How is a planet classified a planet?
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:
- The body must be orbiting the Sun.
- The object must be massive enough for gravity to make it spherical in shape.
- 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.