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NOVA
The 170-Year Search for a Planet Beyond Neptune
Planet Nine’s existence is not the first time astronomers have predicted a planet they couldn’t see. Clyde Tombaugh was looking for Planet X when he accidentally found Pluto. After Voyager 2 flew by Neptune in 1989, new calculations determined there was no Planet X. But three years ago, a disturbance was detected at the far reaches of the Solar System.
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