Ninety years ago today, Clyde Tombaugh, a young astronomer working at Lowell Observatory in Flagstaff, Arizona, discovered Pluto. In doing so he unknowingly opened the door to the vast “third zone” of the solar system we now know as the Kuiper Belt, containing countless planetesimals and dwarf planets—the third class of planets in our solar system.
Source:
https://phys.org/news/2020-02-tombaugh-discovery-pluto-revolutionized-knowledge.html