Twenty years ago, Pluto was demoted. Following a vote by the International Astronomical Union (IAU) in August 2006, Pluto was relegated from major planet status to dwarf planet status. Our solar system went from nine planets to eight. At the time, many scientists were not happy with the decision. Two decades later, the debate is, if anything, even more polarized. “The definition of a planet they created is not usable at all,” says Philip Metzger, director of the Stephen W. Hawking Center for Microgravity Research and Education at the University of Central Florida. “This makes the definition of a planet completely useless in science.” Metzger is one of several scientists and laypeople who consider Pluto’s demotion a big mistake. For some, the degradation of their status is the source of their affection for the former planet. “Pluto represents the underdog that so many people support, the little kid on the block who is being made fun of,” says Kevin Schindler, a historian at Lowell Observatory in Arizona, where Pluto was discovered in 1930. For others, the IAU decision, and the way it was made, is central to the debate. “My involvement in defending Pluto began on voting day,” says writer, actress and amateur astronomer Laurel Kornfeld. “I thought, wait a minute, this is wrong.” When we meet over Zoom, she is wearing a T-shirt that says: “Pluto Endurance, 20 Years: 2006-2026.” Space oddities… dwarf planets (from left) Ceres, Pluto and Eris. Photograph: Stocktrek Images/Alamy The campaign to reinstate Pluto – with its passionate advocates, ongoing debates, occasional protests and petitions – appears to be largely limited to the United States. “It’s more of a problem in the United States,” Metzger says. “This was the one we discovered.” So was the degradation of the planet a source of wounded pride? “I admit there was some of that, among the public, not among scientists.” More recently, the campaign has taken on a populist overtone, with Trump’s NASA administrator Jared Isaacman telling a congressional committee in April that he favored a rethink. “I’m very much in favor of making Pluto a planet again,” he said. The event that sparked the IAU vote in 2006 was the discovery of a distant celestial body that would come to be called Eris. Eris was in the Kuiper belt, beyond Neptune, where Pluto also resides. It was about the same size as Pluto (less volume, but more mass). If Pluto was a planet, so the argument went, so was Eris. And if Eris were not a planet, how could Pluto still be one? Philip Metzger, director of the Stephen W. Hawking Center for Microgravity Research and Education. Photo: Florida Space Institute Eris and Pluto were not alone in the Kuiper Belt. Around the same time, other smaller bodies were discovered, including Haumea, Makemake and Sedna. Were these also planets? There were probably many more similar objects still to be found. How would schoolchildren handle a planetary list that could number in the hundreds? In search of 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 adopt a round shape; and it must have “cleared its neighborhood,” meaning it must gravitationally dominate the area in which it orbits. Nothing beyond Neptune that we knew qualified; Pluto was out. It was the last point in particular that bothered many Pluto advocates, because a body’s ability to clear its neighborhood depends a lot on the neighborhood. “One of the biggest flaws with the IAU definition is that you can have the same object in two different orbits,” Kornfeld says. “In one it is considered a planet and in the other it is not considered a planet.” If Earth were dragged into Pluto’s position, planetary scientist Alan Stern argues, it would also be unable to clear its zone and would no longer meet the IAU definition of a planet. “We could even have small asteroids, too small to be round, clearing an orbit if they are close to a star,” Metzger says. “It just goes to show that orbit cleanup isn’t really that significant of a category.” To its detractors, the IAU definition seemed crafted with a specific intent. “A reason was invented to exclude Pluto,” Kornfeld says. “A reason was invented to exclude Pluto”… the IAU vote on the resolution for the definition of the planet in 2006. Photograph: Michal Čížek/AFP/Getty Images The demotion of Pluto created two opposing scientific teams: the so-called geophysical camp, which believes that the physical properties of a celestial body should count for its planet status; and dynamic ones, which are more concerned with the position a body occupies, its environment, and its direction of travel. One could characterize the divide as planetary scientists versus astronomers. Many planetary scientists, including Metzger, simply do not use the IAU definition. When the New Horizons spacecraft flew by Pluto in 2015, the geophysical field took advantage of what it considered an advantage. They maintained that Pluto definitely had the geophysical characteristics of a planet. It had mountains, evidence of recent geological activity, and hills of water ice floating on frozen nitrogen. It might even be able to sustain life. How could it not be a planet? Those who agitate Pluto’s planetary status generally do not recognize any problems arising from its reinstatement. They welcome a definition broad enough to cover hundreds of new planets, however exhausting that may be for children 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 species of Pokémon, and I’ve never heard a child say that’s too many.” Those against the reinstatement of Pluto say that the problem is not with a greater number of planets, per se, but with a primarily geophysical definition that would encompass, among other bodies, planetary moons, including our moon. Underdog story… the surface of Pluto photographed from NASA’s New Horizons spacecraft. Photograph: AFP/Getty Images “I have no problem with having a higher number of planets,” says astronomer Mike Brown by email. “My argument is simply that we decided a long time ago that the Moon is not a planet and that also excludes Pluto.” Brown is often described as The Man Who Killed Pluto, because he led the team that discovered Eris in 2005, setting all deplanetization in motion. It’s a title he leans toward: he even wrote a book called How I Killed Pluto and Why I Planned It. Brown often says that the moon is larger than Pluto; It is round and has a complex geophysical history, and would certainly meet the criteria for being a planet if any orbit-related requirements were excluded. What he is saying is: if Pluto is a planet, so is the Moon. “I have no problem calling the Moon a planet,” Byrne says. “Because I can be a brother and a husband at the same time, and it doesn’t require me to be one or the other. And that’s why I don’t see why the Moon can’t be a satellite and a planet.” “You could argue that they never officially voted on a planet because they didn’t follow the required procedure,” Metzger says. Brown denies there was anything political in the decision. “The only ones who complained about the voting procedure were those who didn’t like the result.” So the question remains: why does any of this matter? “It was almost like a coup,” Kornfeld says. “And as a result, children learn less. They are learning eight planets, and that’s it.” Cosmic controversy… astronomer Gerard P. Kuiper claimed that Pluto is actually one of Neptune’s satellites. Photo: Bettmann Archive For Metzger, the planetary definition that excludes Pluto is based on an outdated notion of the solar system. “They were creating a reactionary view towards an ordered cosmos, rather than the more modern scientific view where it is chaotic,” he says. “The second problem was that I think they misread the room. Today’s audiences are not the audiences of the 19th century. They are perfectly happy understanding a dynamic, chaotic cosmos, and we missed the opportunity to teach that.” In 1915, astronomer Percival Lowell published calculations based on peculiarities of the orbits of Uranus and Neptune, predicting the existence of a new ninth planet, which he called Planet X, with a mass between that of the Earth and that of Neptune. In 1930, the long search for Planet However, it gradually became clear that whatever Pluto was, it was not Planet X. It did not seem large enough to explain the irregularities in Neptune’s orbit. Scientists offered theories to explain why Pluto seemed so small, but the truth was that it was small, even smaller than the Moon. Only in 1992 did we learn that Lowell’s calculations were based on an overestimation of Neptune’s mass. There was no Planet How would they have voted on Pluto’s planetary status if they had known the truth in 1930? And will another vote be necessary to satisfy Pluto’s defenders? “People have rallied,” Metzger says. “We would have a less useful vote today than in 2006. 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 it again. We’re going to go back to the scientific method.”
