Percival Lowell postulated the existence of a planet beyond Neptune and began in earnest in 1905 to search for it.
He spent his remaining 11 years in a lengthy mathematical and observational search for the elusive “trans-Neptunian” planet. Upon Lowell’s death in 1916, the search was suspended while the Observatory went through some protracted legal wrangling about the estate. In 1927, once the estate was finally settled, Percival’s brother — Harvard University President A. Lawrence Lowell —provided the funds needed to construct the telescope and dome you can still see every day on guided tours here.
The new telescope was completed in 1929, and the Observatory hired a farm boy from Kansas named Clyde Tombaugh, who had impressed director V. M. Slipher with some unsolicited sketches, to renew the search.
The process was laborious: Tombaugh would photograph the same part of sky several days apart and use a Zeiss Blink Comparator, also still on display here, to detect the motion of a nearby planet against the more distant “fixed” stars. Once Tombaugh got going toward the end of 1929, the discovery came remarkably rapidly: on February 18, 1930, he found the distant planet on plates taken on the 23rd and 29th of January. The discovery was announced March 13, 1930 — Percival Lowell’s birthday.