Where is the observatory where Percival Lowell first theorized the existence of Pluto?

The observatory where Percival Lowell first theorized the existence of Pluto was Lowell Observatory in Flagstaff, Arizona. Lowell founded the observatory in 1894 and used it to study Mars, Venus, and the possibility of an undiscovered planet beyond Neptune.

Lowell called the hypothetical object “Planet X” and began a systematic search, but he died in 1916 before it was found. The observatory’s search resumed years later, and Clyde Tombaugh discovered Pluto there on February 18, 1930, by comparing photographic plates taken at different times.

The wording can cause a small historical mix-up: Lowell did not personally discover Pluto. He developed the theory and initiated the search; Tombaugh made the discovery at the Flagstaff observatory fourteen years after Lowell’s death. Pluto was classified as a dwarf planet in 2006.