How is a planet classified a planet?

1 Answer
Mar 23, 2018

A planet is now defined by the International Astronomical Union (IAU).


Everything which was known to orbit the Sun and over a certain size was considered a planet.

Ceres was originally designated a planet when it was discovered. It was then found that Ceres was part of a group of objects which were designated asteroids. As a result Ceres was demoted to an asteroid.

More recently many objects were discovered, particularly beyond Neptune. The problem was should they all be designated planets?

The IAU made a resolution in 2006. The made some definitions. A planet must conform to three criteria:

  1. The body must be orbiting the Sun.
  2. The object must be massive enough for gravity to make it spherical in shape.
  3. The object must have cleared its orbit of all other bodies.

Pluto got demoted to a dwarf planet because it didn't meet the third criterion.

Ironically Jupiter arguably doesn't meet the third criterion because it has many bodies in its orbit at the trojan points.