Farewell to Pluto
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.
Farewell to Pluto
- 26 Sep 2006 By Christopher H. Wortley Page 1 of 3A 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.
Even Gingerich is not satisfied with the new category of 'dwarf' planets and describes the term as clumsy and linguistically preposterous. Likewise, Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, describes the decision as a 'terrible mess' and dislikes the unscientific idea of putting a limit on the number of planets. Although the third criterion is intended to cover objects orbiting in the Asteroid or Kuiper Belts, Stern dislikes it and claims that other planets have also failed to either absorb or knock away their orbital debris.
But coming up with a new definition was no easy matter. The committee was specifically charged with considering social and historical context when coming up with defining criteria. The choice of 'roundness' as a criterion for planetary status reflects the committee's sensitivity to the broad cultural significance and use of the term. Likewise, moons were maintained as distinct entities from planets.
Robin Catchpole of Cambridge University Institute of Astronomy believes that the new definition is the lesser of two evils. Although he would have preferred to keep Pluto on historical grounds, he had been unhappy about the original proposal to allow 12 planets. He would have preferred the term 'minor planet' to the term 'dwarf planet', but believes that the names given to objects are not really that important.
Perhaps the IAU's task was an impossible one. As far as the public was concerned, the scientists were in the familiar position of being unable to provide the certain and watertight definition required of them. Conversely, some astronomers feel they accommodated cultural context at the expense of good science.
With the IAU currently considering a dozen candidate dwarf planets, the argument about how to categorise them may have only just begun.
