What's a Planet?

New riddles beyond the solar system

By Ron Cowen, 15:56 PM April 15, 2008

"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.

But last August, the International Astronomical Union approved the first formal definition of a planet since the Greeks coined the term some 2,000 years ago. Pluto, got the...

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