The politics of Pluto: 10 years later, the bitter debate rages on

More than 10 years after Pluto was demoted from planet to dwarf planet, the debate continues, with leading scientists on both sides becoming more vociferous and maybe a little testy.

What's in a name? Plenty, say scientists who want Pluto classified as a planet

Pluto, seen here with its 'heart,' the informally named plain Sputnik Planum. Scientists have been debating the planetary status of this far-off, icy world since 2006.

It's a gloves-off, no-holds-barred debate: Is Pluto a planet or not?

Lately, the question has elicited some fiery exchanges between the scientists at the forefront of the argument.

Both sides claim to have great respect for the other's work, but when you speak with them, they're quick to mock their opponents' views on the subject.

Take Mike Brown of the California Institute of Technology. He discovered Eris, the celestial body located beyond Pluto that stirred up the debate about what constitutes a planet and ultimately led to the International Astronomical Union's vote in 2006 that demoted Pluto from planet to dwarf planet.

He calls some pro-planet Pluto arguments "insane."
Alan Stern, principal investigator of the Pluto New Horizons mission, believes Pluto should be classified as a planet. (NASA Ames Research Center)

Then there's Alan Stern, principal investigator of New Horizons, the spacecraft that flew by Pluto in 2015. He believes astronomers had no business voting on what constitutes a planet in the first place: "We're called planetary scientists because we study planets. It's part of the title of our profession. We know what we're doing."

Piero Benvenuti, general secretary of the International Astronomical Union (IAU), places the blame for the decade-old debate squarely on Stern and suggests few people actually support his cause. When asked why the debate has lasted so long, Benvenuti told CBC News: "Because of Alan Stern. Because of the Horizons team … Why am I not getting French schoolboys or Italian schoolboys or Iranian schoolboys writing to me about Pluto?"

New definition

A paper Stern co-wrote provided a new attack in the Great Pluto Debate when it was presented earlier this week at the Lunar and Planetary Sciences Conference in Texas. It proposes a new definition for the word "planet."

According to the IAU's criteria, a planet has to orbit the sun; have sufficient mass and self-gravity to form a round object; and clear its neighbourhood in its orbit. But Stern's new simplified definition says a planet only has to form a sphere under its own gravity and hasn't undergone fusion, like a star.

Except, that would add more than 110 planets to our solar system. And it would include our moon.

Brown thinks the idea is ludicrous. "Nobody wants the moon to be a planet," he said with a laugh.

War of the scientists

Stern has criticized the IAU vote for several reasons: only 424 of its 10,000 members voted; they weren't representative of planetary scientists, who are the experts on planets; and, he says, "you don't legislate science."

But Brown says Stern's opposition is only about the result, not the process: "He would have no complaint about the voting if the voting had gone in the direction he wanted."

The orbits of the outer planets, including dwarf planets Pluto and Eris. (NASA)

Along with Brown, Neil deGrasse Tyson, the renowned astrophysicist and director of the Hayden Planetarium in New York City, gets blamed in some circles for Pluto's fall from grace.

In 2000, Tyson removed Pluto from the planetarium's display of the solar system, instead placing it out in its own category (the debate began a year later as a result of a New York Times article). He received plenty of hate mail, but it didn't faze him. For anyone who has a problem with Pluto's dwarf planet status, Tyson usually has the same reply: Get over it.

But Stern will have none of that. 

"Neil has a PhD in astrophysics. Wrong profession," Stern said. "Neil will find, this time, that he's tangling with something he can't stop. And he's going to end up on the wrong side of history, the wrong side of science, and he'll end up having to apologize, because the field is laughing at him."

Kirby Runyon, a planetary geologist and lead author of the new paper with Stern, said Tyson's arguments for not including Pluto as a planet can be proven incorrect. For example, Tyson said Pluto needed to be reclassified because of its odd orbital inclination —  it's on an inclination of 17 degrees compared to the rest of the planets —  its extreme elliptical orbit and because it would vaporize if it were to pass too close to the sun. 

People gather on the New Mexico State University campus to protest Pluto's loss of status as a planet in 2006. (New Mexico State University)

"If you take any planet and move it too close to a star, it's going to vaporize. If Jupiter were in an elongated, elliptical orbit, it would still be considered a planet," Runyon said.

Though Runyon praises Tyson for his work bringing science to the masses, he said his arguments aren't up to his normal scientific standards.

"It's so easily rebutted," Runyon said. "Look, Neil is an astrophysicist … I'm a planetary geologist. I actually study planets."

A rose by any other name

So what's the big deal if Pluto is a planet or a dwarf planet? 

"What we call things affects how we think about them, how we intuit about them, how we compare it to other things that are similar or different," Runyon said.

For example, if Pluto doesn't have the prestige of being a planet, Runyon and Stern argue, that could affect the public's view of whether it's worth funding a mission to explore it.

Brown says that argument makes no sense. 

"If they can't make the case that the object that they sent their billion-dollar spacecraft to is interesting without having to co-opt the word planet, then they should have their spacecraft taken away from them. I mean, that's insane." 

Brown also said the outcry isn't quite as loud as perhaps some people believe. 

I am tired because we are going nowhere.- Piero Benvenuti, general secretary of the International Astronomical Union

"The only reason that it even sounds like there are people who are complaining … is because it's the same small group of people loudly complaining over and over the past decade."

Brown and Benvenuti both say they're tired of the debate and wish it would just end. Benvenuti said he'd consider drafting another electronic vote of all the IAU members if such a proposal was put forward.

"I am tired because we are going nowhere," he said. "I would be happy if that group who wrote the paper would instead make a proposal to us."

But Stern said he has no intention of presenting the new paper to the IAU's general assembly.

Meanwhile, Brown would like the public to think about Pluto in a different light.

"Don't feel like you have lost Pluto as a planet," he said. "Feel like you now suddenly realize that Pluto is one of these many thousands of objects that we know of in the outer solar system that we're just beginning to explore."


Nicole Mortillaro

Senior reporter, science

Based in Toronto, Nicole covers all things science for CBC News. As an amateur astronomer, Nicole can be found looking up at the night sky appreciating the marvels of our universe. She is the editor of the Journal of the Royal Astronomical Society of Canada and the author of several books. In 2021, she won the Kavli Science Journalism Award from the American Association for the Advancement of Science for a Quirks and Quarks audio special on the history and future of Black people in science. You can send her story ideas at