Clyde Tombaugh, American astronomer and academic, discovered Pluto (born 1906)

Clyde William Tombaugh was an American astronomer and telescope maker, best known for discovering Pluto in 1930, marking the first detection of what would eventually be recognized as the Kuiper belt. At the time, Pluto was referred to as the ninth planet in the Solar System, a classification that stood for over seven decades.

Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has roughly one-sixth the mass of the Moon and one-third its volume. Originally considered a planet, its classification was changed when astronomers adopted a new definition of planet.