A true colour image of Pluto A true colour image of Pluto, made by monitoring brightness changes when Pluto was eclipsed by Charon. Pluto's brown colour is thought to result from frozen methane on its surface. Image: Eliot Young (SWRI) et al, NASA. Following from the IAU's decision on 24 August 2006, Pluto has been re-categorised as a 'dwarf planet'.

For 20 years out of its 249 year orbit Pluto is actually closer to the Sun than Neptune. This happened, most recently, between 21 January, 1979 and 11 February, 1999. Pluto will next pass inside Neptune's orbit in 2226. The reason for this is that the orbit of Pluto around the Sun is an ellipse with quite a large eccentricity. This means that it is more 'oval-shaped' than circular. At perihelion, its closest distance to the Sun, Pluto is about 4440 million kilometres from the Sun. At aphelion, its furthest distance from the Sun, it will be about 7395 million kilometres from the Sun.

Pluto's discovery

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory. It was discovered as a result of astronomers comparing the observed positions in the sky of the two planets Uranus and Neptune, with positions predicted from their orbits about the Sun. Small departures from the predicted positions indicated that the paths of these two planets were being disturbed by the gravitational pull of another body.

Moons of Pluto

Pluto, Charon and two new moonsThese two Hubble Space Telescope images show Pluto and its large moon Charon, along with two possible new moons. The new moons are further away from Pluto than Charon so move more slowly in the three days between the first and second image. Image: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI) and the Hubble Space Telescope Pluto Companion Search Team Pluto's very close satellite named Charon was discovered in 1978. Charon orbits Pluto at a distance of 20,000 kilometres in 6.4 days. This allows astronomers to calculate that that Pluto has a mass only 0.2% of the Earth. Its diameter is about 2500 kilometres and so Pluto has a density much less than the Earth.

In 2005 Hubble Space Telescope astronomers announced the discovery of what appeared to be two other moons, provisionally designated S/2005 P1 and S/2005 P2. The two candidate moons were 44000 km from Pluto, or about twice as far away as Charon. If confirmed as satellites, the new objects will be given names by the International Astronomical Union.

Pluto's surface

Pluto is very black and it has been supposed by some astronomers that it is more like a giant comet nucleus than a planet. Its surface temperature is about -230°C, too cold for there to be much of an atmosphere.

Artist's concept of the New Horizons spacecraftArtist's concept of the New Horizons spacecraft during its planned encounter with Pluto and its moon, Charon. Image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI) From recent observations in the infrared Pluto is known to have on its surface solid ices of nitrogen, methane and carbon monoxide. This implies that there will be a thin atmosphere of these gases around the planet. The only information about surface detail comes from an analysis of the variation in the observed brightness over 5 years during which Pluto's satellite, Charon, occulted differing parts of the surface. From these measures it has been deduced that Pluto's south pole has recently received a new layer of methane ice giving high reflectivity of about 90 percent whereas other parts of the surface only reflect less than 30 percent of the sunlight.

Viewing Pluto

Launch of New Horizons spacecraftThe launch of the New Horizons mission to Pluto on 19 January 2006. The spacecraft was at the top of a three-stage rocket and is the fastest ever launched. Image: NASA / KSC 

Pluto is only visible in fairly large telescopes where it appears as a star-like object of 14th magnitude. Because of its great distance from the Sun Pluto only moves very slowly across the sky. At present it lies close to the borders of the constellations Libra and Serpens Caput.

Pluto's orbit has the highest eccentricity and largest inclination to the ecliptic of all the planets.

Due to its great distance no space probes have visited the Pluto-Charon system. This is set to change following the successful launch of NASA's New Horizons spacecraft in January 2006. The probe will pass Jupiter in February 2007 and use the gravity of the giant planet to accelerate with a rendezvous with Pluto in July 2015. New Horizons will then head into the Edgeworth-Kuiper belt and take a close look at some of its many Pluto-like minor planets.