On February 18, 1930, the American astronomer Clyde Tombaugh discovered Pluto!

Before he discovered Pluto, another astronomer named Percival Lowell had spent over a decade trying to find it. He had theorized that a ninth planet existed based on wobbles seen in the orbits of Uranus and Neptune. But it wasn't until Tombaugh started using a new observation technique that Pluto was first spotted. The new technique involved something called a blink microscope, which Tombaugh used to compare photographic plates and look for signs of moving objects.