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10th Planet

A stunning discovery at the far reaches of our solar system raises questions about what makes a planet a planet.
Topic: Space + FlightSpace & FlightBrand: NOVA ScienceNOWNOVA ScienceNOW

(This video is no longer available for streaming.) In 2005, astronomer Mike Brown and his team at Caltech discovered a new planet out beyond Neptune. Slightly larger than Pluto, the new object was nicknamed Xena (after the TV show "Xena: The Warrior Princess"). But is Xena a planet? For that matter, is Pluto a planet? Brown's discovery sparks a controversy over what exactly is meant by planethood.

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