It was not. It is not a planet. It's a small ball of rock in another belt along the outer ring of the solar system called the kupier belt. Also it really doesn't make sense for it to be a planet. It's smaller than even the moon.
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Pluto is now technically a planet again. The IAU hasn't officially recognized the decision yet, but the head of their planet definition committee and the director of Harvard's Origins of Life Initiative won a debate against the associate director of the IAU's Minor Planet Center on September 18, 2014. The audience also voted and agreed that Pluto should be a planet.
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The first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens or even more than 100 dwarf planets awaiting discovery. The IAU recognized Pluto's special place in our solar system by designating dwarf planets that orbit the sun beyond Neptune as plutoids.