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 "List of Halley-type comets"): The following is a list of Halley-type comets (HTCs), which are periodic comet with an orbital period between 20 and 200 years, often appearing only once or twice within one's lifetime.
Relevant evidence: No