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What's a Planet?

New riddles beyond the solar system

By
7:53am, November 28, 2006

"I found a planet!" Caltech astronomer Mike Brown remembers exclaiming during a phone call he made to his wife early in 2005. Little did he know that he'd have to eat his words just 18 months later. Brown had found an outer–solar system object heavier than Pluto, so it seemed reasonable to call the object the tenth planet.

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