The planets that were looked for
Planets predicted from the sky's arithmetic before anyone saw them: Neptune found where the numbers said, Lowell's Planet X and the plates that found Pluto, and Vulcan, the planet inside Mercury that was never there.
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Lowell's Planet X and Tombaugh's Plates of 1930
Planet X began as Percival Lowell's prediction: an unseen planet beyond Neptune, pulling on Uranus, whose position he published in 1915. In February 1930 Clyde Tombaugh, using the telescope and the plan Lowell's observatory had built for the search, found Pluto near one of Lowell's two positions. Here is the hunt, the case that it was a prediction come true, why most astronomers came to call it chance, and what that leaves.