Pluto, which was discovered in 1930, is the most mysterious planet in the solar system. Known as the ninth planet, Pluto actually slips inside the orbit of Neptune for a period of time in each circulation around the Sun. It is the only planet not to have been explored by spacecraft and therefore very little is actually known about it and its weather.

Pluto is a very small planet - it is even smaller than our moon and is around six times smaller than Earth. This has led to much debate about whether Pluto is a planet at all. Some scientists believe it was once a moon of Neptune which has somehow 'escaped'. Its diameter is thought to be around 1,900 miles) and that of its moon, Charon, is not much smaller at around 800 miles in diameter.

The planet's orbit takes some 248 years. However, for 20 years of this time, Pluto actually becomes closer to the Sun than Neptune, as a result of its highly elliptical orbit. Pluto ended a 20 year stint as the eight planet in 1999. Pluto's 'day' (one rotation on its own axis) is six days, 9 hours and 18 minutes. At its furthest, Pluto is 4.6 billion miles from the Sun, and at its nearest it is 2.8 billion miles from the Sun.

Another unusual feature of Pluto is that it is thought to be made up of mainly rock and ice, rather than being a gas planet like those nearest to it. The other planets with a similar composition are those such as Earth, Mercury, Venus and Mars - which are all much closer to the Sun. It was quite a surprise to scientists when they found out both how small the planet is and also how is has nothing in common with the gas planets which are near to it.

Pluto's surface has a red tint to it and it is believed to be made up of snow, which is a combination of methane, nitrogen and carbon monoxide. Its atmosphere is not very dense and therefore its atmospheric pressure is very low - about one millionth of that here on Earth. Many observations of Pluto are conducted through the Hubble Space Telescope, as it is too far away for space travel at the moment. The telescope shows a difference in brightness across the planet, possibly due to craters and basins on the surface. Its also thought it could be due to frosts of methane and nitrogen.

Some observations have noticed the evidence of polar caps on Pluto and also large, dark spots in the area near to its equator.

Temperatures on Pluto are obviously very cold. Surface temperatures are thought to be around -223C (-369F). However it does vary depending on where it is in its orbit. As it moves closer to the sun temperatures raise somewhat, and its atmosphere thickens as the ices turn into a gas. However, during the colder period, it is thought that Pluto's atmosphere collapses causing a snow storm which covers the entire planet. The next collapse of this kind is predicted in 2010. The changes in temperatures are thought to cause massive pressure differences and therefore very strong winds.

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