In addition to "downgrading" Pluto's status, the International Astronomical Union also defined two celestial objects - the asteroid Ceres, Charon (Pluto's moon) and 2003 UB313, nicknamed "Xena" by its discovered Michael Brown of Caltech - as dwarf planets.
Michael Brown (the astronomer, not the FEMA guy) wrote (before the astronomer's vote) that calling Pluto a planet would cause many other celestial objects to fall into that category:
And the solar system would look like this:
So, despite public outcry, astronomers voted to kick Pluto out instead of letting in hundreds of space rocks into the Planet Club.
Link: Michael Brown's Website
This cartoon, drawn by Richard Crowson of the Wichita Eagle, Kansas - source: the always awesome Daryl Cagle's Professional Cartoonist Index.
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