In 2006, the International Astronomical Union changed Pluto's classification from a planet to a dwarf planet because the planet has not cleared its orbit of other objects. When a planet clears its orbit, all other objects on the orbital path are either drawn into the planet's gravitational pull or flung away from it and out of the orbit.
The change in classification came after the discovery of the dwarf planet Eris in 2005. Eris is more massive and Pluto and orbits the sun, so its discovery challenged the notion of a nine-planet solar system. To account for the discovery of Eris and the subsequent discovery of other bodies, the International Astronomical Union created the dwarf planet classification. By definition, Pluto best matches the characteristics of a dwarf planet and was reclassified as one. Pluto could potentially clear its orbit in the future, most likely by colliding with other objects in its orbit and combining with them, which would give it back its planet classification. As of 2014, the other named dwarf planets besides Pluto and Eris are Ceres, Makemake and Haumea. According to NASA, there may be over 100 as-yet undiscovered dwarf planets in the solar system, which covers objects with sufficient mass to achieve a round shape and that orbit the sun.