Leicester University Physics Department

Pluto

picture : Pluto taken from Los Alamos National Laboratory, US

Solar System Index

The Sun

Mercury

Venus

Earth

The Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

The Kuiper Belt

Introduction

Pluto Pluto is now considered to be a dwarf planet, after its recent demotion from planetary status by the International Astronomical Union. Its composition is unknown, but it is expected to consist of about 80% rock and 10% water ice, its surface being covered with ices of methane, nitrogen and carbon dioxide, leading to a very high albedo (reflectance). It is suspected that at near its closest approach to the Sun, which last took place in 1989, some of the gases are released from these ices, forming a slight atmosphere, which may even interact with Pluto's moon, Charon. There has so far been no mission to this fascinating planet, but NASA'a New Horizons mission is due to launch in January 2006. (Image courtesy of NASA).

Pluto's Eccentric Orbit

Pluto's orbit sometimes takes it within the orbit of Neptune. The last occurence of this was from 1979 to 1999. Pluto is also orbitally locked to Neptune. This means that it takes exactly 1.5 times longer than Neptune, and crosses orbits at roughly the same point each time, therefore the two planets will never collide if nothing interrupts their orbits. Even if this was not the case, the odds against such a collision would be astronomical, due to the vast distances and times involved. Pluto's orbit is curious in two other respects. Firstly its orbit does not lie in the same plane as the other planets and secondly the plane of Pluto's equator is at almost exactly right-angles to the plane of its orbit, which is a similar situation to that of Uranus.

Charon

Charon & Pluto Charon is Pluto's only known moon, being approximately one sixth of its mass. A picture of their relative sizes is shown on the right. Charon is now also considered to be a dwarf planet. Its composition is unknown, but its lower density suggests that it is similar in nature to Saturn's icy moons, such as Rhea. Its surface seems to be covered with ice, explaining its high albedo.