PRAGUE, Czech Republic - Pluto, beloved by some as a cosmic underdog but scorned by astronomers who considered it too dinky and distant, was unceremoniously stripped of its status as a planet Thursday.

The International Astronomical Union, dramatically reversing course just a week after floating the idea of reaffirming Pluto's planethood and adding three new planets to Earth's neighborhood, downgraded the ninth rock from the sun in historic new galactic guidelines.

The shift will have the world's teachers scrambling to alter lesson plans just as schools open for the fall term.

"It will all take some explanation, but it is really just a reclassification and I can't see that it will cause any problems," said Neil Crumpton, who teaches science at a high school north of London. "Science is an evolving subject and always will be."