Pluto’s planetary demotion by the International Astronomical Union (IAU) in August 2006 sparked a scientific and cultural debate that remains polarized two decades later. While the IAU relegated the celestial body to dwarf planet status due to its failure to clear its neighborhood, a vocal faction of researchers and space advocates continue pushing for reinstatement.
The 2006 IAU Vote and the Discovery of Eris
The event that sparked the IAU’s August 2006 vote was the discovery of a distant celestial body named Eris, located in the Kuiper Belt beyond Neptune, according to historical accounts of the decision. Eris was roughly the same size as Pluto, possessing less volume but more mass. Astronomers argued that if Pluto was a planet, Eris had to be one too, prompting the IAU to establish a strict three-point planetary definition. According to the IAU resolution, a planet must orbit the Sun, have sufficient mass to pull itself into a round shape, and have “cleared its neighbourhood” gravitationally. Pluto failed the final criterion, dropping our solar system from nine planets to eight.
“The definition of a planet they created, you can’t use it at all,” says Philip Metzger, director of the Stephen W Hawking Centre for Microgravity Research and Education at the University of Central Florida. “It makes the definition of a planet completely non-useful in science.”
Geophysical Camp Versus Dynamicists
The 2006 demotion created two opposing scientific factions: the geophysical camp and the dynamicists, according to researchers in the field. Planetary scientists in the geophysical camp maintain that physical properties should dictate planethood, pointing to data from the 2015 New Horizons spacecraft flyby that revealed mountains, water ice hills, and geological activity on Pluto. Conversely, dynamicists like Mike Brown—who led the team that discovered Eris in 2005 and wrote the book How I Killed Pluto and Why It Had It Coming—argue that orbital characteristics remain the defining metric. According to Brown, accepting Pluto as a planet based solely on roundness would also force science to classify Earth’s moon as a planet.
Did you know?
Mike Brown is frequently described as “The Man Who Killed Pluto” following his 2005 discovery of Eris, a celestial body in the Kuiper Belt that triggered the International Astronomical Union’s reevaluation of planetary criteria.
Populist Pushback and Political Advocacy
Opposition to the IAU ruling extends far beyond academic circles, drawing involvement from historians, writers, and public figures. Kevin Schindler, historian at the Lowell Observatory in Arizona where Pluto was discovered in 1930, notes that Pluto functions as an enduring underdog symbol for the public. Writer and amateur astronomer Laurel Kornfeld has campaigned against the decision since the day of the vote, arguing that the IAU procedure was flawed. More recently, the movement has attracted populist support; according to congressional testimony, Trump’s NASA administrator Jared Isaacman told a congressional committee in April that he supports a rethink, stating he is in the camp of “Make Pluto a Planet Again.”
Frequently Asked Questions
Why was Pluto demoted from being a planet?
Pluto was reclassified as a dwarf planet because it failed to meet the International Astronomical Union’s 2006 criterion to “clear its neighbourhood” of other orbiting bodies in the Kuiper Belt.
Who discovered Pluto and when?
Pluto was discovered in 1930 at the Lowell Observatory in Arizona, following orbital calculations initially sparked by astronomer Percival Lowell’s search for a hypothetical Planet X.
Do all scientists agree with Pluto’s demotion?
No. Many planetary scientists, including Philip Metzger and Paul Byrne, reject the IAU definition, arguing that a geophysical definition based on a body’s physical properties is more scientifically useful.
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