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Surprisingly, until the triennial meeting of the International Astronomical Union (IAU) addressed that question in August 2006, there was no official scientific definition of the word, "nor was there much concern over this deficiency," write astronomers Laurence A. Marschall and Stephen P. Maran in Pluto Confidential: An Insider Account of the Ongoing Battles Over the Status of Pluto.
True, ambivalence among astronomers about the status of Pluto had been growing almost from the time of its 1930 discovery. But as Marschall and Maran note, everyone seemed prepared to accept that the term "planet," like "pornography," was "very hard to define, but you know it when you see it."
That uneasy situation prevailed until 2005, when a Solar System object that had been discovered in 2003 in the region beyond Neptune called the Kuiper Belt was found to be larger than Pluto. It had been designated 2003 UB313 and nicknamed Xena, but an object of that size needed an official name. (It was eventually named Eris.)
The IAU has various conventions for naming Solar System objects, which meant it first had to decide whether UB313 was a planet or not. And if not, then its fellow Kuiper Belt Object (KBO) named Pluto, was in for a demotion. So the IAU members meeting in Prague in 2006 had little choice but to try to define the previously undefinable.
The discussion was contentious, to say the least. Members of the committee charged with developing a definition to vote on were accused of not getting sufficiently broad input from a range of planetary astronomers. Their first attempt at a definition, which would have included Pluto and its moon Charon as a double planet plus Eris and the largest asteroid, Ceres, was rejected.
By the time they returned with a new definition on the last day of the meeting, most of the 2400 attendees had left for home. Only 424 cast their votes. That may have been a representative sampling of the membership, but it cast a shadow on the legitimacy of the outcome.
The reaction of non-astronomers, most of whom learned through the news media that the IAU vote "demoted" Pluto to dwarf planet status, ranged from bemusement to outrage. What happened in Prague that hot summer day, they wondered.
Marschall, who voted in favor of the IAU resolution, and Maran, who would have voted against it had he been present, join forces to answer that question. Those readers who want a raucous disagreement between the authors will be disappointed. But those who enjoy a good historical tale will be rewarded.
The authors lay out the contentious history of planet hunting from Uranus to Ceres and other large asteroids, and from Neptune to the non-existent Vulcan, as background to today's arguments over the status of Pluto and Eris.
Those stories are rich with scientific and political disagreement. Yet in the end the authors present a picture of the process of scientific discovery at its finest--not pitched battles for glory and supremacy (though there are some), but rather lively disagreements in the quest of understanding. Who could ask for more?