Planet Profile

Mass (kg)............................................1.02 x 10^26
Diameter (km)........................................49528
Mean density (kg/m^3) ...............................1640
Escape velocity (m/sec)..............................23300
Average distance from Sun (AU).......................30.06
Rotation period (length of day in Earth hours).......19.1
Revolution period (length of year in Earth years)....164.8
Obliquity (tilt of axis in degrees)..................29.6
Orbit inclination (degrees)..........................1.77
Orbit eccentricity (deviation from circular).........0.009
Mean temperature (K).................................48
Visual geometric albedo (reflectivity)...............0.51
Atmospheric components...............................74% hydrogen, 25% helium, 1% methane (at depth) Rings................................................Rings are narrow, and contain concentrations of particles called ring arcs.

Neptune Image Gallery


Neptune (planet), major planet in the solar system, eighth planet from the Sun, and fourth largest in diameter. Neptune maintains an almost constant distance, about 4.5 billion km (about 2.8 billion mi), from the Sun. Neptune revolves outside the orbit of Uranus and for most of its orbit, moves inside the elliptical path of the outermost planet Pluto (see Solar System). Every 248 years, Pluto's elliptical orbit brings the planet inside Neptune's nearly circular orbit for about 20 years, temporarily making Neptune the farthest planet from the Sun. The last time Pluto's orbit brought it inside Neptune's orbit was in 1979. In 1999, Pluto's orbit brought it back outside Neptune's orbit.

Astronomers believe Neptune has an inner rocky core that is surrounded by a vast ocean of water mixed with rocky material. From the inner core, this ocean extends upward until it meets a gaseous atmosphere of hydrogen, helium, and trace amounts of methane. Neptune has four rings and eight known moons. Even though Neptune's volume is 72 times that of Earth's volume, its mass is only 17 times Earth's mass. Because of its size, scientists classify Neptune—along with Jupiter, Saturn, and Uranus—as one of the giant or Jovian planets (so-called because they resemble Jupiter).

Mathematical theories of astronomy led to the discovery of Neptune. To account for wobbles in the orbit of the planet Uranus, British astronomer John Couch Adams and French astronomer Urbain Jean Joseph Leverrier independently calculated the existence and position of a new planet in 1845 and 1846, respectively. They theorized that the gravitational attraction of this planet for Uranus was causing the wobbles in Uranus's orbit. Using information from Leverrier, German astronomer Johann Gottfried Galle first observed the planet in 1846.

After its discovery, Leverrier proposed that the new planet be named after the sea god Neptune from Greek and Roman mythology. The appropriateness of this name was confirmed in the 20th century when astronomers learned about Neptune's watery interior.


The orbit of Neptune traces out a flat region of space called the planet's orbital plane. The orbital plane of Neptune (and of all the planets in our solar system except for Pluto) lies close to Earth's orbital plane. As a result, Neptune always crosses the same region of Earth's sky. Neptune is barely visible to the naked eye and is so faint that even through binoculars it appears as a dim star. Through a large telescope, the planet appears from Earth as a small greenish disk with a diameter of about 2.3 arc seconds. Astronomers use the unit arc second to describe the size of objects in the night sky. Arc seconds give the angle an object blocks out in the sky (a quarter held at arm's length is approximately 7000 arc seconds).

Because Neptune is so far from Earth (4.49 x 109 km/2.79 x 109 mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, 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 the planet accelerated the spacecraft enough to help it reach the next planet, until it reached Neptune more than ten years after its launch.

As Voyager 2 passed by Neptune, it recorded and transmitted images of the planet, its rings, and its moons. Astronomers studying these images discovered four rings and six previously undiscovered moons. Four of these newly discovered moons include the inner moons of Neptune, the largest of which measures only 180 km (112 mi) in diameter—small enough to fit in a large crater of Earth's Moon.


Neptune takes 164.79 years to complete a single revolution around the Sun, so a year on Neptune is 164.79 times longer than a year on Earth. The planet spins in place once every 16 hours, just as Earth spins once every 24 hours. Neptune spins around its axis, an imaginary line that runs down the middle of the planet. The ends of the line mark the north and south poles of Neptune, much like Earth's axis marks the North Pole and South Pole on Earth. The axis of rotation on Neptune tilts 29.6° into its orbital plane (the plane created by Neptune's orbit around the Sun). This tilt gives Neptune almost Earthlike season. (Seasons on Earth result from our planet's 23.5° tilt into its orbital plane.)


Neptune contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Neptune formed from frozen water and rock supplied by icy comet-like material found in the outer regions of the solar system. As the planet grew in size, pressures and temperatures in the planet's interior increased, heating the planet's frozen water into hot liquid.

Astronomers believe that Neptune has a solid core no larger than Earth (Earth's diameter is 12,756 km/7,926 mi) and that this core is composed primarily of iron and silicon. Neptune's core may be small because most of the rock composing the planet remains mixed with the vast ocean that extends upward from the core to the atmosphere.

Neptune's vast body of liquid accounts for most of its volume. Scientists think this ocean is composed mostly of water as well as molecules of methane and ammonia. Neptune's ocean is extremely hot (about 4700° C/about 8500° F). The ocean remains liquid at this temperature instead of evaporating because the pressure deep in Neptune is several million times higher than the atmospheric pressure on Earth. Higher pressure holds molecules in liquid closer together and prevents them from spreading apart to form vapor.

The gaseous atmosphere of Neptune contains hydrogen, helium, and about 3 percent methane. It extends about 5,000 km (about 3,000 mi) above the planet's ocean. Light reflected from Neptune's deep atmosphere is blue, because the atmospheric methane absorbs red and orange light but scatters blue light. In 1998 astronomers also identified molecules of methyl in Neptune's atmosphere.

Methyl molecules each contain one carbon atom and three hydrogen atoms. Methyl molecules are known as hydrocarbon radicals because they are short-lived and highly reactive. They combine to form ethane (C2H6), a flammable, colorless gas. The discovery of methyl in Neptune's atmosphere marked the first observation of a hydrocarbon radical in the atmosphere of the outer planets. Astronomers hypothesize that great storm systems on Neptune eject methane into the upper atmosphere. Once in the upper atmosphere, the Sun's energy breaks the methane down into methyl molecules.

Neptune emits about 2.7 times the amount of heat it absorbs from the Sun. Astronomers believe the excess heat that Neptune radiates comes from comets that crashed into Neptune as the planet formed. Due to the force of gravity in the planet's interior, the material in Neptune's core is continually being pulled inward. As the material compacts, the molecules strike each other more frequently and with more force, releasing energy in the form of heat. Neptune's core, which reaches temperatures of 5149° C (9300° F), is hotter than the Sun's surface.

Neptune has an active atmosphere, with winds and massive storms that may be caused by heat escaping the planet's interior. Neptune's winds, which blow in a latitude direction, are faster at the planet's polar regions than they are at Neptune's equator. Neptune has the fastest winds in the solar system, reaching speeds of 2,000 km/hour (1,200 mi/hr).

Using the Hubble Space Telescope, astronomers have observed storms thousands of kilometers across in Neptune's atmosphere. These storms are often visible as dark spots that appear and disappear in Neptune's atmosphere over many months. The largest storm, known as the Great Dark Spot, appeared in the planet's southern hemisphere and was photographed extensively in 1989 by the Voyager 2 spacecraft. Scientists estimated that the Great Dark Spot was as large in diameter as Earth is. By 1994 images transmitted to Earth by the Hubble Space Telescope showed that the Great Dark Spot had disappeared. Scientists believe this dark spot was an immense storm that either dissipated or was covered by other atmospheric features. Later in 1994, astronomers observed the emergence of another large dark spot in Neptune's northern hemisphere, indicating that the planet's atmosphere changes rapidly.

Although Neptune is one of the giant planets, it is smaller and has a different chemical composition than those of Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Neptune captured a much smaller amount of these elements as the solar system formed. Instead, Neptune captured mostly water. Because water is more dense than are hydrogen and helium, Neptune is more compact than either Jupiter or Saturn is. Jupiter, for example, has a radius of 71,355 km (44,338 mi), while Neptune has a radius of 24,766 km (15,389 mi).


From Voyager 2 spacecraft images, astronomers identified four rings of debris encircling Neptune's equator. These rings range in width from 15 km (9.3 mi) to 5,800 km (3,600 mi). All of these rings completely encircle the planet, but the outermost ring includes three or more arcs of concentrated debris, some of which had been detected from Earth before the Voyager 2 encounter. Astronomers speculate that the gravitational pull from nearby moons may cause smaller particles to form these arcs.

Eight moons orbit Neptune. Only two of these moons— Triton and Nereid—are large enough to be observed from Earth. Triton was discovered in 1846 by British astronomer William Lassell, and Nereid was discovered in 1949 by Dutch-born American astronomer Gerard Kuiper. Scientists discovered another moon in 1981 when the moon occulted (moved in front of) a star, and discovered the other five moons of Neptune from images transmitted to Earth by the Voyager 2 spacecraft.

Nereid is the outermost moon from Neptune and has the most elliptical orbit of any planet or moon in the solar system, varying in distance around Neptune from 1.4 x 106 km (8.7 x 105 mi) to 9.6 x 106 km (6.0 x 106 mi). Triton is the second-outermost moon from Neptune and is the largest of the planet's moons, measuring 2,700 km (1,700 mi) in diameter. Triton, which is composed of about one-quarter ice and three-quarters rock, has few craters on its surface, suggesting that this moon is relatively young.

Proteus is the sixth-outermost moon from Neptune. This satellite is the largest irregularly shaped moon in the solar system, measuring 436 km (262 mi) through its widest diameter and 402 km (241 mi) through its narrowest diameter. Larissa is the fifth-outermost moon from Neptune. It is heavily cratered and irregular in shape. Its density, chemical composition, and internal structure are unknown.

The four innermost moons of Neptune are the smallest, ranging in diameter from 58 km (36 mi) to 180 km (110 mi). From the farthest out from Neptune inward, these innermost moons are Naiad, Thalassa, Despina, and Galatea.

Neptune, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Scientists believe that the slow escape of heat from the planet's core circulates currents of electrically charged particles in Neptune's deep ocean, generating a magnetic field. Neptune's magnetic axis, the line indicating the direction of the force the planet's magnetic field exerts, is aligned at an angle of 47° to Neptune's axis of rotation. The influence of Neptune's magnetic field extends for several hundred thousand kilometers above the planet.

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