by Mike Brown, Professor of Planetary Science, Caltech

On August 24th 2006 the word "planet" was given its first-ever scientific definition by a vote of the International Astronomical Union. With the raising of a few yellow cards in Prague, Pluto was demoted from full-fledged planet to "dwarf planet." Eris, originally called  2003 UB313, sometimes called Xena, sometimes called the "10th planet," which in many ways precipitated this final debate, becomes the largest known dwarf planet. Unless astronomers revisit this issue at some point in the future, it is unlikely that there will ever be more than eight planets. While some still continue to mourn the loss of Pluto from the pantheon of planets, it is much more interesting to consider this new class of dwarf planets, instead.