Science

Twenty years after Pluto's demotion, scientists remain split on its planetary status

Proponents of the geophysical definition argue for reinstatement, while astronomers maintain that Pluto does not meet orbital criteria.

Updated Aug 31, 2026 · 04:18 UTC1 reports

The short version

The debate over Pluto's planetary status remains highly polarized 20 years after the International Astronomical Union (IAU) voted to reclassify it as a dwarf planet in August 2006.
Advocates of the "geophysical" camp argue that Pluto's complex physical properties, highlighted by the 2015 New Horizons flyby, justify its status as a planet.
Proponents of the "dynamical" classification defend the 2006 decision, warning that removing the orbital clearing requirement would classify moons and numerous small celestial bodies as planets.

In August 2006, the International Astronomical Union voted to relegate Pluto from a major planet to a dwarf planet, reducing the solar system's planet count from nine to eight.[The Guardian]
The IAU definition of a planet requires a celestial body to orbit the Sun, have sufficient mass to be nearly round, and have cleared the neighborhood around its orbit.[The Guardian]
The demotion was prompted by the discovery of Eris, a Kuiper Belt object of similar size to Pluto, which raised questions about whether many other newly discovered bodies should also be designated as planets.[The Guardian]
The 2015 New Horizons flyby of Pluto revealed mountains, active geology, and water ice hills, which proponents of Pluto's planethood cite as physical proof of its planetary nature.[The Guardian]
Jared Isaacman, a NASA administrator, expressed support for revisiting Pluto's planetary status during a congressional committee appearance in April.[The Guardian]

Critics of the IAU decision argue that the 2006 vote was procedurally flawed, a claim that other astronomers dispute, attributing the procedural complaints to dissatisfaction with the final outcome.[The Guardian]