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By Jason Snell for
The unlikely dream of a large “Planet X” at the far reaches of the solar system — the same dream that motivated Clyde Tombaugh to find Pluto 100 years ago — is still alive. Scientists scanning the icy belts of material way beyond Pluto keep looking for signs that those distant objects are being influenced by a larger object.
Astronomers have already ruled out finding Jupiter- or Saturn-sized planets in the extreme reaches of the solar system. But finding even an Earth-sized body would be a dramatic moment that would change our understanding of the solar system forever.