The eighth major planet from the Sun. Discovered in 1846, it was named after the Roman god of the sea.
A brief treatment of Neptune follows.
Neptune travels around the Sun in an elliptical orbit at an average distance of 4.504 billion km (2.799 billion miles). At this distance, it takes the planet 165 years to complete one orbit. Neptune is more than 17 times as massive as the Earth, and its volume exceeds that of the latter by 44 times. It has an equatorial diameter of roughly 49,528 km (30,775 miles). Neptune consists largely of hydrogen and helium. It has no apparent solid surface, but it may have a core of ice and rocky material. The planet's atmosphere, particularly the outer layers, contains substantial amounts of methane gas. Absorption of red light by the atmospheric methane is responsible for Neptune's deep blue colour.
Considerable atmospheric turbulence was detected by the U.S. Voyager 2 space probe during its 1989 flyby of Neptune. Winds moving at 300 metres per second (700 miles per hour) and higher were observed by the spacecraft, as was an enormous storm system--the so-called Great Dark Spot--in the planet's southern hemisphere. Above this system drift delicate clouds resembling those of the cirrus variety, and closer to the south pole are broad cloud bands of varying shades of blue.
Because of its great distance from the Sun, Neptune receives only a limited amount of solar radiation. Temperature measurements, however, suggest that the planet has an internal heat source just like Jupiter and Saturn. Even though Neptune receives about 2.5 times less sunlight than neighbouring Uranus, its effective radiating temperature is roughly the same as that of the latter (about 59 K).
As in the case of the other giant outer planets and the Earth, Neptune is surrounded by a magnetic field. Though not nearly as strong as those of the other planets, Neptune's field traps solar wind and galactic cosmic-ray particles (e.g., energetic protons and electrons) in a belt around the planet that is somewhat akin to the Earth's Van Allen radiation belts. By analyzing radio emissions from the Neptunian magnetic field, astronomers have determined that the rotational period of the planet is 16.11 hours. Neptune's axis of rotation is tilted approximately 29 to the plane of the planet's orbit around the Sun, which is similar to those of the Earth and Mars (23.5 and 25, respectively).
Voyager 2 observations revealed that Neptune is encircled by at least four rings, which are populated largely by dust-sized particles. The particles of the outermost ring are not distributed uniformly; there are five bright arclike regions of greater density distributed along a 45 segment of the ring. (see also Index: ring system)
Neptune also has eight known satellites. Only two of these, Triton and Nereid, had been observed prior to the Voyager 2 flyby. Triton is the largest of the eight satellites and is almost as big as the Earth's Moon. The other Neptunian satellites range in diameter from 58 to 416 km (36 to 258 miles). Apart from Triton, the moons of Neptune are irregularly shaped and have very dark surfaces.
©Copyright Andrew Do