Eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. When Uranus didn't travel exactly as astronomers expected it to, two mathematicians, working independently of each other, proposed the position and mass of another, as-yet unknown planet that could account for Uranus' orbit. Although "the establishment" ignored the predictions, a young astronomer decided to look for the predicted planet. And so was Neptune discovered in 1846. Seventeen days later its largest moon, Triton, was discovered.

Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once in 165 years, and therefore has made not quite a full circle around the Sun since it was discovered. It is invisible to the naked eye due to its extreme distance. Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the farthest planet from the Sun for a 20-year period out of every 248 Earth years. Pluto will remain closer to the Sun until the end of the 20th century.

In 1989, the Voyager 2 spacecraft sped through the Neptune system, providing a wealth of new information about this distant planet.

Neptune is the smallest of our solar system's gas giants (including Jupiter, Saturn and Uranus), so-called because they have no solid surfaces. Even so, its volume could hold nearly 60 Earths. Neptune's hydrogen/helium/methane atmosphere extends to great depths, gradually merging into water and other "melted ices" over a heavier, approximately Earth-sized liquid core.

Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun. Its seasons last an incredible 40 years: during the southern summer the south pole is in constant sunlight for about 40 years, and in northern summer the north pole is in constant sunlight for about 40 years.

Similar to the case at Uranus, Neptune's magnetic field is tilted 47 degrees away from the planet's spin axis and offset from the planet's center by about half its radius.

Neptune's atmosphere is made up of hydrogen, helium, and methane, the last of these giving the planet its blue color (since methane absorbs red light). Winds here are the fastet on any body in the solar system, blowing at speeds over 2,400 km (1,500 miles) per hour.

In 1989, at the time of Voyager 2's flyby, there was a large oval dark spot in Neptune's southern hemisphere. This hurricane-like " Great Dark Spot" was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1200 km (745 miles) per hour. Unlike Jupiter's Great Red Spot, which has been seen in telescopic views since Galileo first pointed a telescope at Jupiter nearly 400 years ago, Neptune's Great Dark Spot appears to be relatively short-lived. Recent images from the Hubble Space Telescope show no sign of the southern hemisphere Great Dark Spot, although a comparable spot appeared in 1997 in Neptune's northern hemisphere. Voyager 2 also noted a smaller dark spot and a fast-moving cloud dubbed the "Scooter," as well as high-altitude clouds over the main hydrogen and helium cloud layer.

The planet has four rings of varying widths, confirmed by the Voyager 2 flyby of Neptune in 1989. The 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 the moon Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally Galle. Neptune's rings are believed to be relatively young and relatively short-lived.

Neptune has eight known moons, six of which were discovered by Voyager 2. The largest moon, Triton, orbits Neptune in a direction opposite to the planet's rotation direction, and is gradually getting closer until it will collide with the planet in about 10 to 100 million years, forming vast rings around Neptune that will rival or exceed Saturn's extensive ring system, whose total mass is a tiny fraction of the mass of the moon Triton. Voyager 2 also found great geysers of gaseous nitrogen. The geysers may be caused by solar melting of the material below the surface's nitrogen ice. The ice turns to gas, and explodes into space through vents in the crust. As the gas rushes toward the vent, bits and pieces of dust from the subsurface are carried along and shot into the sky. The dust then falls back to the surface, forming the dust streaks found on many parts of Triton's surface. Triton is the coldest body yet visited in our solar system: temperatures on the surface of Triton are about -235°C (-391°F)!