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The 10th Planet
In the beginning of the 20th century, it seemed to astronomers that the outermost planets didn't move quite as they ought to. Perhaps there was a ninth planet out beyond Neptune, and perhaps its gravitational pull was making the outermost planets behave oddly. They searched for many years and finally found Pluto in 1930. That seemed to settle everything. But then, as more years passed, it turned out that Pluto was small, its gravitational pull couldn't possibly affect the outermost planets. Does that mean there is another planet beyond Neptune, a large one? We don't know.
Planet 10th. We think it might be out there, beyond Pluto, tugging away at the outermost planets of our Solar system. But if there is such a thing as Planet 10th, it would be so far away and so fait that it would be hard to detect.
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