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.  It orbits 4,504,000,000 kilometers from the Sun, has a diameter of 49,532 kilometers, and a mass of 1.0247x10 to the 26th kilograms.  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.  Neptune was first observed by Galle and d'Arrest on September 23rd, 1846 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus.
Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly.  Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.  Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989.  Almost everything we know about Neptune comes from this encounter.

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 the result of absorption of red light by methane in the atmosphere.  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 kilometers/hour.  Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.

At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot in the southern hemisphere.  It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth).  Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph).
 

Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter".  It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.
 

However, the Hubble Space Telescope observations of Neptune in 1994 show that the Great Dark Spot has disappeared.  It has either simply dissipated or is currently being masked by other aspects of the atmosphere.  A few months later the Hubble Space Telescope discovered a new dark spot in Neptune's northern hemisphere.  This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.

                                                                                                                                                 Neptune also has rings.  Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps.  One of the rings appears to have a curious twisted structure.  Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.  Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring coorbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.  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.