Neptune is the eighth planet from the Sun and the most remote of the gas giants of the outer Solar System. During 1845 and 1846 the Englishman John Couch Adams and the Frenchman Urbain Jean Joseph Leverrier, unknown to each other, independently calculated where an eighth planet would have to be in order to explain slight perturbations in the orbit of Uranus. In Berlin on the night of Sept. 23, 1846, Johann Gottfried Galle and Heinrich Louis d'Arrest found a new planet within one degree of the position sent them by Leverrier. The equally good prediction of Adams, made a year earlier, met with unfounded skepticism in England and was not published until after the planet had been discovered.

During the months following the announcement of the discovery, an international controversy developed between English and French astronomers as to whom credit belonged and what the planet should be named. (Leverrier wanted to name it after himself.) Eventually the new planet was named Neptune, for the Roman sea god, and credit was given to both Adams and Leverrier for their calculations. Galileo actually may have spotted Neptune more than two centuries earlier, but he did not recognize it as a planet.

Neptune reaches a maximum brightness in the Earth's night sky of magnitude 7.8, about five times too faint to be seen by the naked eye. In a large telescope the planet appears as a small blue disk, 2.3 seconds of arc in diameter. The best pictures of Neptune from Earth show discrete bright clouds and a bright haze over the south pole of the planet. The U.S. Voyager 2 spacecraft confirmed these sightings when it reached Neptune in 1989, flying less than 5,000 km (3,100 mi.) above the planet's cloud tops on August 25. Its cameras revealed many features, including a large, dark storm system in the southern hemisphere. This feature, named the Great Dark Spot, had disappeared by 1994, but a similar Great Dark Spot now exists in the northern hemisphere.

Voyager 2 also saw a smaller dark storm system with a bright core of feathery clouds, a small bright cloud feature named the "Scooter," well-defined banding on the planet, and numerous wispy cirruslike clouds. Some of these latter clouds cast shadows on deeper cloud decks below. This observation marked the first detection of vertical relief in the atmosphere of an outer planet. The cirruslike clouds changed rapidly, often forming and dissipating over periods of several hours. These surprisingly fast changes imply that the weather of Neptune is perhaps as dynamic and variable as that of the Earth. The scale, however, is immense by Earth standards, however, since Neptune is nearly four times as wide as the Earth.

The orbit of Neptune around the Sun is even more nearly circular than the Earth's orbit. The planet's average distance from the Sun is 4,497,000,000 km (2,794,000,000 mi.), with an eccentricity of only 0.0086. The orbit is inclined 1 degrees 46' to the ecliptic, or plane of the Solar System, and the planet takes 164.793 years to make one trip around the Sun. Neptune's axis of rotation is tipped only 28 degrees 48', which is not greatly different from Earth's 23 degrees 30'.

The rotation period of Neptune's magnetic field, which is presumed to trace the rotation of the planet's core, was found by Voyager 2 to be 16.11 hours. Most of the clouds on Neptune have longer periods of rotation, however, ranging from about 16 hours near the planet's south pole to more than 18 hours near the equator. This means that the jet-stream wind speeds on Neptune reach 2,400 km (1,500 mi.) per hour, moving in a retrograde direction--that is, opposite to the direction of rotation. These are the strongest retrograde winds seen on any planet in the solar system.

Neptune has a diameter of 49,500 km (30,750 mi.) and a mass 17.22 times that of the Earth. This means that the planet is slightly smaller and heavier than Uranus. It has an average density of 1.67/cu cm, compared to Uranus's density of 1.21 g/cu cm.

The atmosphere of Neptune consists mainly of hydrogen and helium, but about 2.5-3 percent of the atmosphere is methane (CH4). The cirrus clouds seen in the atmosphere probably consist of crystals of methane rather than of water ice, as seen in cirrus clouds on Earth. Methane's strong absorption features dominate the spectrum of the planet, giving Neptune its deep blue color. Also in Neptune's spectrum are features due to molecular hydrogen (H2) and, possibly, stratospheric ethane (C2H6). Observations in the microwave region of the spectrum suggest the probable presence of ammonia (NH3), and they indicate that temperatures on Neptune rise with increasing depth, as on Uranus. Scientists had expected the effective temperature of Neptune to be about - 228 degrees C ( - 378 degrees F), but infrared measurements made by Voyager 2 indicated a higher temperature of at least - 218 degrees C ( - 360 degrees F). Thus Neptune, like Jupiter and Saturn but unlike Uranus, appears to have an internal energy source.

Voyager 2 discovered that the magnetic field of Neptune is tilted more than 50 degrees from the planet's rotation axis, and it is offset from the center of the planet. This means that the magnetic field strength varies across the surface of Neptune. The unexpected orientation of the field resembles that of Uranus. Until the observation of Neptune's field, the orientation of Uranus's magnetic field had been thought to be linked to that planet's unusual orientation, with a rotation axis nearly parallel to the plane of the ecliptic. Now some other explanation has to be developed for this shared characteristic.

The English astronomer William Lassell detected Neptune's largest satellite, Triton, less than a month after the discovery of the planet in 1846. In 1949, Dutch-American astronomer Gerard Kuiper discovered Nereid, a second Neptunian satellite. Both satellites had unusual orbits. Triton, unlike any other of the solar system's large satellites--it has a diameter of 2,705 km (1,680 mi.), slightly smaller than Earth's Moon--moves in a retrograde direction around its primary, Neptune. Nereid, in turn, has the most eccentric orbit of any moon in the solar system. Its distance from Neptune varies from 1,400,000 to 9,700,000 km (900,000 to 6,000,000 mi.). Voyager 2 found that Nereid is about 340 km (213 mi.) in diameter and reflects about 12 percent of the sunlight that falls on it. The unusual nature of Triton is discussed in the article of that title.

Voyager 2 also discovered six new satellites during its passage, giving Neptune a total of eight moons. One, Proteus, is the largest of the newly discovered satellites. It has an irregular shape with an average diameter of about 400 km (250 mi.). This makes it slightly larger than Nereid, but it is a much darker body, reflecting only about 6 percent of the sunlight that strikes it. It is also closer to Neptune, which is why it remained undiscovered while Nereid could be observed from Earth. The moon is gray in color, and hints of craterlike forms and groovelike lineations are seen on its surface.

Another satellite, Larissa, is an irregularly shaped, dark object about 210 by 180 km (130 by 112 mi.) in size. It reflects only 5 percent of the sunlight that falls on it, and it appears to have several craters 30-50 km (19-31 mi.) across. The irregular outlines of Proteus and Larissa suggest that they remained cold and icy throughout much of their history. The two satellites orbit at distances of about 117,600 km (73,500 mi.) and 74,000 km (46,300 mi.) from Neptune, respectively. Little is known about the remaining small satellites. Despina and Galatea orbit at distances of about 62,000 km (38,000 mi.) and 52,000 km (32,000 mi.), respectively. Thalassa circles Neptune every 7.5 hours at a distance of 50,000 km (31,000 mi.). Naiad, with its 7.1-hour orbit, has a noticeable inclination, being tilted 4.5 degrees to the equatorial plane of Neptune.

The presence of rings around Neptune had been a subject of debate prior to the Voyager encounter. Several ground-based observations had suggested that irregular arcs, or strands of partial rings, orbited the planet. Studies of the probe's photographs, however, eventually revealed that five rings surround Neptune: two bright, narrow rings and three fainter, fuzzier sheets of orbiting materials. Some sections of the bright rings have significantly higher densities than others, and it was these "arcs" of higher density that had first been detected by Earth telescopes. The bright rings are located roughly at distances of 53,000 km (33,000 mi.) and 63,000 km (39,000 mi.). One broad ring is located at 42,000 km (25,000 mi.), and another in a zone between the bright rings, while a third extended sheet perhaps fills the system between the planet and the inner broad ring.

View a close-up image of Neptune.