On this day - 24 August

Pluto was traditionally known as the ninth planet within the solar system and held the status of planet from its discovery in 1930 until August 24th 2006.

The International Astronomical Union (IAU) set out three rules for something to be considered a planet. While Pluto fulfils the first two criteria - that it is is orbit around the sun and that it is massive enough to be rounded by its own gravity - it does not fulfil the third criteria. The third criteria states that it must have cleared the neighbourhood around its orbit. Pluto is substantially smaller than many of the other objects surrounding it. The IAU therefore reclassified Pluto as a dwarf planet.