Neptune is the eighth planet from the sun and the fourth largest (although its mass is slightly larger than Uranus'). Like the other gas giants, it has an atmosphere rich in hydrogen and helium, but also contains about 1% methane which is responsible for its blue colour. In 1846, French astronomer Urbain Jean Joseph Leverrier calculated that Neptune must exist from perturbations in Uranus' orbit. Johann Gottfried Galle, a German astronomer observed the planet the same year within 1� of its calculated position.

It is not visible with the naked eye, but you may be able to spot it with binoculars if you know where to look. Most of what we know today about Neptune comes from an encounter that Voyager 2 had with the planet in 1989. In 1979, it was actually the farthest planet from the sun, because Pluto's eccentric orbit cut inside that of Neptune. In 1999 it once again placed rank as the eighth planet when Pluto moved back outside of Neptune's orbit.

In many ways Neptune is similar to Uranus, but unlike Jupiter and Saturn its structure is less layered. It likely has a very thick and roughly uniform composition of water, ammonia and methane ice mixed together with hydrogen, rock and small amounts of helium surrounding a small rocky core about the size of Earth. Since its surface temperature is higher than expected for a body so far away from the sun, it must have an internal heat source. Mysteriously, it also has a significant magnetic field, but it is tilted considerably from the planet's rotational axis and is off center.

Typical of any gas giant, Neptune has bands of strong winds reaching 2000 km/hr (1250 mph), the fastest in the solar system. Voyager photographed a Great Dark Spot of swirling gases in its southern hemisphere that is similar to Jupiter's Great Red Spot. Observations from the Hubble Space Telescope however, indicate that it has since disappeared. Late in 1994 Hubble discovered that another dark spot had appeared in its northern hemisphere.

The atmospheric composition is approximately 80% hydrogen, 19% helium and 1% methane. It is the methane that is responsible for Neptune's deep blue colour.

Neptune has at least four rings which are far too faint to be seen from earthly telescopes. The Voyager 2 spacecraft is credited with identifying them. The outermost ring, called Adams, is narrow and thin and its material tends to clump into "ring arcs".