Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. Orbit: Diameter: Mass: 4,504,000,000 km (30.06 AU) from the Sun 49,532 km (equatorial) 1.0247e26 kg

Astronomy

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. In 1846 the German astronomer, Galle, at Berlin observatory, discovered the planet Neptune, whose existence had been postulated in the calculations of Leverrier.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights, Neptune's motion would have been obvious to him. Unfortunately, cloudy skies prevented observations on those few critical days.

Neptune's composition is probably similar to that of Uranus. Both are four times the size of the Earth, each is circled by dark, thin rings and has an atmosphere containing poisonous methane gas. Neptune has various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but may be more or less uniform in composition. But there is most likely a small hot core (about the mass of the Earth) of rocky material which probably stirs up the cloud tops and causes strong winds and storms. Its atmosphere is mostly hydrogen and helium with a small amount of methane. Neptune's blue color is largely the result of the absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore (or chemical combination that produces color in a compound), which gives the clouds their rich blue tint.

Like a typical gaseous planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Voyager 2 photographed a storm on Neptune as big as the Earth and known as The Great Dark Spot. Five years later, it had disappeared. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. Like Jupiter and Saturn, Neptune's internal heat source radiates more than twice as much energy as it receives from the Sun. Like that of Uranus, Neptune's magnetic field is oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk.

If you would like to know more about the solar system, Starscine suggests the following websites:

www.nineplanets.org
www.nasa.gov