Question: Who was the American astronomer involved in the search for a planet thought to explain apparent discrepancies in the orbits of the giant planets?
Sentence (from "Planets beyond Neptune"): The search began in the mid-19th century and culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
Relevant evidence: Yes