The International Astronomical Union (IAU) voted in August 2006 to reclassify Pluto from a major planet to a dwarf planet, reducing the number of major planets in our solar system from nine to eight. This decision, made twenty years ago, continues to be a source of contention among scientists and the public.
Philip Metzger, director of the Stephen W Hawking Centre for Microgravity Research and Education at the University of Central Florida, stated that the IAU's definition of a planet is "completely non-useful in science." He is among those who view Pluto's demotion as a significant error.
The IAU's definition requires a planet to orbit the Sun, have sufficient mass to be round, and have "cleared its neighbourhood" gravitationally. Critics, including writer and amateur astronomer Laurel Kornfeld, argue this last point is flawed, suggesting an object's planetary status could change based on its orbit.
The debate has created two main scientific camps: the geophysical camp, which focuses on a celestial body's physical properties for planethood, and the dynamicists, who prioritise a body's position and surroundings. Some planetary scientists, such as Metzger, reportedly do not use the IAU definition.