Neptune, the eighth planet in our solar system, was the first planet whose existence was mathematically calculated before the planet was actually observed. In 1845 British astronomer John C. Adams predicted that there was an eighth planet based on disturbances in the orbits of the known planets, but his theories received little attention. At the same time in France, another astronomer named Le Verrier was making the same findings, but his work got much more notice. In 1846, two astronomers in Berlin identified the planet, but Adams and Le Verrier are recognized with Neptune's discovery.
Neptune is so far away that it is very difficult to learn more about it with current astronomical technology. Neptune may even have rings, but we cannot tell as yet. The existence of Triton may make it too unstable for rings to form.
Neptune has many similarities to Uranus, there are also several differences. Neptune and Uranus are considered to be "twin planets" because they are close to the same size and are composed of similar elements. Neptune, while slightly smaller than Uranus, is the denser and heavier of the two. Neptune does not share Uranus's peculiar polar tilt. The climate on Neptune is almost exactly the same as that on Uranus, mainly because of their atmospheres, which are both composed of hydrogen and methane gases.
Neptune has a large moon named Triton. Triton is so big that it pulls back on Neptune with its gravity force, which alters Neptune's orbit. The two end up circling each other in a retrograde orbit. Triton and Neptune are slowly pulling closer to each other, so someday Triton may be torn apart if it gets close enough. Neptune also has a second (and much smaller) moon named Nereid. Nereid's orbit is very eccentric, that is, it travels in an ellipse around Neptune rather than a perfect circle. In 1981, researchers at the University of Arizona made observations which suggested the existence of a third, very tiny moon.
Neptune - Images, data & info
The Planet Neptune - Data
Hubble Views Planet Neptune - Hubble discussion
* Photo credit - NSSDC Photo Gallery