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The fuss over Pluto's planethood started a few years ago when astronomers began to discover large, icy bodies in the outer solar system, some of them not much smaller than Pluto. Then in 2005 came the killer: the object designated 2003 UB313, popularly known as Xena, is 2400 kilometres across, making it slightly bigger than Pluto. Should it be called the tenth planet? If so, how many more planets might there be? Nobody had a clear definition of the term.