Tombaugh’s task was to search for a ninth planet by photographing a selected region of the sky, one small piece at a time, and trying to detect any object moving beyond Earth’s orbit. The main tool he used was a machine called a blink comparator—an instrument that could “blink” back and forth between two pictures of the same patch of sky to see if any particular object moved compared to the background.

Tombaugh worked on this project for ten months until, on February 18, 1930, his hard work paid off. He discovered a small, moving solar system body. He was able to rule out the possibility that the body was a comet or asteroid by comparing his results against a third photograph of the same area. Looking back at photographs taken years earlier by Percival Lowell, Tombaugh found further confirmation that the object was there. Lowell had indeed found it; but the object was so small that Lowell’s assistants had apparently overlooked it.