IAU criteria for a planet:
- In 2006, the International Astronomical Union (IAU) established a definition of a planet according to which for an object orbiting its star to be classified as a planet, it needs to “clear” its orbit; in other words, it must be the largest gravitational force in its orbit.
- By the IAU definition, Pluto did not qualify because
- it is influenced by Neptune’s gravity, and
- it shares its orbit with frozen gases and other objects.
New Research:
- Researchers, in a study published in the journal Icarus, have however, opposed this standard for classifying planets.
- They reviewed scientific literature from the past 200 years and found only one publication (1802) that used the clearing-orbit requirement to classify planets, and suggested it was based on reasoning that has since been disproved.
- Instead they recommend classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.
