Scientific Colloquium
April 20, 2007

"Is Pluto a Planet?  The Great Planet Debate"

Is Pluto a planet?  And just what *is* a planet anyway?  At first glance it seems these simple questions should have simple answers.  But like other seemingly simple questions (What is life?  What is "true color"?), the question of how to define "planet" has been a devilishly difficult one to answer.  

Pluto has long been a problematic focal point for anyone contemplating this issue.  The discovery of numerous objects orbiting beyond Neptune, what we now know as the Kuiper Belt, turned up the heat on the long simmering planet-definition debate.  Many of these newly-discovered objects had orbits that were dynamically indistinguishable from Pluto's.   

The debate finally came to a boil with the July 2005 discovery of 2003 UB313, a transneptunian object larger than Pluto.  Had the tenth planet been discovered?  Would there be more?  What would it be named and who would name it?  The dramatic conclusion to this debate (for now) was played out at the August 2006 International Astronomical Union General Assembly where a partial (and controversial) definition of what constitutes planethood was hammered out in back rooms and in front of cameras amidst the medieval spires and cobbled alleyways of Prague.

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