#OTD in 1930.
While studying photographs taken in January, Clyde Tombaugh discovers Pluto, making it by far the first known object in the Kuiper belt. It was immediately hailed as the ninth planet.
However, its planetary status was questioned when it was found to be much smaller than expected. In 2006, the International Astronomical Union (IAU) formally redefined the term planet to exclude dwarf planets such as Pluto.
https://en.wikipedia.org/wiki/Clyde_Tombaugh
