NEPTUNE

Neptune is the eighth planet from the Sun and the fourth largest by diameter at 49,532 km.. It is smaller in diameter but larger in mass than Uranus. Its orbit is 4,504,000,000 km from Sun and mass is 1.0247e26 kg.

Neptune was first observed by Galle and d'Arrest on Sept 23 1846.

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. If he had seen it on the previous few nights Neptune's motion would have been obvious to him, but cloudy skies prevented obsevations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter.

Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.

Neptune's composition is probably similar to Uranus': 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 rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.

Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.

Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.

Like Jupiter and Saturn, Neptune has an internal heat source and it radiates more than twice as much energy as it receives from the Sun.

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