Why is Pluto not a planet? Pluto, the former ninth planet in our solar system, was reclassified as a "dwarf planet" in 2006 by the International Astronomical Union (IAU). This decision was based on new scientific findings that revealed Pluto did not meet the criteria for a planet. One of the key factors in this decision was Pluto's size. While it was once believed to be a sizable planet, scientists have since discovered that it is much smaller than originally thought. In fact, it is smaller than