Two decades ago, Pluto got the celestial equivalent of being unceremoniously kicked out of the cool kids' table. In August 2006, the International Astronomical Union (IAU) voted to demote our favorite underdog from major planet to dwarf planet, shrinking the solar system's planetary roster from nine to eight. And if you think people have gotten over it, you clearly haven't met the Pluto Resistance, which has been gearing up for its 20th anniversary like it's a grudge match.
Philip Metzger, director of the Stephen W Hawking Centre for Microgravity Research and Education at the University of Central Florida, is not subtle about his feelings: "The definition of a planet they created, you can't use it at all. It makes the definition of a planet completely non-useful in science." Metzger is among a growing chorus of scientists and laypeople who think the demotion was a cosmic mistake.
For some, Pluto's downgrade is precisely why they love it. "Pluto represents the underdog that so many people root for, the small kid on the block that is being picked on," says Kevin Schindler, historian at the Lowell Observatory in Arizona, where Pluto was discovered back in 1930. For others, it's the process that stings. Laurel Kornfeld, a writer, actor, and amateur astronomer, was so incensed on the day of the vote that she's been advocating for Pluto ever since, wearing a T-shirt that declares "Pluto Resistance, 20 years: 2006-2026."
The campaign to reinstate Pluto - complete with passionate advocates, ongoing debates, occasional protests, and petitions - seems to be a uniquely American phenomenon. Metzger admits as much: "It is more of an issue in the United States. This was the one we discovered." So, the demotion was a wound to national pride? "I'll admit there was some of that - among the public, not the scientists."
Recently, the movement has even picked up a populist tinge, with Trump's NASA administrator, Jared Isaacman, telling a congressional committee in April that he's all for a rethink. "I am very much in the camp of Make Pluto a Planet Again," he said, proving that even the cosmos isn't safe from campaign slogans.
The whole mess started with the discovery of Eris, a distant body in the Kuiper Belt beyond Neptune - Pluto's neighborhood. Eris was roughly Pluto's size (less volume, but more mass), and suddenly the question became: if Pluto's a planet, why isn't Eris? And if Eris isn't, how can Pluto be? The Kuiper Belt turned out to be a crowded place, with other bodies like Haumea, Makemake, and Sedna popping up. Were they planets too? Schoolchildren could end up memorizing hundreds of planets, and no one wants that.
So the IAU crafted a three-point definition: a planet must orbit the Sun, be round, and have "cleared its neighbourhood" - meaning it must gravitationally dominate its orbit. Pluto, being a small fry in a busy neighborhood, failed the third test. Cue the outrage.
Critics point out that the "cleared its neighbourhood" criterion is deeply flawed. "One of the biggest flaws with the IAU definition is that you can have the same object in two different orbits," says Kornfeld. "In one, it's considered a planet, and in the other, it's not considered a planet." Planetary scientist Alan Stern famously argued that if Earth were towed out to Pluto's position, it wouldn't clear its zone either and would lose its planetary status. Metzger adds, "You could even have small asteroids, that are too small to be round, clearing an orbit if they're close to a star. It just goes to show that orbit clearing isn't really that meaningful of a category."
To its detractors, the definition seemed tailor-made to exclude Pluto. "A reason was invented to exclude Pluto," says Kornfeld.
The demotion split the scientific community into two camps: the geophysical camp (physical properties matter) and the dynamicists (position and surroundings matter). It's basically planetary scientists versus astronomers, and many planetary scientists, including Metzger, simply refuse to use the IAU definition.
When NASA's New Horizons spacecraft flew by Pluto in 2015, the geophysical camp felt vindicated. Pluto had mountains, evidence of recent geological activity, and water ice hills floating on frozen nitrogen. It might even support life. How could that not be a planet?
Those pushing for Pluto's reinstatement are unfazed by the prospect of hundreds of new planets. "The more the merrier," says Paul Byrne, associate professor of earth, environmental and planetary sciences at Washington University in St Louis. "There are 1,025 official Pokémon species, and I've never heard a kid say that's too many." A compelling argument, honestly.
Opponents, like astronomer Mike Brown - the self-proclaimed "Man Who Killed Pluto" for discovering Eris - argue that a purely geophysical definition would include moons, including our own. "I have no problem with having a larger number of planets," Brown says. "My argument is simply that we decided long ago that the moon is not a planet and that also precludes Pluto." He wrote a book called How I Killed Pluto and Why It Had It Coming, which is either a confession or a brag.
Byrne is fine with calling the moon a planet: "I can be a brother and a husband at the same time, and it doesn't require that I be one or the other. And so I don't see why the moon can't be a satellite and a planet." Try explaining that at your next dinner party.
The debate also wades into procedural weeds, with Plutonians claiming the IAU vote was improperly conducted and therefore void. "You could make the argument that they never officially voted on a planet because they didn't follow the procedure that was required," says Metzger. Brown dismisses this: "The only people who complain about the voting procedure were the ones who didn't like the outcome."
So why does any of this matter? "It was almost like a coup," says Kornfeld. "And as a result, kids are learning less. They're learning eight planets, and that's it." Metzger sees it as a missed opportunity to teach the public about a dynamic, chaotic cosmos instead of an orderly one. "They were creating a reactionary view towards an orderly cosmos, rather than the more modern scientific view where it's chaotic," he says. "The public of today is not the public of the 1800s. They're perfectly happy understanding a dynamic, chaotic cosmos, and we missed the opportunity to teach that."
Pluto's history is full of mistaken identity. In 1915, Percival Lowell predicted a ninth planet, Planet X, based on oddities in Uranus and Neptune's orbits. In 1930, Pluto was found at Lowell Observatory, but it was way too small to be Planet X. Turns out, Lowell's calculations were based on an overestimation of Neptune's mass. There was no Planet X, just our favorite underdog, punching above its weight for decades.
Would Pluto have been a planet in 1930 if they'd known the truth? And will it take another vote to satisfy its defenders? Metzger thinks another vote would be even less useful: "People have locked in. We would have a less useful vote today than we did back in the 2006 timeframe. So we don't want another vote. 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." Good luck with that.