As of 2006, when Pluto was demoted from planet status, Neptune became the furthest planet from the Sun. Neptune is similar in size and composition to Uranus, but has the fastest winds in the Solar System, blowing at almost 2,000 km per hour. When Voyager two flew past Neptune in August 1989, it observed the planet’s rings, which were not proven to exist prior to the encounter. Neptune’s rings do not completely encircle the planet, and hence are known as ring arcs. The Great Dark Spot was also photographed by Voyager two. It is similar to Jupiter’s Great Red Spot in the sense that it is an enormous cyclonic storm system, which is around the size of the Earth. However, resent observations by the Hubble Space Telescope show that the Great Dark Spot seems to have disappeared.

Neptune’s largest moon, Triton, is the coldest world in the Solar System. At 2,700 km in diameter, it is larger than Pluto and has geysers of nitrogen which shoot kilometres above the moon’s surface before raining back down. Triton has what is known as a retrograde orbit, it orbits in the opposite direction to the other moons and Neptune’s rotation. This suggests that it may have been a captured planet or a stray Kuiper Belt Object.

Image of Triton's south pole taken by Voyager Two.

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