Twenty years on, Pluto's demotion remains deeply polarizing among scientists

Twenty years have passed since the International Astronomical Union voted to strip Pluto of its planetary status, and the wound has not healed. In August 2006, the IAU's decision reduced our solar system from nine planets to eight with a single vote. What was meant to settle a growing problem—the discovery of increasingly similar objects beyond Neptune—instead created a schism that has only widened with time.

The catalyst was Eris, a distant body in the Kuiper Belt roughly the same size as Pluto but with greater mass. If Pluto qualified as a planet, the logic went, then so did Eris. And if not Eris, then what about Haumea, Makemake, and Sedna? Astronomers faced the prospect of a planetary roster that might eventually number in the hundreds. The IAU responded with a three-part definition: a planet must orbit the Sun, possess enough mass to be round, and gravitationally dominate its orbital neighborhood. Pluto failed the third test.

But the definition itself became the problem. Philip Metzger, director of the Stephen W Hawking Centre for Microgravity Research and Education at the University of Central Florida, argues the criterion is scientifically useless. A body's ability to clear its neighborhood depends entirely on where it orbits—the same object could be a planet in one orbit and not in another. If Earth were moved to Pluto's position, planetary scientist Alan Stern has pointed out, it would fail the IAU test too. The rule, critics contend, was invented specifically to exclude Pluto rather than derived from sound scientific principle.

Two camps emerged. The geophysical school, largely composed of planetary scientists, believes a body's physical properties—mountains, geological activity, water ice—should determine its status. The dynamicists, more aligned with traditional astronomy, prioritize orbital mechanics and position. Many planetary scientists, including Metzger, simply ignore the IAU definition in their work. When the New Horizons spacecraft flew past Pluto in 2015, revealing a geologically complex world with evidence of recent activity, the geophysical camp saw vindication. How could something so dynamic not be a planet?

The reinstatement campaign has taken on unexpected dimensions. It began as scientific objection but evolved into something more populist. Laurel Kornfeld, a writer and amateur astronomer, began advocating for Pluto on the day of the 2006 vote, eventually wearing a T-shirt declaring "Pluto Resistance, 20 years: 2006-2026." Kevin Schindler, a historian at the Lowell Observatory in Arizona where Pluto was discovered in 1930, frames it differently: Pluto represents the underdog, the small kid being picked on. The campaign has remained largely confined to the United States, where wounded national pride plays a role—this was the planet America discovered. In April 2026, Jared Isaacman, NASA's administrator under Trump, told Congress he favored reconsidering Pluto's status, declaring himself in the camp of "Make Pluto a Planet Again."

Opponents worry that a geophysical definition would classify Earth's moon as a planet. The moon is larger than Pluto, round, and geologically complex. Mike Brown, the astronomer who discovered Eris and is often called "the man who killed Pluto," argues that if you exclude orbital requirements, the moon would qualify. Paul Byrne, an associate professor at Washington University in St Louis, counters that an object can hold multiple classifications simultaneously—the moon can be both satellite and planet, just as a person can be both brother and husband. Brown denies the 2006 vote was political, noting that only those unhappy with the outcome complained about procedure.

Metzger and others reject the idea of another vote. They argue the 2006 decision reflected an outdated, reactionary view of an orderly cosmos rather than the modern scientific understanding of a dynamic, chaotic solar system. The public, Metzger suggests, is ready for that complexity. Instead of voting again, he says, scientists should acknowledge the mistake and return to the scientific method. Pluto's history adds irony to the debate: it was discovered in 1930 based on calculations by Percival Lowell predicting a massive ninth planet. Only decades later did scientists realize Pluto was far too small to account for the orbital irregularities Lowell had observed, and only in 1992 did they learn his mass estimates for Neptune were wrong. Pluto had been misclassified from the start, punching above its weight for seventy-six years. Whether another reclassification will come remains uncertain, but the advocates show no sign of surrender.