Neptune was discovered in 1846.
The planet is so far away that even with the most powerful telescoop the only thing of the planet that's visible is a small disk. Sometimes Neptune is the last planet of the solar system, this happens when Pluto's orbit is closer to the sun. This lasts 19 years.

Neptune was visited by only one spacecraft., Voyager 2 (1989).

Neptune's composition is probably very similar to Uranus. Various ices and rock with about 15% oxygen and a little helium. Like Uranus, Neptune might not have a distinct internal leyering, but is rather uniform in composition. It's core is most likely small (about the mass of earth) made of rocky material. The atmosphere is mostly hydrogen and helium, with a small amount of methane. The blue color of the planet is a result of absorption of red light by the methane in the atmosphere. 
Like a typical gas planet, Neptune also has high velocity winds, reaching 2000 km/hour
Neptune also has an internal heat source, just like jupiter and saturn. The planet radiates almost twice as much energy as it receives from the sun.

Neptune also has rings. Observations from earth showed only faint arcs in stead of complete rings. But observations from Voyager 2 showed that the planet has complete rings. One of the rings seems to have a strange twisted structure.
Neptune's rings are Difuse, inner, Plateua and main.

Neptune has 8 known moons, 7 small ones and 1 larger one, Triton.
The other 7 are Naiad, Thalassa, Despina, Galatea, Larissa, Proteus and Nereid.

Neptune's diameter is 30,454 miles. The rotation around it's axis takes 18-20 hours.The distance to the sun is 4,504 million km (2,794 million miles). The rotation time around the sun is 164.8 years. The mass is 1.0274e27 kg

Pictures of neptune  (Click to see the large image):