American astronomer who discovered Pluto in 1930. Beyond the orbit of Neptune, the existence of a planet known as Planet X, had been predicted some years earlier by the American astronomer Percival Lowell based on apparent perturbations in the orbits of Neptune and Uranus. Tombaugh's careful analysis of celestial photographs did reveal a new celestial body, although it was too small to have caused the perturbations. Planet X itself was never found.
