Pluto was discovered by Clyde Tombaugh in 1930. It is not considered planet anymore because in 2006, Astronomers of IAU gave a new definition to the word world 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.
A planet should have removed asteroids and dwarf planets, out of its path. Now Pluto is in orbit around the sun and spherical in shape. But since Pluto neighborhood has any other in objects and it has not cleared in its neighborhood it is not a planet anymore.
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