Is Sedna really a planet?
This week, astronomers tiptoed delicately around the precise classification of Sedna, the mysterious distant object far beyond the orbit of Pluto. Headline writers enthusiastically reached for phrases like Planet X. Nasa press officers appeared to endorse planet status for a lump of icy rock the size of a European country with a wildly eccentric orbit that takes 10,500 years to complete. But was planet the right word? The Greeks invented the word (it means “wanderer”) and distinguished planets from fixed stars because they behaved differently. So Venus, Mars, Jupiter and Saturn have been called planets for 2,500 years. But the definition is flexible. “You can set your definition of a planet to include those you don’t wish to exclude,” says Robin Catchpole, senior astronomer at the Royal Greenwich Observatory. “You can set a definition about it being a large body in almost circular orbit about the sun, in the same plane, and then you might say: that is very nice, but that excludes Pluto
