Pluto was declared a dwarf planet by the International Astronomical Union (IAU) in 2006. The IAU is an internationally recognized organization that is responsible for classifying celestial objects and defining various astronomical terms and concepts.
The decision to reclassify Pluto was made during the IAU General Assembly in Prague, Czech Republic, on August 24, 2006. At that time, the IAU revised its definition of a planet and introduced the new category of "dwarf planet." According to the new definition, a planet is defined as a celestial body that orbits the Sun, is spherical in shape, and has cleared its orbit of other debris. Since Pluto shares its orbit with other objects in the Kuiper Belt, it did not meet the criteria to be classified as a planet and was designated as a dwarf planet instead.