Neptune in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and the dwarf planet Pluto; its period of revolution is about 165 years. (Pluto has such a highly elliptical orbit that from 1979 to 1999 it was closer to the sun than Neptune; it will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit.) Neptune was discovered as the result of observed irregularities in the motion of Uranus and was the first planet to be discovered on the basis of theoretical calculations. J. C. Adams of Britain and U. J. Leverrier of France independently predicted the position of Neptune, and it was discovered by J. C. Galle in 1846, the day after he received Leverrier's prediction. Neptune has an equatorial diameter of about 30,700 mi (49,400 km), nearly four times that of the earth, and a mass about 17 times the earth's mass. It is much like Uranus and the other giant planets, with a thick atmosphere of hydrogen, helium, methane, and ammonia, a relatively low density, and a rapid period of rotation. On Aug. 24-25, 1989, the U.S. spacecraft Voyager 2 observed Neptune and its moons. It discovered that Neptune's atmosphere has zones like Jupiter's as well as giant storm systems as dark spots on its surface. Although Neptune receives a much smaller fraction of the sun's radiation than does Uranus, its surface temperature is similar: -350°F (-212°C).This may indicate a possible internal heat source. Neptune's largest moon, Triton, was discovered in 1846, a month after the discovery of the planet itself. Triton has a diameter of about 1,700 mi (2,700 km), and its motion is retrograde (see retrograde motion ), i.e., opposite to that of the planet's rotation. Its surface temperature is -400°F (-240°C), making it one of the coldest objects in the solar system. Nereid, discovered in 1949, has a diameter of about 210 mi (338 km), is very faint, and has a highly elliptic orbit; it may be of asteroid origin. Voyager discovered six smaller dark moons orbiting between the planet and Triton: Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus—all irregularly shaped, ranging from 35 to 260 mi (58-418 km) in diameter. Since Neptune was named for the Roman god of the sea, its moons were named for various lesser sea gods and nymphs in Greek mythology. Five additional moons, as yet unnamed, were discovered using earth-based telescopes in 2002 and 2003. Voyager also found a faint ring system with three bands. These are named Adams, Leverrier, and Galle in honor of the planet's discoverers. Composed of small rocks and dust, the rings are not uniform in thickness or density. Adams, the outermost, contains three prominent arcs named Liberty, Equality, and Fraternity.

Bibliography: See P. Moore, Planet Neptune (1989); E. Burgess, Far Encounter: The Neptune System (1992); G. E. Hunt et al., Atlas of Neptune (1994).

Neptune Ordinarily the eighth planet of the solar system, but the ninth when Pluto's eccentric orbit carries it inside the orbit of Neptune. Neptune is at 30.06 AU from the Sun and between 4305.6 × 10⁶ km and 4687.3 × 10⁶ km from Earth. Its equatorial radius is 24 766 km; polar radius 24342 km; volume 6254 × 10¹⁰ km³; mass 102.43 × 10²⁴kg; mean density 1638 kg/m³; gravity 11 (Earth = 1); visual albedo 0.41; black-body temperature 33.2 K. Neptune has a dense atmosphere, with a surface pressure greater than 100 bars, composed of molecular hydrogen (89%), helium (11%), and traces of methane, with aerosols of ammonia ice, water ice, ammonia hydrosulphide, and possibly methane ice (similar to that of Uranus). The average surface temperature is about 58 K, and winds blow at up to 200 m/s. Neptune has eight known satellites (see NEPTUNIAN SATELLITES).

Neptune

Neptune is the most distant giant planet, circling the Sun at an average distance of almost 6 billion kilometers (3.7 billion miles; thirty-nine times the distance from Earth to the Sun). Neptune is a near twin to Uranus in size (with a radius of 24,764 kilometers [15,354 miles] at the equator), in composition (about 80 percent hydrogen, 15 percent helium, and 3 percent methane, with other trace elements), and in internal structure (a rocky core surrounded by a methane-and ammonia-rich watery mantle topped by a thick atmosphere).

The icy particles in the upper cloud decks of Neptune differ slightly from those of Uranus. Their color, combined with the atmospheric methane that absorbs red light, gives Neptune a rich sky-blue tint compared with the more greenish Uranus. Neptune has the strongest internal heat source of all the giant planets, radiating almost three times more heat than one would expect. Like Jupiter and Saturn, which radiate about twice as much energy than expected, Neptune is thought to have excess heat from the time of the planet's formation and from continued gravitational contraction . Neptune's rotational axis is inclined only 29 degrees, compared with Uranus's more than 90 degrees.

A Saga of Discovery

The discovery of Neptune was a mathematical triumph and a political nightmare. After Uranus was discovered in 1781, astronomers inferred the presence of another planet from the shape of the Uranian orbit. In England, astronomer John Adams made meticulous but unpublished calculations of the planet's likely position in 1845. Shortly thereafter, French astronomer Urbain Leverrier independently determined the suspected planet's position, which nearly matched Adams's prediction. After Leverrier's work was published in 1846, English astronomers realized that Adams's work warranted a more serious look. But by then, the French astronomer had sent his prediction to observers in Berlin. Almost immediately, German astronomer Johann Galle discovered Neptune near the predicted location. For years, debates raged across national boundaries over who deserved credit for the discovery of Neptune. We now credit both Leverrier and Adams for the prediction and recognize Galle for the actual observation.

Unusual Cloud Features

In 1989, Voyager 2 flew by Neptune and detected numerous cloud features. The biggest was the Great Dark Spot, a hurricane-like storm that was about half the size of Earth. The next to be discovered was a small white spot, which appeared to race rapidly around the planet when compared with the lumbering Great Dark Spot. It was named the "Scooter." Many more spots were found, many of which were rotating even faster than Scooter. A small dark spot in the south developed a bright core, and a bright clump near the south pole was observed to be composed of many fast-moving bright patches.

Rotation and Magnetic Field

Voyager 2 measured Neptune's 16.11-hour internal rotation period by monitoring the planet's magnetic field. The atmosphere rotates with periods ranging from over 18 hours near the equator to faster than 13 hours near the poles. In fact, the winds of Neptune are among the fastest in the solar system; only Saturn's high-speed equatorial jet is faster. Like the Uranian magnetic field, Neptune's magnetic field is also offset from the planet's center and significantly tilted with respect to the planet's rotation axis. Neptune's field is about 60 percent weaker than that of Uranus.

The Moons of Neptune

Neptune's largest moon, Triton, has a retrograde and highly inclined orbit. This suggests the moon may have been captured rather than formed around Neptune. Triton has a thin atmosphere of primarily nitrogen gas, thought to be in equilibrium with the nitrogen ice covering Triton's surface. Because of Triton's unusual orbit, however, the surface ice is thought to change with time, leading to the possibility that Triton's atmosphere also varies. Recent occultations (observations of stars glimmering through Triton's tenuous atmosphere) suggest that Triton's atmosphere may have expanded by nearly a factor of two since the Voyager 2 encounter. Triton's northern hemisphere looks much like the surface of a cantaloupe. The southern hemisphere is dominated by a polar ice cap, probably composed of nitrogen. In the highest resolution images, active geysers (ice volcanoes) were seen spewing columns of dark material many kilometers into the thin atmosphere. Triton's surface has relatively few impact craters , suggesting that it is young.

Nereid (the only other Neptune moon known prior to the Voyager 2 mission) also has an unusual orbit that is highly elliptical and tilted nearly 30 degrees, again suggesting a capture origin. Little is known about it other than its irregular shape. Voyager 2 discovered six additional moons around Neptune. These are all in circular prograde orbits near Neptune's equatorial plane, and they probably formed in place. One of these, Proteus, is larger than Nereid; it had not been discovered prior to the Voyager 2 encounter because it is so close to Neptune. Proteus is irregular in shape. A particularly large impact crater suggests that it came close to destruction in an earlier collision.

The Rings of Neptune

Astronomers used occultations to search for rings around Neptune, because that technique had been successful for discovering the rings of Uranus. The results were odd: some events seemed to clearly show rings, but others clearly did not. The Voyager 2 encounter solved the puzzle. There were three complete rings, but the rings were variable in their thicknesses (the three distinct rings were named Adams, Leverrier, and Galle, after the astronomers who were involved in the discovery saga). The thickest parts—dubbed rings arcs—were seen during occultations; the other parts of the rings were too thin to be detected. Scientists are not sure what causes Neptune's rings arcs. Some of the smallest moons appear to "shepherd" the inner edges of two of the rings, but no moons were found at locations that would explain the clumps through a shepherding mechanism. Despite their clumpiness, Neptune's rings are very circular, unlike the rings of Uranus.

Recent Hubble Space Telescope images have continued to show remarkable changes in Neptune's atmosphere: the Great Dark Spot discovered by Voyager 2 in 1989 had disappeared, and a new Great Dark Spot developed in the northern hemisphere. From the dynamics of Neptune's clouds, to the expanding Triton atmosphere, to the forces creating the clumpy rings, many interesting puzzles remain to be solved in the Neptune system.

see also Exploration Programs (volume 2); Nasa (volume 3); Robotic Exploration of Space (volume 2); Uranus (volume 2).

Heidi B. Hammel

Bibliography

Cruikshank, Dale P., ed. Neptune and Triton. Tucson: University of Arizona Press,1995.

Moore, Patrick. The Planet Neptune. Chichester, UK: Ellis Horwood, 1988.

Standage, Tom. The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker and Company, 2000.