August 29, 2006. Fun fact: because Pluto was the outermost planet, its orbit was obviously the biggest. This also means it was the newest planet, it was the planet to be discovered last in our solar system. Makes sense right? But this also means that from the date it was discovered on February 18, 1930 to the date it was demoted as a planet on August 29, 2006, it did not even create a single full orbit. Pluto has an orbital period of 247 years. It only lasted as a planet for 76 years.