What Makes a Planet a Planet?

How do you solve a problem like a planet?
Meeting in Prague in August of 2006, the IAU (International Astronomical Union) ruled that, in order for a celestial body to be referred to as a planet, it must meet three criteria: 1) it must be in orbit around the sun, 2) it must have sufficient mass for its gravity to force it into a nearly round shape, and 3) it must be able to clear the neighborhood around its orbit. The controversial ruling placed Pluto into the classification of dwarf planet, causing editors of astronomy-related books to bang their heads on their desks incessantly.

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