Updated at 4:15 p.m. ET on August 27, 2021.
In 2006, astronomers gathered in Prague to consider a very basic question: How many planets are in our solar system? Was it nine, or was it actually eight, or perhaps as many as 12? By the end of the conference, after several polite debates and “lots of heated hallway discussions,” the verdict was in. Under the new rules of planethood, the solar system had eight planets, and Pluto wasn’t one of them.
The wider public doesn’t usually get riled up about the solar system, but this decision was quite shocking. For many Americans, the names of planets were some of the earliest scientific facts we learned, and that there were nine of them seemed like a basic truth of existence. Despite the millions of miles that separate us, the planets feel close to home, and the news that one of them had been kicked out of the group felt a bit destabilizing.
It didn’t help that the decision to give Pluto a new designation—dwarf planet—was described in the press as a “demotion,” even though the announcement from the International Astronomical Union, the organization that held the vote, said nothing of the sort. Our sappy human brains and their tendency to anthropomorphize inanimate objects kicked in. Poor Pluto hadn’t even completed a full orbit around the sun between the time of its discovery and the occasion of its downgrade. Have a heart!
This week marks 15 years since that meeting, and people still feel some way about Pluto—including the people who actually work in astronomy. There was no consensus among them then, and there is none now. Ask one astronomer and they’ll sigh, before saying that it’s time to stop dredging up the past and move on. Ask another and they’ll say the matter desperately needs a do-over. They agree on so much about the cosmos, but on a matter that seems as though it should be straightforward, some of them might as well exist in two different solar systems.
The story of what to call Pluto began during breakfast at Venetia Burney’s house in Oxford, England. In 1930, Burney’s grandfather read aloud the news that the American astronomer Clyde Tombaugh had discovered a mysterious planet beyond the orbit of Neptune, and wondered what it would be named. Burney, just 11 years old, knew that the other planets were named after mythical figures, so she proposed the Roman god of the underworld: Pluto. Burney’s grandfather, a respected librarian, passed the suggestion along to the Lowell Observatory, in Arizona, where Tombaugh had made the discovery.
By that point, the definition of a planet had already experienced some shifts. In 1801, astronomers deemed Ceres, a rocky object they had spotted between Mars and Jupiter, a planet, but 50 years later, after further observations, they designated it as an asteroid. Percival Lowell, the namesake of the Arizona observatory where Pluto was first photographed, had predicted that a ninth planet lurked somewhere deeper in the solar system, and Tombaugh and his colleagues believed they had found it.
But Pluto quickly turned out to be different than they’d expected. It was not, as Lowell had imagined, seven times more massive than Earth, but a tiny thing, smaller than the moon. And compared with the other planets, the specifics of its 248-year orbit were weird too. Still, Pluto stayed a planet.
Then, in the 1990s, astronomers started finding icy objects beyond Neptune. “People who were paying attention immediately said, Oh, we get it. This is what Pluto is,” Mike Brown, a Caltech astronomer who discovered one of these celestial bodies, told me. “Pluto is not this oddball at the edge of the solar system; Pluto is part of this larger population.” In 2000, when the American Museum of Natural History, in New York, opened a new space wing, curators tagged Pluto as part of the Kuiper belt, the disc of objects floating past Neptune, a decision that prompted a flurry of hate mail from Pluto fans.
Which brings us to that fateful meeting in Prague. At the start of the conference, some astronomers suggested a definition of planethood that would not only preserve Pluto’s status, but also include some other non-planet objects, such as Ceres, the onetime planet turned asteroid. But by the end of the week, the idea was out, and the attendees had settled on a new set of criteria for a planet: It must orbit the sun, be big enough for gravity to have smoothed it into a sphere, and be gravitationally dominant enough to clear other objects out of its orbital zone. The last rule disqualified Pluto.
The decision was supposed to mark the end of the Pluto discussion, not spark more debate. To Brown, the International Astronomical Union’s judgment was brave, considering how the public would perceive the loss of a beloved world. “Pluto was finally classified correctly,” Brown said. For Phil Metzger, a planetary scientist at the University of Central Florida, the call was cowardly. “They were afraid the public would not accept it if there were too many planets,” Metzger told me. “There were people arguing that children have to be able to memorize the planets.” A few days later, some 300 planetary scientists—about as many who voted in favor of the new planet rules—signed a letter decrying the decision.
In the ensuing years, the arguments have settled into two camps. Some astronomers, particularly those who study the dynamics of celestial bodies, emphasize the puny influence of Pluto’s gravity in its cosmic neighborhood. Some planetary scientists, whose work doesn’t center on such orbital details, say that there’s much more to a planet than that. They argue that Pluto has a host of characteristics that qualify it for planethood: an atmosphere, geological activity, even five of its own moons.
“No formal poll has been taken, but I think most planetary scientists, especially those who study Pluto, consider Pluto a planet,” Bonnie Buratti, a planetary scientist at NASA’s Jet Propulsion Laboratory, told me. “Astronomers tend to be less favorable to that status, because they place a lot of emphasis on the dynamic criterion of Pluto not having ‘cleared’ its orbit.” (Buratti, for the record, is both an astronomer and a planetary scientist, two different specialties that occasionally overlap.)
The experts I talked with for this story had a kaleidoscope of Pluto opinions. Some said that the scientists who disagree with the reclassification are in the minority, while others said that they’re surrounded by people who consider Pluto a planet. Some have sentimental feelings about Pluto; others are simply over it. For many of them, the results of the New Horizons flyby in 2015 make discounting Pluto difficult. The mission gave humankind its first close-up of Pluto, uncovering stunning topography, such as a massive heart-shaped glacier made of nitrogen. Fran Bagenal, a planetary scientist at the University of Colorado at Boulder, emailed me a picture from the flyby and wrote, “Doesn’t this look like a planet to you?” The image showed a slice of Pluto curving against the darkness of space. There was a wispy atmosphere, and below that a textured landscape dotted with mountains, their peaks capped with snow made of frozen methane. To this, some astronomers say that if the complexity of a world defines the criteria, wouldn’t the moons around Jupiter have to count as planets, and maybe even our own moon too? Let gravity decide.
The New Horizons mission has since moved deeper into the Kuiper belt, toward the outer regions of the solar system, where another planet-related debate has unfolded in recent years. Astronomers have detected objects moving rather strangely in the distant reaches beyond Pluto, and they believe that a giant, unseen planet, about six times the mass of the Earth, must be tugging at them. Brown, the astronomer whose work contributed to Pluto’s status change, is leading the search. One day, this object could join the solar system’s official lineup as the ninth planet, and 11-year-olds around the world can start brainstorming names.
After talking with members of both Pluto camps, I wonder, as Lisa Grossman suggested in Science News this week for the anniversary of the Prague meeting, whether having multiple definitions is really such a bad thing. We’re a bunch of people on one rock in space trying to figure out what another rock in space means to us. Of course the answers aren’t always going to be neat. For all their disagreements, everyone I spoke with was on the same page about one thing: that the question of planethood has no bearing on whether Pluto is a fascinating place to study.
The New Horizons flyby found evidence that Pluto—little Pluto!—might even have an ocean beneath its surface. “The name doesn’t matter,” Sanjay Limaye, a planetary scientist at the University of Wisconsin at Madison, told me. “It doesn’t matter what we call it, as long as we can explore it and learn from it.” Regardless of what we decide here on Earth, Pluto will still be there, doing its thing, blissfully unaware that some aliens a few rocks down are mesmerized by its existence.