Why Pluto isn't Classified as a Planet
The scientific community had three options on how to deal with Eris, Ceres, and subsequently Pluto. They could boost the number of planets in our solar system to 11. They could simply ignore the fact that Eris and Ceres existed. They could drop the number of planets in the solar system to 8 and reclassify Pluto. This last option is what was chosen and the new classification of ‘dwarf planet’ was given to Pluto and Eris with Ceres remaining a large asteroid even though it had been considered a planet back in the 1800s shortly after its discovery.
They based this decision by qualifying the three celestial bodies based on the definition of a planet. The three criteria that had to be met were as follows:
1. Does it orbit the sun? Pluto and the others clearly orbit the sun.
2. Does it have enough gravity to pull itself in a spheroid? Pluto is spherical in shape.
3. Does it ‘clear the neighbourhood’ of its orbit? Pluto does not.
Number 3’s question is clarified by stating that as planets develop, they either pull smaller masses into its orbit consuming them or their gravity slings them away from the planet as it forms. They become the dominant gravitational body in their orbit. Ceres does not meet criteria number three because of the asteroids that are in orbit with it. With Eris’ orbit being so close to Neptune’s and crossing Pluto’s, it does not have a clean orbit of it's own. Pluto does not meet the criteria of the planet definition for the same reason because Eris crosses it's path. When this happens, the celestial body becomes a classified as a ‘dwarf planet’. As long as there are other objects sharing Pluto’s, Eris’s and Ceres’ – orbits, they will never be given full planet status.