The debate over whether Pluto should again be called a planet has resurfaced 20 years after the International Astronomical Union reclassified it as a dwarf planet in August 2006. The IAU decision followed growing questions about Pluto’s status, especially after discoveries of similar distant objects beyond Neptune and the July 2005 discovery of Eris, which is comparable in size to Pluto and farther from the Sun.

The IAU established three conditions for planet status: an object must be nearly round through its own gravity, orbit the Sun, and gravitationally clear its orbital neighbourhood. Pluto meets the first two but not the third, because it shares its region with many Kuiper Belt objects and has an orbit linked with Neptune. Critics argue that the clearing requirement contains ambiguities and has shifted public discussion away from Pluto’s scientific features.

Washington University in St. Louis planetary astronomer Paul Byrne says Pluto, Ceres, Eris, Makemake and Haumea should be considered planets. He favors classification based on an object’s physical and geological nature, while acknowledging that any reversal should rest on scientific grounds rather than nostalgia or nationalism.