American astronomer who discovered Pluto in 1930 after a systematic search for a ninth planet instigated by the predictions of other astronomers. He also discovered several clusters of stars and galaxies, studied the apparent distribution of extragalactic nebulae, and made observations of the surfaces of Mars, Venus, Jupiter, Saturn, and the Moon.

During his career, Tombaugh served at Lowell Observatory, Flagstaff, Ariz., as assistant astronomer; Arizona State College as science instructor; White Sands Missile Range as optical physicist and astronomer; and New Mexico State University as associate professor of the department of earth sciences (1961–65), full professor (1965–73), and professor emeritus from 1973. Among his publications are The Search for Small Natural Earth Satellites (1959) and Out of Darkness: The Planet Pluto (1980), with Patrick Moore.