Farewell to Pluto

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.