A major planet of the solar system, normally the eighth in order from the Sun. (Between 1979 and 1999 the eccentric orbit of Pluto brings it temporarily closer than Neptune.).
Viewed from Earth, Neptune is a seventh or eighth magnitude object and so not visible to the naked eye. With high magnification and larger telescopes, it is seen as a faintly bluish disc, the colour coming from methane in the upper atmosphere. Neptune was discovered by J. G. Galle of the Berlin Observatory on 23 September 1846 following predictions made independently by John Couch Adams in England and Urbain J. J. Leverrier in France. Their calculations were based on discrepancies between the observed and predicted orbits of Uranus since its discovery in 1781, which were attributed to the gravitational perturbations of an unknown planet.
Surface features are not detectable by ground-based optical observation, though bright spots are observed in the infrared. However, close-up images were obtained by Voyager 2 during its flyby of Neptune in August 1989. In many ways, such as size and structure, Neptune is similar to Uranus. It has a small rocky core surrounded by an icy mantle of frozen water, methane and ammonia. The outer atmosphere is mainly molecular hydrogen with 15-20 per cent helium (by mass) and some methane.
But unlike Uranus, Neptune proved to have a number of distinctive cloud features. The most prominent of these has been termed the Great Dark Spot, and appears to be similar in nature to Jupiter's Great Red Spot. Located about 20í south of the equator, it rotates anticlockwise in a period of about 16 days. Bright cirrus-like clouds form over this and other small dark spots. Subtle banding parallel to the equator is also apparent.
There are two main cloud layers in the upper atmosphere. The highest consists of crystals of methane ice, and this lies over a lower opaque blanket of cloud that may contain frozen ammonia or hydrogen sulphide. There is also a high-altitude haze of hydrocarbons produced by the action of sunlight on methane.
Regular radio bursts detected by Voyager 2 revealed that Neptune has a magnetic field and is surrounded by a magnetosphere. The bursts occurred at intervals of 16.11 hours, apparently the rotation period of the planetary core. The atmospheric features rotate at different rates, also moving in latitude. Wind speeds up to 2,200 km/h were measured. The magnetic axis is tilted at 47í to the rotation axis and it is thought that the asymmetric field may originate in the mantle rather than the core.
Based on the total radiated energy, the average temperature is 59 K, though it will of course vary from place to place. It is not understood why Neptune radiates 2.7 times more energy than it receives from the Sun.
Observations made from the ground during occultations by Neptune had suggested the presence of incomplete ring ╘arcs'. Voyager 2 detected four tenuous rings, one of which is ╘clumpy' in a way that can account for the occultation observations. The mission also discovered six new moons around Neptune, bringing the total number known, with Triton and ⌐Nereid, to eight. One of the newly discovered moons, Proteus, is more than twice the size of Nereid with a diameter of about 400 kilometres.
