Pluto

Following from the IAU's decision on 24 August 2006, Pluto is to be re-categorised as a 'dwarf planet'. Please note that some of the text in these pages does not yet reflect Pluto's new status.

Introduction

A Hubble Space Telescope image of Pluto showing light and dark areas. Image: Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA Clyde Tombaugh discovered Pluto in 1930. Tombaugh was working at the Lowell Observatory in the United States, searching for the ninth planet that astronomers believed responsible for irregularities in Neptune's movement in the same way that the gravitational pull of Neptune affects Uranus. Coincidentally, Pluto was found near this object's predicted position. The search was based on a mistake - modern calculations indicate that Neptune's orbit can be explained without the need for a planet further out and in any case Pluto is far too small to have a significant effect.

Pluto is at a mean distance of 39.5 AU or 5900 million km from the Sun and takes 248 years to complete one revolution. However its orbit is very eccentric and it sometimes comes closer to the Sun than Neptune. This last happened between January 1979 and February 1999.

Artist's impression of the New Horizons spacecraft due for launch in 2006. Image: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute As well as being the most distant planet from the Sun, Pluto is also the smallest with a diameter of only 2274 km. It remains the only planet not visited by a space probe. The best images come from the Hubble Space Telescope but still show little detail.

NASA is planning to launch a spacecraft to Pluto in January 2006. This New Horizons mission is due to arrive there in 2015.

The surface temperature of Pluto ranges between 38K and 63K, which may be just warm enough for it to have an atmosphere when closest to the Sun.

Surface features

A NASA Earth-based image showing the frozen methane deposits that give Pluto its brownish colour. Image: Eliot Young [SwRI] et al., NASA Pluto has a surface with bright areas and dark areas probably consisting of differing amounts of ices of nitrogen, ethane, methane and carbon dioxide. Like the Neptunian moon Triton it may be about 70% rock and 30% water ice.

Charon

Charon is the only known Plutonian satellite and was discovered in 1978. It has a diameter of 1172 km. This large size in relation to Pluto means the Pluto-Charon system can be described as a 'double planet', even more so than the Earth and Moon.

This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by the Hubble Space Telescope. Image: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA The two objects are close enough to each other that both rotate synchronously - Pluto keeps the same face to Charon and Charon to Pluto. Charon has a water-ice surface quite different to Pluto.

Pluto – a large comet?

Pluto's composition, size and distance from the Sun make it quite different from the other planets in the solar system. Some scientists suggest that it should be re-classified as an asteroid or comet, but this is unlikely to happen in the near future.

Observing Pluto

At magnitude 14, Pluto is hard to observe without a large amateur telescope. It appears as a faint point, moving very slowly against the background stars from night to night. Even the largest ground-based telescopes show virtually no detail other than the disc itself.

Questions to think about

• Pluto and Charon are the best example of a double planet. Which other planet and moon in the solar system meet this description?