The issue can bring conversations to a screeching halt, or turn them into shouting matches. “Sometimes,” Runyon said, “it's just easier not to bring it up.”
But Runyon will ignore his own advice this week when he attends the annual Lunar and Planetary Science Conference in Houston. In a giant exhibit hall crowded with his colleagues, he's attempting to reignite the debate about Pluto's status with an audacious new definition for planet — one that includes not just Pluto, but several of its neighbors, objects in the asteroid belt, and a number of moons. By his count, 102 new planets could be added to our solar system under the new criteria.
“It's a scientifically useful bit of nomenclature and, I think, given the psychological power behind the word planet, it’s also more consumable by the general public,” Runyon said.
“A classification has to be useful, or else it’s just lipstick on a pig,” countered planetary scientist Carolyn Porco. Runyon's definition “is not useful at all.”
If aliens arrived at our solar system tomorrow, they would not see planets laid out in the orderly parade depicted in textbooks. Instead, they'd encounter hundreds of constantly moving bodies engaged in a complex dance around a brightly burning star.
It's hard to know what would immediately catch their attention. Probably Jupiter, the largest body in our solar system. Next they'd spot Saturn, Neptune and Uranus — other giant worlds — and the two belts of debris that orbit the sun inside Jupiter's orbit (the asteroid belt) and beyond Neptune (the Kuiper belt).
If they peered a bit closer, they'd spot the small, rocky spheres of the inner solar system: Mercury, Venus, Earth and Mars. They might see the dozens of worlds that circle the larger bodies or Ceres in the asteroid belt. And finally, if they searched near the Kuiper belt, they'd discern Pluto, tinier than Earth's moon but undeniably captivating, with water ice mountains and a heart-shaped plain.
Which of these would they consider a “planet” — or whatever the alien term for “planet” might be?
When the IAU voted in 2006, scientists came to the conclusion that gravitational dominance is what distinguishes the eight planets from the solar system’s other spheres. From giant Jupiter to tiny Mercury, each is massive enough to make them the bullies of their orbits, absorbing, ejecting or otherwise controlling the motion of every other object that gets too close. According to the definition, planets must also orbit the sun.
Pluto, which shares its zone of the solar system with a host of other objects, was reclassified as a “dwarf planet” — a body that resembles a planet but fails to “clear its neighborhood,” in the IAU's parlance.
“If you look at the solar system with fresh eyes, it is really hard to not realize that there are eight big things dominating the solar system and millions of tiny things flitting around,” said Caltech astronomer Mike Brown, whose discovery of the dwarf planet Eris, announced in 2005, precipitated the IAU vote a year later.
Brown was not at that vote, but he said that a definition based on orbital dynamics “is the most profound classification you can come up with.”
“That’s the one that asks the question we’re asking as planetary scientists,” he explained. “Why did the solar system form with these eight giant things and all these other things around them?”
But to Runyon, that distinction is less important than what dozens of solar system worlds have in common: geology.
“I’m interested in an object's intrinsic properties,” he said. “What it is on its surface and in its interior? Whether an object is in orbit around another planet or the sun doesn’t really matter for me.”
Runyon calls his a “geophysical” definition. A planet, he says, is anything massive enough that gravity pulls it into a sphere (a characteristic called “hydrostatic equilibrium"), but not so massive that it starts to undergo nuclear fusion and become a star.
“It’s only about one force and one property, the mass,” said Alan Stern, who led NASA's New Horizons mission to visit Pluto in 2015. Stern is a co-author on the paper outlining Runyon's new definition. “I think that's very elegant, as a physicist.”
Within that definition, Runyon and Stern say, scientists can divide planets into subcategories: moon planets like Europa and Titan; rocky planets like Earth and Mars; gas giant planets like Jupiter and Saturn; icy planets like Eris and Pluto.
But making “planet” more inclusive would formalize something many scientists already do: use the term when comparing geologic features. Runyon said he found dozens of examples in the scientific literature of researchers referring to “the planets Pluto, Earth and Mars” to talk about glacial processes on their surfaces, or “a planet-wide haze layer” when discussing the moon Titan's atmosphere.
“As planetary scientists we feel like the situation got really badly mangled back in 2006,” Stern said. “It was time somebody write this all down … and start a new conversation.”
Questions about the definition of “planet” go back much further than the debate over Pluto. The moon was considered a planet until the 17th century, after Copernicus placed it in orbit around the Earth. Galileo initially referred to the four largest moons of Jupiter as planets, but astronomers eventually adopted the term “satellite” (“attendant”) instead.
Ceres was considered a planet for several years after it was discovered circling between Mars and Jupiter in 1801. But when astronomers realized it was just the largest of thousands of objects inhabiting that stretch of sky, they renamed Ceres an “asteroid” and called its crowded home orbit “the asteroid belt.”
When Pluto was named the ninth planet in 1930, astronomers vastly overestimated its size, suggesting it could be even larger than Jupiter. It would also take more than 50 years for them to realize that Pluto had plenty of company in its far-flung orbit.
Jean-Luc Margot, a planetary scientist at the University of California at Los Angeles who voted in favor of the IAU resolution in 2006, said Pluto might have gone the way of Ceres if scientists had found other Kuiper belt objects sooner.
“An aspect of science is that we revisit our ideas,” he said. “We have to be able to acknowledge when we make a mistake.”
That moment came in 2006. When astronomers arrived at IAU meeting in Prague that year, they were surprised to hear that a group had been working in secret to devise a formal definition for planet — something that had never been done before.
“Planet,” the group proposed, was any object made round by its gravity that's in orbit around a star. Though many bodies in the solar system met this requirement, only Ceres and Eris would be made new planets; Pluto and its moon Charon would be called a binary planet system. The group also suggested a new classification, “pluton,” for bodies like Pluto whose orbits around the sun took 200 years or more.
The draft definition made almost no one happy. It was criticized as awkward and arbitrary, and the secrecy in which it was developed meant that researchers who wanted to improve the definition had little time to do so. Scientists spent the next two weeks of the conference rushing to come up with terminology they liked better.
Sara Seager, an exoplanet researcher at MIT who did not attend that year's meeting, recalled watching the chaos from afar.
“It actually was very confusing,” she said. “Everyone was asking me what was going on. … I'd be in a taxi and the taxi driver would say, 'I really want Pluto to be a planet. Will it still be a planet?' And I couldn't say.”
The vote happened on the meeting's last day. Despite the rushed circumstances, the resolution passed with a large enough majority that no one counted the votes. Pluto had lost.
Stern, who missed the 2006 meeting to help his daughter move into her college dorm, said it felt like astronomers who study black holes and stars had hijacked the most important concept in his field. He hadn't even known that a definition was being formulated — otherwise, he might have tried to attend the meeting. Now, suddenly, he was a planetary scientist whose object of study was no longer a planet.
He scoffed at Pluto's new classification, “dwarf planet” — “How can an adjective in front of a noun not describe the noun?” Stern asked. “There are dwarf stars but they're still considered stars.”
Runyon, who was a 21-year-old college student at the time of the IAU vote, said that the result never sat well with him. In 2015, he was on the data analysis team for the New Horizons spacecraft as it flew past Pluto. That December, the stunning new images of that distant world fresh in his mind, he sat down and “in a fit of creative passion” drafted his definition.
The paper that Runyon will present this week isn't a formal proposal, like the one that was devised at the IAU. He's not putting his definition up to a vote, or even suggesting that it should replace the IAUs. If he did, it's unlikely that the IAU would adopt it.
But it's sure to spark debate. Porco, who is one of the lead scientists for NASA's Cassini mission to Saturn, pointed out that she is a planetary scientist and has no problem with the IAU's orbital dynamics-based definition.
She also noted that astronomers already have a perfectly serviceable term for the kind of body Stern and Runyon are trying to describe: “world.” In her view, the only scientists who want to make those places planets are people who study Pluto.
Nearly everyone agrees that the IAU definition is imperfect. Margot, the UCLA planetary scientist who voted for the resolution, has tried to refine it. But the debate over Pluto was so “traumatic” for the community, he said, that he doubts that the IAU would be willing to revisit the issue anytime soon.
If you talk to enough scientists on either side of this debate, you'll notice that their arguments start to echo each other. They use the same terms to criticize the definitions they don't like: “not useful,” “too emotional,” “confusing.” Both groups want the same thing: for the public to understand and embrace the science of the solar system. But each is convinced that only their definition can achieve that goal. And each accuses the other of confusing people by prolonging the debate.
But Seager, the exoplanet researcher, said the opposite might be true. In her experience, the debate over Pluto's status has given her more opportunities to talk about the solar system than ever before.
“What I love about it is it’s a teaching moment,” she said. “If someone asks about Pluto … you use that as an opening to say, look whatever you want to call it, here’s what’s going on in our system today.”
Seager has no dog in this fight. Her gaze extends far beyond the Kuiper belt, to worlds that orbit stars light-years from our own. In the years since the IAU resolution, scientists have found thousands more planets outside our solar system. Many are like nothing astronomers have ever seen before — giant “hot Jupiters” that orbit tightly around their stars; “rogue planets” that rocket through the galaxy independent of any sun. For scientists like Seager, the age of planet discovery is just beginning.
“What else is out there? What’s beyond Pluto?” she asked. “There’s so much we still don’t know.