Pluto is, on average, the most distant planet from the Sun. For the next few years, however, it is in fact closer to the Sun than is its closest rival, Neptune. 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 present Pluto is near perihelion, its closest distance to the Sun, and is about 4440 million kilometres away. At its furthest from the Sun, aphelion, which will be reached in about 124 years time (half its orbital period) it will be 7395 million kilometres away.
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.
in 1978 it was discovered that Pluto has a very close satellite, now called Charon. 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 230C, 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.
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 very little, apart from the facts given above, is known about Pluto.
Produced by the Information Services Department of the Royal Greenwich Observatory.
