Pluto is a mysterious planet, now classified a dwarf planet, about which most of us know very little. Even S. Alan Stern, one of the foremost experts on planetary science agrees.

"I can tell you everything we know for sure about Pluto on three post cards," says Stern. "That leaves a lot of room for discovery."

So unsurprisingly there was great excitement amongst Stern's team as they gathered at Cape Canaveral, in October 2005 - the final formal meeting before the launch of NASA's New Horizons in January 2006, the first mission to explore Pluto and the outer solar system. Just days earlier a team of astronomers led by Stern had reported the discovery of two more, smaller moons orbiting Pluto.

Undoubtedly there will be more surprises to come. "We don't even know what we don't know," says Stern. Because Pluto is so distant and so utterly unknown, any findings could radically change our thinking. New Horizons' discoveries could help us understand many things such as how the solar system formed and how Earth's atmosphere came to be.

Pluto is made of a mix of rock and frozen water, methane and other volatile compounds - that would melt, evaporate and even boil of into space in the warmth of the inner solar system. But at the cold outer solar system, these compounds have probably been unchanged since the formation of the planets.

"It's not just that it's ice, it's pristine ice that's been in that form for the entire age of the solar system," says Richard Binzel, a professor at Massachusetts who has specialized in studying the composition of asteroids. Binzel says this primordial ice means the mission "is not only a trip in space, it's also a trip in time".

Until the discovery of Pluto in 1930, people knew only of two kinds of planets: the terrestrial planets - small, rocky worlds Earth, Venus, Mars and Mercury - and the gas giants Jupiter, Saturn, Uranus and Neptune, made mostly of hydrogen with little or no solid surface.

Pluto is very different. Less than half the diameter of Mercury; the Sun's rays take four hours to reach it; and it's 1000 times darker than Earth. The surface temperature on Pluto is around -233 degrees Celsius. Pluto's atmosphere is about 100,000 times thinner than the Earth's, and is mainly nitrogen - it sometimes freezes up.

Pluto and its largest moon, Charon - discovered 1978 - are called ice dwarves, to separate them from terrestrial planets and gas giants. Pluto might not be the last-found member of a limited number of planets, it may be the gateway to a whole new class of bodies known as Kuiper belt objects, or KBOs. Pluto is known to be one of maybe hundreds of similar sized objects in the Kuiper belt. The Kuiper belt is the largest and outermost zone of the Solar System and the source of most comets visible on Earth.

KBOs are the leftovers from the formation of the planetary solar system 4.5 billion years ago, some of which stuck together to form the terrestrial planets and the cores of the gas giants. Deep frozen in space far from the sun, KBOs may be relics from the beginning of the solar system. The relatively undisturbed remnants we may find when studying Pluto and its environs may hold a chemical and structural record of the beginning of the planets.

So Pluto may tell us much about the early days of Earth. Pluto and Charon are very close in space and of a similar size, often they are termed a double planet. They may actually exchange atmospheres. A long term study of atmospheric changes on Pluto may help to reveal how quickly the hydrogen in the Earth's atmosphere escaped into space.

The Baby Planet

Pluto may be an unfinished planet. Computer simulations show that Pluto should have grown to at least the size of Earth. There should have been enough material in Pluto's region of space to form a big planet. Something shut off the supply of that material but nobody knows what.

New Horizons' discoveries about the number of KBOs and size and density of craters on Pluto and Charon may strengthen the unfinished planet theory. Computer simulations can do a reasonable job in predicting planet formation but the early stages are mostly inference. New Horizons could provide substantial new evidence to bolster, or change theories of planetary formation.

As well as being important to the theories of planetary formation, study of KBOs will help explain the nature of the comets we see from Earth, building on the information obtained from the Deep Impact mission in 2005.

New Horizons will also enable a deeper understanding of the chemical make up of KBOs. Limited observation from Earth-based spectroscopes has shown some variability indicating the presence of complex organic molecules.

Impacting bodies from the Kuiper belt may have been sources of the Earth's water, atmosphere and complex hydrocarbons helping to establish life. So detailing the composition and distribution of KBOs may help us understand the history of Earth and the origins of life.

But New Horizons will take 10 years to reach Pluto and will map the planet and its moon as it flies by in 6 days. The resolution of the information received will be thousands of times more than we can presently achieve.

The craft is about the size of a golf buggy and boasts a large amount of monitoring technology on its relatively small frame - visible, infrared, ultraviolet, particle and plasma spectrometers and a long range telescope, all powered by a nuclear power source.

However the most limiting factor on the amount of data received is the strength of the signal that can be beamed to Earth. A network of receivers in California, Spain and Australia will be listening for the signals. Once New Horizons is well past Pluto the network will transmit signals for the craft to analyse as to how they are absorbed and bent by Pluto's atmosphere. This analysis will give important information about the atmosphere density, temperature and composition.

New Horizons will still be sending important data as it travels further into the Kuiper belt, a target KBO will be chosen for the craft to study after its encounter with Pluto.