Why is Pluto now classified as a dwarf planet? Why was it downgraded?

1 Answer
Nov 17, 2017

Pluto was downgraded from planet to dwarf planet because the definition of a planet was changed.


Pluto was discovered by the astronomer Clyde Tombaugh in February 1930. It was given the status of the ninth planet of the solar system.

As telescopes, particularly in on satellites, improved, more objects were discovered which caused a problem that they were quite small and some astronomers didn't think they qualified as being planets.

The International Astronomical Union (IAU) had a vote which was very close. They defined three criteria which a planet must satisfy.

  1. It must be large enough for gravity to overcome structures of materials and make it spherical. Most bodies are flattened spheroids due to rotation.
  2. It must orbit the Sun.
  3. It must have cleared its orbit of other bodies other than moons.

The IAU created a new definition of an object called a dwarf planet which only satisfies the first two criteria. Pluto fails the third criterion, so it was demoted to a dwarf planet.

Many people, including myself, still consider Pluto to be the ninth planet.

To be pedantic, Jupiter has a lot of asteroids in its orbit at its two Lagrange points. They are called trojan asteroids. So, this means that Jupiter fails the IAU's third criterion and should be a dwarf planet, which it is certainly not!