Farewell To Pluto

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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 Harvardastronomer and historian Owen Gingerich met in July to draft a newplanetary definition. The much-publicised proposal to add three newplanets to our solar system failed to gain approval by astronomers. A celestialbody in our solar system must now meet three conditions to beconsidered a planet. It must (a) be in orbit around the sun, (b) havesufficient mass for its self-gravity to overcome rigid body forces sothat 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 Ithas long been clear that Pluto is different from the other planets. Notonly is it much smaller - about 1600 miles in diameter - but itselongated orbit is tilted in relation to the other planets, causing itto 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). Thediscovery of an object nicknamed 'Xena' (but now officially named'Eris') by a team led by Mike Brown of CalTech University put Plutounder pressure. Using the Hubble Space Telescope, they showed that thisicy Kuiper Belt object, which is 10 billion miles from the sun, wasslightly larger than Pluto. Astronomers reasoned that if Pluto is aplanet, then so is Eris. But what if, as is likely using theever-improving instruments, they make further discoveries in the KuiperBelt? To avoid ending up with a large and confusing number of planets,many of which might not merit the title, the committee chose a morerestrictive 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 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.