Pluto - the smallest planet

General Information

Average distance from the Sun

5,913 million km
(3,675 million miles)

Length of year

249 Earth years

Average rotation period

6 days 9 hours 17 minutes

Diameter at equator

2,280 km

Approximate surface temperature

- 230 0C


Pluto's existence was predicted in the early twentieth century as astronomers tried to find a reason for the orbit of Uranus not following the path they had predicted. The American astronomer Clyde Tombaugh spent thousands of hours searching photographs with million of stars before he found Pluto in 1930. It appears as a speck even in the largest telescopes, and has not been visited by any space probes, so astronomers have had to work out as best they can what Pluto might be like. Observations from Earth show that its surface is covered with frozen water and methane at about -2300C.

Pluto has one moon, Charon, which was discovered in 1978 as a small bulge in an image of Pluto, made by J. Christy, an astronomer at the United States Naval Observatory. It is about 1,190 km in diameter, about half the size of Pluto itself. Its surface is icy but darker than Pluto's, and it seems to have water ice on it but no methane.

In 1987 and 1988, Charon's orbit carried it behind and directly in front of Pluto on each circuit, as seen from Earth. This is a rare series of events that can only be witnessed every 200 years. Observations made at this time were a great help of astronomers, in working out the size and nature of this remote pair.

Pluto rotates once on its axis, in exactly the same time that it takes Charon to complete one orbit: 6 days 9 hours and 17 minutes. This means that Charon is always in the same place in the sky above Pluto.

Pluto is usually described as the planet furthest from the Sun, but this is not always true. Its orbit is such a stretched-out ellipse that the planet comes to within 4 million kilometers of the Sun, closer than the orbit of Neptune. Its furthest point from the Sun is over 7 million kilometers away.

For twenty years of each orbit Pluto is closer to the Sun than Neptune. During the years 1979 to 1999, Neptune is the outermost planet of the Solar System.