In 2006, the International Astronomical Union (IAU), which promotes and safeguards the science of astronomy, passed a resolution that classified all bodies (except satellites) in our solar system into three distinct categories – planets, dwarf planets, and Small Solar System Bodies. To qualify as a planet, the body had to orbit around the sun, have sufficient mass for its self-gravity to pull it into a round shape, and have cleared the neighborhood around its orbit. Since Pluto did not meet the third criterion, it was downgraded to a dwarf planet.