Uranus
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 6,650°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.
Scientists think this ocean consists mostly of water molecules, which are
mixed with silicate, magnesium, nitrogen-bearing molecules, and hydrocarbons
(molecules composed of carbon and hydrogen). Uranus’s ocean is extremely hot
(about 6,650°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.
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. Astronomers believe this atmosphere is relatively calm and inactive,
with few storms or clouds. 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.
V SPACE AROUND URANUS
Astronomers have identified ten rings of debris encircling Uranus’s equator.
These extremely dark, narrow rings orbit the planet in the plane of its
equator at distances from 3.8 × 104 km (2.4 × 104 mi) to 5.1 × 104 km (3.2 ×
104 mi). Many of these rings are made of ice and rock boulders about the
size of large beach balls. American astronomer James L. Elliot detected five
of the ten rings in 1977. Starting from the innermost ring, these five rings
are called Alpha, Beta, Gamma, Delta, and Epsilon. In 1986 images taken by
the Voyager 2 spacecraft helped scientists discover five more rings
encircling Uranus.
Astronomers have confirmed that at least 18 moons orbit Uranus, and they
have sighted 3 other possible moons. Sixteen of the confirmed moons revolve
about the planet’s equator, moving with the planet in an east-to-west
direction. The other two confirmed moons orbit Uranus from west to east at a
large angle to the planet’s equator.
Seventeen of Uranus’s confirmed moons are named for characters in the works
of English playwright William Shakespeare and English poet Alexander Pope.
The two largest and brightest moons, Oberon and Titania, were discovered by
Sir William Herschel in 1787. British astronomer William Lassell detected
the two next largest moons, Umbriel and Ariel. American astronomer Gerard
Peter Kuiper discovered Miranda in 1948. Voyager 2 helped scientists
discover Uranus’s 11 innermost moons, each with a diameter of less than 100
km (60 mi). The ninth moon from Uranus was discovered in 1999 from photos
that Voyager 2 took in 1986. This moon does not yet have an official name,
but is known as 1986U10. In order of their distance from Uranus, these 11
moons are Cordelia (which is closest), Ophelia, Bianca, Cressida, Desdemona,
Juliet, Portia, Rosalind, 1986U10, Belinda, and Puck. Two other 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 two distant moons were subsequently
named Caliban and Sycorax. Unlike the planet’s 16 other moons, these 2 moons
do not orbit in the plane of Uranus’s equator. Astronomers believe that
Caliban and Sycorax are captured asteroids instead of satellites that formed
from the same cloud of planetary nebula (dust and gases that condense into
planets) that formed Uranus. The surfaces of Uranus’s four largest
moons—Ariel, Umbriel Titania, and Oberon—are old, heavily cratered, and
geologically inactive. Astronomers believe that these four moons consist of
half ice and half rock (see Hale Observatories). In 1999 astronomers
discovered three other bodies that are possibly moons of Uranus. These
bodies are known as S/1999U1, S/1999U2, and S/1999U3.
VI ROTATIONAL AND MAGNETIC AXIS
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 make these charged particles move through the
ocean, 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 the 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 Uranus’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 planet’s center. The
influence of Uranus’s magnetic field extends for several hundred thousand
kilometers above the planet.