Published : 01 Sep 2026, 11:41 PM
Updated : 08 Sep 2026, 09:37 AM
Twenty years after Pluto was stripped of its status as a planet, scientists are still arguing over whether the decision was right -- and a growing number say it was not.
In August 2006, the International Astronomical Union (IAU) voted to demote Pluto from major planet to dwarf planet, shrinking the solar system's official planetary count from nine to eight.
The trigger was the discovery of Eris, a distant body in the Kuiper Belt roughly the same size as Pluto, reports The Guardian.
If Pluto was a planet, astronomers reasoned, so was Eris -- and potentially hundreds of similar objects yet to be found.
The IAU settled on a three-part definition: a planet must orbit the Sun, pull itself into a round shape, and "clear its neighbourhood" of other objects. Pluto failed on the third count.
Two decades on, that definition remains deeply contested.
"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."
Critics of the ruling argue that a body's ability to clear its neighbourhood depends heavily on where it orbits. Planetary scientist Alan Stern has pointed out that if Earth were moved to Pluto's position, it too would fail the IAU test.
When NASA's New Horizons spacecraft flew past Pluto in 2015, it revealed mountains, evidence of recent geological activity, and water ice hills floating on frozen nitrogen -- fuelling the argument that Pluto has all the hallmarks of a planet regardless of its orbital neighbourhood.
The campaign to reinstate Pluto has recently gained an unlikely ally.
NASA Administrator Jared Isaacman, appointed under President Donald Trump, told a congressional committee in April that he supported a rethink. "I am very much in the camp of Make Pluto a Planet Again," he said.
Not everyone is convinced. Astronomer Mike Brown, whose team's discovery of Eris set the demotion in motion, argues the case is simple: the Moon is larger than Pluto, round, and geologically complex -- and no one calls it a planet.
"If Pluto's a planet, so is the moon," he says.
For now, neither side shows any sign of backing down. Metzger says another IAU vote is not the answer.
"What we want is for people to recognise that voting was a mistake, and we're not going to do that any more. We're going to go back to the scientific method."
