April 20, 2007
SPACE TELESCOPE SCIENCE INSTITUTE
"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
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.