Twenty years ago, Pluto was demoted. In August 2006, the International Astronomical Union (IAU) voted to change Pluto’s status from a major planet to a dwarf planet, shrinking our solar system from nine planets to eight. At the time, many scientists were unhappy with the decision. Two decades later, the debate is even more divided.
“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 useless in science.”
Metzger is one of several scientists and non-experts who believe Pluto’s demotion was a big mistake. For some, Pluto’s downgraded status is actually why they love the former planet. “Pluto represents the underdog that so many people root for, the small kid on the block that is being picked on,” says Kevin Schindler, a historian at the Lowell Observatory in Arizona, where Pluto was discovered in 1930.
For others, the IAU’s decision—and how it was made—is at the heart of the debate. “My involvement with advocating for Pluto started on the day of the vote,” says writer, actor, and amateur astronomer Laurel Kornfeld. “I thought: wait a minute, this is wrong.” When we meet on Zoom, she’s wearing a T-shirt that says: “Pluto Resistance, 20 years: 2006-2026.”
The campaign to reinstate Pluto—with its passionate supporters, ongoing debates, occasional protests, and petitions—seems to be mostly limited to the US. “It’s more of an issue in the United States,” says Metzger. “This was the one we discovered.” So, was the demotion a blow to national pride? “I’ll admit there was some of that—among the public, not the scientists.”
Recently, the campaign has taken on a populist tone. In April, Trump’s NASA administrator, Jared Isaacman, told a congressional committee he supported a rethink. “I am very much in the camp of Make Pluto a Planet Again,” he said.
What sparked the IAU’s 2006 vote was the discovery of a distant celestial body that would later be named Eris. Eris was in the Kuiper Belt, beyond Neptune—where Pluto also sits. It was roughly the same size as Pluto (less volume, but more mass). If Pluto was a planet, the argument went, then so was Eris. And if Eris wasn’t a planet, how could Pluto still be one?
Eris and Pluto were not alone in the Kuiper Belt. Other smaller bodies were found around the same time, including Haumea, Makemake, and Sedna. Were these also planets? There were likely many more similar objects waiting to be discovered. How would schoolchildren handle a planetary list that might number in the hundreds?
Looking for a solution, the IAU came up with a three-point definition of a planet: it must orbit the Sun; it must have enough mass to pull itself into a round shape; and it must have “cleared its neighborhood”—meaning it must gravitationally dominate the area it orbits. Nothing beyond Neptune that we knew of qualified; Pluto was out.
It was the last point that especially upset many Pluto supporters, because a body’s ability to clear its neighborhood depends a lot on the neighborhood itself. “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.” If Earth were moved out to Pluto’s position, planetary scientist Alan Stern argued, it wouldIt wouldn’t be able to clear its zone either, and so it would no longer meet the IAU’s definition of a planet.
“You could even have small asteroids, too small to be round, clearing an orbit if they’re close to a star,” says Metzger. “That just shows that orbit clearing isn’t really a meaningful category.”
To its critics, the IAU definition seemed designed with a specific goal in mind. “A reason was invented to exclude Pluto,” says Kornfeld.
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“A reason was invented to exclude Pluto” … the IAU vote on the planet definition in 2006. Photograph: Michal Čížek/AFP/Getty Images
Pluto’s demotion created two opposing scientific camps: the so-called geophysical camp, which argues that a celestial body’s physical traits should determine whether it’s a planet, and the dynamicists, who focus more on where a body sits, what surrounds it, and how it moves. You could sum up the split as planetary scientists versus astronomers. Many planetary scientists, including Metzger, simply don’t use the IAU definition.
When the New Horizons spacecraft flew past Pluto in 2015, the geophysical camp saw a chance to press its case. Pluto clearly had the geophysical features of a planet, they argued. It had mountains, signs of recent geological activity, and water-ice hills floating on frozen nitrogen. It might even be able to support life. How could that not be a planet?
Those pushing for Pluto’s reinstatement generally don’t see any downside to giving it back its planetary status. They welcome a definition broad enough to include hundreds of new planets, even if that makes things harder for kids and their wall charts. “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.”
Those against Pluto’s reinstatement say the issue isn’t the larger number of planets itself, but a mostly geophysical definition that would also include planetary moons, like our own.
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Underdog story … the surface of Pluto pictured from Nasa’s New Horizons spacecraft. Photograph: AFP/Getty Images
“I have no problem with having a larger number of planets,” says astronomer Mike Brown, via email. “My argument is simply that we decided long ago that the moon is not a planet, and that also rules out Pluto.” Brown is often called The Man Who Killed Pluto, because he led the team that discovered Eris in 2005, which set the whole de-planetisation in motion. It’s a title he embraces: he even wrote a book called How I Killed Pluto and Why It Had It Coming.
Brown often points out that the moon is bigger than Pluto; it’s round and has a complex geophysical history, and would easily meet the criteria for a planet if you dropped any requirement about orbit. His point is: if Pluto’s a planet, so is the moon.
“I don’t have an issue with calling the moon a planet,” says Byrne. “Because I can be a brother and a husband at the same time, and it doesn’t require that I be one or the other. So I don’t see why the moon can’t be both a satellite and a planet.”
To an outsider, this debate can often seem stuck on political or procedural questions, with Pluto supporters arguing that the IAU vote was carried out incorrectly and is therefore invalid. “You could argue they never officially voted on a planet because they didn’t follow the required procedure,” says Metzger.
Brown denies there was anything political about the decision. “The only people who complain about the voting procedure are the ones who didn’t like the outcome.”
So the question remains: why does any of this matter?
“It was almostIt was like a coup,” says Kornfeld. “And as a result, kids are learning less. They’re learning eight planets, and that’s it.”
Cosmic controversy … astronomer Gerard P Kuiper claimed that Pluto is actually one of Neptune’s satellites. Photograph: Bettmann Archive
For Metzger, the planetary definition that excludes Pluto is based on an outdated view of the solar system. “They were pushing a reactionary idea of an orderly cosmos, rather than the more modern scientific view where it’s chaotic,” he says. “The second problem was that I think they misread the public. Today’s public isn’t the public of the 1800s. They’re perfectly fine with understanding a dynamic, chaotic cosmos, and we missed the chance to teach that.”
Pluto’s nature has always been hard to pin down. In 1915, astronomer Percival Lowell published calculations based on oddities in the orbits of Uranus and Neptune, predicting a new, ninth planet, which he called Planet X, with a mass somewhere between Earth’s and Neptune’s. In 1930, the long search for Planet X paid off: the new object, found at the observatory bearing Lowell’s name, was named Pluto.
Over time, though, it became clear that whatever Pluto was, it wasn’t Planet X. It didn’t seem nearly large enough to explain the irregularities in Neptune’s orbit. Scientists came up with theories to account for why Pluto looked so small, but the truth was, it was small—even smaller than the moon. Only in 1992 did we learn that Lowell’s calculations were based on an overestimate of Neptune’s mass. There was no Planet X.
Pluto’s reputation as the underdog, the picked-on kid, may come from the 2006 vote, but you could argue it was punching above its weight for years. How would they have voted on Pluto’s planetary status if they had known the truth in 1930? And will it take another vote to satisfy Pluto’s defenders?
“People have locked in,” says Metzger. “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 recognize that voting was a mistake, and we’re not going to do that anymore. We’re going to go back to the scientific method.”
Frequently Asked Questions
Here is a list of FAQs about the Pluto debate written in a natural and informative tone
Beginner Questions
1 Wait why isnt Pluto a planet anymore
In 2006 the International Astronomical Union created a formal definition for a planet To be a planet an object must meet three criteria it must orbit the Sun it must be round due to its own gravity and it must have cleared its neighborhood of other debris Pluto orbits the Sun and is round but it fails the third test because its orbit is full of other objects in the Kuiper Belt
2 What exactly is a dwarf planet
A dwarf planet is a celestial body that orbits the Sun is round but hasnt cleared its orbital neighborhood So its like a planet in many ways but it shares its space with lots of other rocky and icy bodies Pluto is the most famous dwarf planet but there are others like Eris and Ceres
3 Is Pluto still there Did it disappear
Haha no Pluto is definitely still there Its just a matter of classification Think of it like renaming a file on your computerthe file still exists it just has a different label now Pluto is still a fascinating icy world at the edge of our solar system
4 So was the demotion a huge mistake
Thats the big debate Many scientists argue it was a necessary correction to our understanding of the solar system Others including some planetary scientists think the definition is flawed and too narrowly focused and that it was a mistake to demote a beloved world like Pluto
The Science The Debate
5 Whats the biggest argument for saying the demotion was a mistake
The main argument is that the clearing the neighborhood rule is too vague and doesnt apply to other planets For example Earth shares its orbit with asteroids and Jupiter has the Trojan asteroids By that strict rule Earth and Jupiter might not clear their zones either Critics say the rule is arbitrary and doesnt reflect the complex geology of a world like Pluto which has mountains glaciers and an atmosphere
6 Whats the strongest argument for saying the demotion was correct
The demotion was correct because it finally gave astronomers a clear scientific definition of
