Pluto

Pluto is a tiny icy rock skirting the outer solar system at a distance as far as 7.5 thousand million kilometres from the Sun. At that distance the Sun appears as little more than a bright star, and the temperature falls to under -230°C, cold enough for a surface of frozen methane to have formed on Pluto. Observations of Pluto are still blurred because of its great distance but recent pictures indicate that the surface has more regions of contrast than any planet excluding the Earth.

Pluto is in many ways the oddball planet in the solar system. It is a tiny world, smaller even than our moon, and its location and composition do not fit the current models of how the solar system formed. These facts along with its highly elongated orbit have lead some scientists to believe that it may not be a planet, but rather an escaped moon of Neptune. This theory would help to explain some of the peculiar orbits of Neptune's other moons.

Pluto has a single moon, known as Charon, which orbits at a distance of 19 thousand kilometres every six days. The Pluto-Charon system is the most closely matched planet-moon pair in the solar system, with Charon measuring around half the size of Pluto. This similarity seriously effects Pluto's orbit, and both objects are locked around a common centre of mass like a dumbbell, with the same face fixed towards each other. Astronomers have used the gravitational properties of the system to calculate the masses, and the repeated eclipses that occur to try to map the surface.

Apart from the basic properties of the planet, very little is known about Pluto and its moon. This may change with the arrival of a probe known as Pluto Express. Scheduled to arrive at Pluto around 2007 two small probes will pass within 15 thousand kilometres of Pluto, providing the first high resolution mapping of its surface.