Pluto: The Demoted Dwarf Planet

It was the only planet to have been discovered by an American, but it's no longer classified as a planet. Who found Pluto, and how did astronomers even know to look for the so-called Planet X on the edge of our solar system?

Pluto photographed by the LORRI and Ralph instruments aboard the New Horizons spacecraft

Topics in this Podcast: science history, NASA, astronomy