Look! Up in the sky! It's a bird! It's a planet. It's a very large ball of ice!

It's Pluto, with its moon, Charon

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You might call Pluto the black sheep of the planet family. Or, its favorite child. Of late, the solar system's smallest planet is experiencing something of an identity crisis as it orbits at the center of an impassioned debate. A handful of planetary scientists insist that Pluto should be expelled from the family of planets. "It's merely a large comet," says Brian Marsden of the Smithsonian Astrophysical Observatory at Harvard University, succinctly expressing the demotion view.

But other astronomers insist that a planet is exactly what Pluto is. "It's a planet, and always has been," says Alan Stern, director of the Southwest Research Institute's Department of Space Studies.

One fact is indisputable: Pluto has always been the odd planet out. While the other eight commonly recognized planets fall into two clear categories—four rocky, terrestrial bodies (Mercury, Venus, Earth and Mars), and four gas giants (Jupiter, Saturn, Uranus and Neptune)—Pluto stands alone: an ice ball rich in frozen water and nitrogen.

More important, the debate underscores the strangely unsettling fact that astronomers have great trouble defining the term "planet." The arguments over Pluto's planetary status may seem merely semantic, but they reflect groundbreaking thinking about our solar system, how it developed and the range of objects that call it home.

Stern and others hope to unmask the mysteries of lonely Pluto when NASA's New Horizons spacecraft reaches the outer solar system—if all goes according to plan—in 2016. Planet or not, Pluto will undoubtedly continue to surprise us.

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