Farewell to Pluto

A new definition for 'planet' that excludes Pluto is causing much controversy.

Photo credit: International Astronomical Union/Lars Holm Nielsen What is a planet? Astronomers voted for a new defintion of a planet during the 2006 General Meeting of the International Astronomy Union

Schoolchildren now have it easier: they only have to memorise the names of eight planets. On August 24, 2006, a vote by the International Astronomical Union (IAU) at their 2006 General Assembly in Prague saw Pluto stripped of its planet status. For fans of Pluto, it was a sad occasion. But it all came about because astronomers have never really formalised what a planet actually is - until now.

The debate started after the recent discovery of new objects in our solar system that were larger than Pluto. A Planet Definition Committee, comprised of historians, writers and astronomers and chaired by Harvard astronomer and historian Owen Gingerich met in July to draft a new planetary definition. The much-publicised proposal to add three new planets to our solar system failed to gain approval by astronomers.

A celestial body in our solar system must now meet three conditions to be considered a planet. It must (a) be in orbit around the sun, (b) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) have enough mass to have cleared the neighbourhood around its orbit.

Pluto does not have enough mass to satisfy the third condition, but it does fit into a new category of 'dwarf' planet, which describes a non-satellite object with not enough mass to clear its orbit. Of the three new planet contenders - Ceres, Eris and Charon - Ceres and Eris have also been placed in this category and Charon simply remains Pluto's moon.

Pluto's Status

It has long been clear that Pluto is different from the other planets. Not only is it much smaller - about 1600 miles in diameter - but its elongated orbit is tilted in relation to the other planets, causing it to be nearer to the sun than Neptune for part of its 248-year journey.