- The eighth planet of the Solar System and the last of significant size, being the outermost of the gas giants. Neptune was discovered by Johann Galle and Louis d’Arrest in Berlin in 1846, on the basis of calculations by Leverrier. Numerous cases have since come to light of earlier unrecognised observations of Neptune, including two by Galileo, who in 1612 noted the ‘star’s’ motion but failed to realise that he was looking at a new planet. Neptune has a diameter of 48,600km and a mean distance from the Sun of 3OAU, and takes 165 years to make one orbit of the Sun. Neptune weighs 17 times as much as the Earth. Neptune seems to have an atmosphere dominated by hydrogen, helium and methane, but probably has a core of heavier materials, perhaps including iron and silicates, to account for its density of 1.66 gm/cc. This core may be surrounded by a mantle with water ice and other frozen materials including methane and ammonia. Neptune’s upper atmosphere rotates in a period of some 17 hours. Observations of an occultation of a faint star by Neptune in 1984 led to the discovery of a thin ring some 20km wide about 76,000km from the planet’s centre. Neptune was the last planet to be visited by a space probe, Voyager 2 in August 1989. It found since new satellites including Proteus, the second largest after Triton and larger than Nereid, the satellites discovered telescopically from Earth. The green-blue clouds of Neptune seem when observed by Voyager – or Hubble – to be accompanied by high, bright clouds and to be darkened by spots, one named the Great Dark Spot by analogy with Jupiter’s Great Red Spot.