The eighth planet from the Sun, the outermost of the four ‘gas giant’ planets; discovered in 1846 as a result of a prediction by Leverrier to explain anomalies in the observed orbit of Uranus; encountered by Voyager 2 (24 Aug 1989). There are 13 known moons, including Triton, which has a thin atmosphere, and Nereid. Neptune’s main characteristics are: mass 1·02x1026 kg; equatorial radius 24 766 km/15 389 mi; mean density 1·7 g/cm3; rotational period 16·11 hours; orbital period 164·8 years; inclination of equator 28·3°; eccentricity of orbit 0·010; mean distance from Sun 30·07 AU. It is an apparent twin of Uranus internally, composed of hydrogen and helium, but with much more carbon, nitrogen, and oxygen than Jupiter and Saturn. It is thought to lack sharp internal boundaries between a rock-rich core, an ice-rich mantle, and a deep atmosphere. Its bluish-green coloration is produced by methane in the upper atmosphere. It has cirrus clouds of methane above high-level clouds of methane and ammonia, with lower water clouds. There are storm systems resembling Jupiter’s, including a Great Dark Spot in the S hemisphere seen by Voyager. It has a magnetic field of 0·1 gauss, tilted 47° from the rotation axis. There are five rings between 42 000 km/26 000 mi and 63 000 km/39 000 mi from Neptune’s centre. The outer ring has clumpy sections, termed arcs, probably associated with small moonlets.
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