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Pluto was discovered by Clyde Tombaugh in 1930. It is not considered a planet anymore because in 2006, astronomers of IAU (International Astronomical Union) gave a new definition to the word 'planet'.

According to the new definition, a planet is a celestial body which is in orbit around the sun, is spherical in shape and it has cleared the neighborhood around its orbit.

What does this mean?

It basically means that using its gravity, a planet should have removed asteroids and dwarf planets, out of its path, thus cleared its neighborhood.

Now, Pluto is in orbit around the sun and is spherical in shape. But since Pluto's neighborhood has many other objects and it has not cleared its neighborhood. It is not called a planet anymore.