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

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.

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!

Pluto was reclassified as a dwarf planet because the International Astronomical Union (IAU) redefined what constitutes a planet in 2006. According to the new definition, a planet must clear its orbit of other debris, which Pluto does not do. Therefore, it was reclassified as a dwarf planet.