Why is Pluto not a Planet?
There were nine planets in the solar system till the year 2006. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. Since then, it is no longer a planet of the solar system.
According to IAU, the planets in the solar system should meet the following three eligibility criteria:
- A planet should be in orbit around the Sun.
- It should have nearly a round shape. That is, a planet should be large enough to have large enough gravity to force itself in to a spherical shape.
- It should have large enough gravity such that it could clear the neighbourhood around its orbit. That is, there should not be any other celestial bodies of comparable size other than its own satellites in its vicinity.
Pluto did not meet the third eligibility criteria set by IAU for being a planet: That is, it has not cleared the neighbourhood around its orbit. In other words, it is not dominant gravitationally.
Now, Pluto is added to the list of “dwarf planets”, as it did not meet the above criterion.