Pluto is not considered a planet because it does not meet all of the three criteria that the International Astronomical Union (IAU):
1. It is in orbit around the Sun.
2. Due to its sufficient mass, it can overcome rigid body forces and achieve a hydrostatic equilibrium (nearly round) shape.
3. It has "cleared the neighborhood" around its orbit.
Pluto and many other objects in the solar system are now classified as dwarf planets.
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