On this day in 1930, Clyde Tombaugh discovered Pluto.
The existence of a planet beyond Neptune was first proposed by Percival Lowell, who posited that orbital perturbations in the outer planets were being caused by an undiscovered body. Using Lowell’s calculations, astronomer Clyde W. Tombaugh searched for the missing planet for months. Using a new technique of photographic plates combined with a blink microscope, Tombaugh finally discovered Pluto on February 18, 1930.
While the International Astronomical Union formally declared Pluto a dwarf planet in 2006, it has nevertheless been one of the most important finds of the 20th century. In fact, Pluto’s unique characteristics have served to further define what a planet is, and is helping scientists search for extrasolar bodies today.
NASA’s New Horizons spacecraft performed a flyby of Pluto in 2015 and returned some awe-inspiring photos, which show that a dwarf planet once thought to be a dead rock in space is actually an incredibly dynamic body with unique geography, weather patterns and possible subsurface motion.
Learn more about the discovery of Pluto at the Lowell Observatory website.