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Pluto is, on average, the most distant planet from the Sun.

This image of Pluto was obtained by the Hubble Space Telescope in 1994. At the time Pluto was at a distance of 4.4 billion km from the Earth and Charon was at a distance of 19640 km from Pluto.(Image credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA.)
For 20 years out of its 249 year orbit, however, it is in fact closer to the Sun than is its closest rival, 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 4,440 million kilometres from the Sun. At aphelion, its furthest distance from the Sun, it will be about 7,395 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.

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. From these facts we can determine 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. It is also 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.

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
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.

An artist’s impression of Pluto, Charon and the Pluto-Kuiper Express spacecraft.(Image credit: Jet Propulsion Laboratory.)

Due to its great distance no space probes have visited the Pluto-Charon system. NASA is now in the process of planning such a mission, the Pluto-Charon Express. It will use Jupiter’s gravity to accelerate the satellite towards Pluto, arriving there after 2010 and then continuing into the Edgeworth-Kuiper Disk (see our leaflet on 'The Furthest Object in the Solar System’). This contains many minor planets which have yet to be investigated.

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