In 1929 he became an assistant at the Lowell Observatory, and began a search for the trans-Neptunian planet that had been predicted by P. Lowell. To do this, he took pairs of photographs a week apart and compared them using a blink comparator. Pluto was discovered on 1930 February 18, on a pair of plates exposed the previous month. Tombaugh continued the search for another ten years, finding star clusters, clusters of galaxies, and nearly 800 asteroids, but no further planets. He later made a similar search for small moonlets of the Earth, but found none.