In 2006 the International Astronomical Union (IAU) demoted the much-loved Pluto from its position as the ninth planet from the Sun to one of five “dwarf planets.” The IAU had likely not anticipated the widespread outrage that followed the change in the solar system's lineup.

Which planet has been downgraded?

Believe it or not, each year on August 24, the international scientific community recognizes Pluto's historic downgrade with a holiday called Pluto Demoted Day. But just because Pluto lost its planet status doesn't mean it isn't fascinating, says Cathy Olkin, a planetary scientist at the Southwest Research Institute.

Why was Pluto demoted?

Bottom line: August 24 is the anniversary of Pluto's demotion to dwarf planet status. The International Astronomical Union demoted Pluto largely because it is has not “cleared the neighborhood around its orbit.”