Mike Brown

Mike Brown honestly doesn't care whether Pluto is a planet or not. "It's like asking whether Australia is a continent," says the Caltech astronomer. "The word has no formal scientific definition." Nevertheless, Brown brought that long-simmering issue to a head last summer by discovering an object he nicknamed Xena. It's similar to Pluto—only bigger. If Pluto is a planet, as astronomers have classified it for more than 70 years, then we have at least 10 planets in the solar system. If Xena isn't, we have only eight.

Fortunately for Brown, 40, defining the word planet is the job of the International Astronomical Union (IAU), which has been wrestling with the question for months now. Brown's forte is finding heavenly bodies, and there's nobody better at it than he. Since the late 1990s, Brown has been scanning the skies with an old 48-in. telescope that is too small for exploring distant galaxies but perfect for the sort of patient, methodical search needed to find dim worlds out beyond Neptune.

Brown and his team have already found at least five such bodies, including Santa and Easter Bunny (like Xena, these are nicknames) and Quaoar and Sedna (these names are real). None of them are as big as Pluto, but they are all substantial worlds in themselves—and all challenge the simple nine-planet model of our solar system that most of us grew up with. And there are undoubtedly more to come. Any number of small planets could still be lurking out there. If the IAU ends up broadening the definition of what counts as a planet, Brown will, in that instant, become the most successful planet hunter in the history of our solar system.

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