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

by Christopher H. Wortley

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.

Photo credit: NASA/JPL/Caltech An artist's rendition of Eris, one of the new dwarf planets (left) with the sun shining in the distance (right).

The discovery of an object nicknamed 'Xena' (but now officially named 'Eris') by a team led by Mike Brown of CalTech University put Pluto under pressure. Using the Hubble Space Telescope, they showed that this icy Kuiper Belt object, which is 10 billion miles from the sun, was slightly larger than Pluto. Astronomers reasoned that if Pluto is a planet, then so is Eris. But what if, as is likely using the ever-improving instruments, they make further discoveries in the Kuiper Belt? To avoid ending up with a large and confusing number of planets, many of which might not merit the title, the committee chose a more restrictive definition.

Definition Doubts

By declaring Pluto the first in a special subcategory, astronomers may have hoped to console Pluto fans, but online petitions indicate that they may not have succeeded. At the meeting, disagreements were also evident, most notably between dynamicists and geologists. The meeting failed in a close vote to approve the name 'Plutonians' for the dwarf planets orbiting beyond Neptune, which was an alternative to the original suggestion, 'Plutons' to which geologists objected. Gingerich said that the process of arriving at a workable resolution was like trying to do diplomacy in the Middle East. Of 10,000 astronomers, only 428 were present for the vote, and on the last day of the conference, a minority voted to add the third clarifying criterion.

Photo credit: International Astronomical Union/Martin Kornmesser Our new solar system, with eight classic planets and three dwarf planets (the status of Pluto's moon Charon is still being considered).

For 100 hours after the decision was made, a petition circulated amongst planetary scientists and astronomers as evidence of the strength of feeling. It said, 'We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.' The petition was signed by 300 prominent planetary science experts.

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.

For more information:

Petition Protesting the IAU Planet Definition
http://www.ipetitions.com/petition/planetprotest/

Pluto Petition
http://www.plutopetition.com/