This Day in History: Pluto Is Discovered February 18, 1930. Once believed to be the ninth planet, Pluto was discovered at the Lowell Observatory in Flagstaff, AZ, by
astronomer Clyde W. Tombaugh. Tombaugh discovered the tiny, dwarf planet using a new astronomic technique of photographic plates combined with a blink microscope. Pluto was given the Roman name for the god of the underworld in Greek mythology, due to its surface temperature of -218 ° C. More than six billion kilometres away from the sun, it takes Pluto approximately 248 years to complete one orbit. Pluto's only known moon, Charon, has a diameter of a mere 1186 kilometres. In 2006, it was announced that Pluto would no longer be considered a planet due to the fact that its orbit crosses into the orbit of planet Neptune.