at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.

The existence of an unknown ninth planet was first proposed by Percival Lowell, who theorized that wobbles in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planetary body. Lowell calculated the approximate location of the hypothesized ninth planet and searched for more than a decade without success. However, in 1929, using the calculations of Lowell and W.H. Pickering as a guide, the search for Pluto was resumed at the Lowell Observatory in Arizona. On February 18, 1930, Tombaugh discovered the tiny, distant planet by use of a new astronomic technique of photographic plates combined with a blink microscope. His finding was confirmed by several other astronomers, and on March 13, 1930—the anniversary of Lowell’s birth and of William Herschel’s discovery of Uranus—the discovery of Pluto was publicly announced.