What really happened when Pluto was demoted? Twenty years after the International Astronomical Union's 2006 vote, Dean Regas revisits the decision that reshaped our solar system, with insider accounts from science writer Dava Sobel and astronomer Bob Stencel.
Episode Transcript: Looking Up is transcribed using a combination of AI speech recognition and human editors. It may contain errors. Please check the corresponding audio before quoting in print.
[Karl Wells interviews Geo Centre's Paul Dean and kids about Pluto]: Well, it seems there's a space controversy brewing.
[BBC Archive 2006: Pluto declared a dwarf planet]: For 76 years it was regarded as the ninth planet in our solar system.
[Aggie Almanac; Protesting for Pluto]:Pluto is a planet! Size doesn’t matter!
Dean Regas: Happy Pluto Demotion Day! Wait, what? Am I the only one who celebrates Aug. 24 every year as the day in 2006 that the International Astronomical Union declared that Pluto was no longer a planet?
[BBC Archive 2006: Pluto declared a dwarf planet]: Today, scientists have downgraded Pluto. It's just too small.
[Science Friday]:Pluto is gone. It's no longer a planet. That's the latest decision by the folks who decide what a planet is. That's the International Astronomical Union.
Dean Regas: Can you believe it's been 20 years? Can you believe that after all this time, you still have feelings on the subject? Some scientists have started a petition to get Pluto reinstated as a full-fledged planet. Admit it. In stark contrast, you probably don't have any feelings about the planet Mercury, for instance. What is it about Pluto's reclassification that resonates with people even today? From the studios of Cincinnati Public Radio, I'm your host, Dean Regas, and this is Looking Up, that takes you deep into the cosmos or just to the telescope in your backyard to learn more about what makes this amazing universe of ours so great. Today we're going back to Aug. 24, 2006. Author Dava Sobel returns to take us inside the group that tried to define a planet, as well as astronomer Bob Stencel, who was in the room and actually cast his vote on planethood.
So in case you have been doing more important things than following how an elite group of international astronomers have defined the word planet, here is the official, and I use that term loosely here, official definition of a planet adopted 20 years ago. A planet is an object that: one, is in orbit around the sun; two, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium, a nearly round shape; and three, has cleared the neighborhood around its orbit.
Well, yeah, that's a definition, I suppose. And so here's one thing I've noticed. Whenever Pluto comes up at any one of my talks or media interviews, people have strong feelings one way or the other. Pluto is definitely a planet. No, Pluto is still a planet. Pluto will always be a planet to me. Pluto is a planet, but Pluto isn't.
No one, and I mean no one, has ever said, "Dean, you know what, who cares?" Darn it, we care about this little icy world so far from the sun. The so-called Pluto debate was raging in the astronomical world for decades. It's definitely different than the classical eight planets. You cannot argue that, no matter who you are. Size, composition, orbit, origin, all different. But it is round.
Some planetary scientists really like touting roundness as important to planet-ness. Yeah, well, roundness makes a planet in their mind. For a solar system object to form a nearly spherical shape to become round, the object had to have enough mass to be big enough to end up that way in the vacuum of space. Round objects can have geologic activity, weather, erosion, glaciers, water, and atmosphere. Yeah, they can, but not all do.
In 2006, as the international astronomical community gathered in Prague for their once every three year meeting, they dared to tackle the thorny issue once and for all: What is a planet? And out of all the things, an object's roundness became central in both the special committee tasked with proposing the definition, oh wow, and we're going to talk with one of its seven members, Dava Sobel, shortly, as well as the final vote and definition that still stands today.
Roundness, man.
Dava Sobel: My name is Dava Sobel. I am a science writer and I have been the editor of the poetry column Meter in Scientific American since its inception in 2020.
Dean Regas: Well, Dava, thanks so much for joining me today. I always enjoy our conversations so much.
Dava Sobel: Me too.
Dean Regas: So let's go back more than 20 years ago where you were selected by the International Astronomical Union to be among a really small and really illustrious group charged with the unenviable task of defining a planet. So how did that even come about?
Dava Sobel: I think it came about because a much larger community of planetary scientists had been trying to come up with a good scientific definition of that ancient word in the light of new discoveries such as exoplanets and Kuiper Belt objects. And the word planet just means wanderer. So there was a big effort to make it more specific. I think it was two years ongoing when the executive committee of the International Astronomical Union said something to the effect of, let's get a smaller group and lock them in a room for a few days till they come up with a definition. And that's what happened.
Dean Regas: You all get locked up into this room in Paris, so you all brought your own experiences and your own ideas. So were there any behind-the-scenes drama? Were there any like troublesome arguments or did you all kind of align pretty quickly?
Dava Sobel: We all had different opinions at the beginning and it looked to me as though we might not come to any kind of agreement, that this was going to be a bust. But we talked and everybody listened to everybody else. And Owen Gingerich was so good as a discussion leader and made everyone feel heard. And my goodness, we came to a unanimous decision.
Dean Regas: So are you or were you a member of the IAU and?
Dava Sobel: No, I am not now and have never been. I'm not an astronomer. I'm a science writer. So I was not a member and I did not go to the meeting, but the vote was contentious. I guess I should explain what our definition was. We said a planet is a body in orbit around a star and massive enough to have shaped itself round. But our definition would have kept Pluto as a planet, and its large moon Charon and the asteroid Ceres would have become a planet. We have 12 planets by our definition. But at the vote, some astronomers introduced a third criterion. So not just an orbit around a star and not just massive enough to be round, but the object had to have cleared its orbit. And on that point, Pluto flunked out.
Dean Regas: Were you surprised by the reaction that the group had to the definition you all made?
Dava Sobel: Yes. Maybe I should have known better, but I was surprised. There was just so much excitement around the whole thing that I remember that too. And I remember the way it got reported in the popular press. You would have thought that the planet definition was the only thing that went on at the IAU General Assembly. It just took up all of the news.
Dean Regas: Well, and knowing how it all turned out, if you could go back to those early days in the committee with Owen Gingerich there in Paris and the others, would you do anything differently?
Dava Sobel: I guess so. Yeah, I guess we would try to come up with a way to fend off that argument and to keep the definition more general.
Dean Regas: In your mind, I guess, if an ideal world, how many planets are there in our solar system?
Dava Sobel: And 12. 12.
Dean Regas: OK, here we go.
Dava Sobel: There was such emotion around Pluto on all sides. I was really struck by that. And I was struck by how many people who were not usually interested in science, just walking in my neighborhood, I'd actually overhear people talking and terms like barycenter coming out of conversations. It was remarkable. And in that sense, it was very good, because emotions were it was a strange time. Emotions were running high in a weird way. I had all sorts of feelings that I didn't know I had and that came out and the whole I don't know if I was in the room. I don't know what I would do. I'd probably go crazy, too. I think it was a no-win situation that they might have been in. Yeah. Yeah. I guess ultimately I would have to say that certain words defy definition and planet may be one of them.
Dean Regas: Wow. Well, it's been great chatting with you about a story that's not really been told enough. And so we're glad to have you here and tell a little more to our listeners as well.
Dava Sobel: Well, thanks. As I said, it was definitely a highlight of my professional life.
Dean Regas: Looking Up right after this. Bob Stencel, retired professor of astronomy, who was also present at the 2006 meeting where Pluto was promoted. Promoted. Oh, interesting. So not demoted, promoted. Well, you got to tell me more about that, because that's a whole new take on this.
Bob Stencel: Absolutely. It has its own special category now. It's the leading member. Much like Dean Regas has his own special category as celebrity, Pluto is known more than it might have been.
Dean Regas: Well, shoot, I was going to start off with a controversial question and say, Dr. Bob, why do you hate Pluto so much? But I guess that's not quite the right answer.
Bob Stencel: Well, it's a little too cold for my liking, but, you know, it's a respectable object in the solar system.
Dean Regas: Well, so you were there in the room as a member of the International Astronomical Union in Prague in 2006. Tell us, what did you observe, you know, as the planet definition was first proposed, the one that was from the special committee?
Bob Stencel: Well, I was in the auditorium where the votes happened ultimately. And it was interesting at the time to wander the halls and hear some pretty heated arguments among some of the pro and con personalities.
Dean Regas: You would say that the 12-planet definition that came through the committee was met with a little bit of, I don't know, discomfort, I suppose.
Bob Stencel: Right. Well, where would it end in that case? Because with discovery of more Kuiper Belt objects that are bigger than Pluto and bigger than the moon of Earth, in some cases, will we end up with 15, 20, 30 plus?
Dean Regas: What and much has been talked about regarding the actual vote. So some astronomers say that most of the members had already left and others say that maybe the vote was closer than you thought. And others say it was rushed and some say it wasn't even their right to make a definition. What did you see as a person in the room? How did the vote actually go?
Bob Stencel: Right. Well, it was frankly toward the end of the second week of a pretty long, busy meeting. So it was understandable that maybe the audience started to thin out. However, we started with a couple thousand people in the room for the first round of voting. And I don't think we dropped below 1,200 to 1,500 later in the game. And in terms of legitimacy, sure, it's the International Astronomical Union. By definition, we're adopting the terms and conditions and definitions that matter in astronomy that everyone in the world can work with or learn about. What other forum could consider this really at this point?
Dean Regas: Were you surprised by the public's reaction to all of this, both positive and negative?
Bob Stencel: It's been fascinating. And since I was teaching large astronomy classes throughout the couple of decades, it always provided a good starting point for discussion of how science gets done. And after enough debate, most people came around understanding that our view of the solar system has matured enough that we could begin to make additional distinctions. So in that spirit, again, it became a teachable moment, if you will.
Dean Regas: Well, Dr. Bob, this has been great talking with you and taking us back to that day 20 years ago and the part that you played in it as well. Thanks so much.
Bob Stencel: You're very welcome. Keep looking up.
Dean Regas: Ultimately, Dava Sobel and her committee were charged with an impossible task. I mean, a planet is not easy to define, and coming up with a definition that everyone would agree to was, you know, probably not going to happen. Like when planetary scientists state that Pluto is a planet, they mean something different than what the general public thinks. The public, who laments Pluto's demotion, only wants things to go back to how they were during their childhood.
[Karl Wells interviews Geo Centre's Paul Dean and kids about Pluto]: Now, Nathan, tell me, what do you think of Pluto being downgraded? I think it should not be downgraded because I like the planet a lot and it's one of my favorite planets.
Dean Regas: They want nine planets again, period. Sorry to tell you, nearly every planetary astronomer does not want that. They want most, if not all of those round objects to be considered planets. Some think that all dwarf planets should be planets. That includes Pluto and its moon Charon, Eris, Haumea, Makemake, and Ceres. Other astronomers want to include some marginally round objects that live far from the sun like Gonggong, Orcus, Quaoar, Salacia, Ixion, and Sedna. Yeah, you've heard all those, right?
Then we've got newly found objects that are so far away that we can't be sure that they're not round. They haven't even been given official names yet, but are catalogued as 2017 OF201, 2010 RF43, and 2021 DR15. They are every bit of a planet that Pluto is.
And what about some moons like Triton, Pluto's twin? That could be a planet. What about all round moons? What about our moon? It's round. So you see where this is going. I mean, you ask a "Pluto is a planet" scientist how many planets are in their definition, you'll get anywhere from 13 at the low end to 36 if they want to include all the round things.
The public doesn't want any of that. It wants to go back to how it was. I'm sorry to inform you, we will never have nine planets in the solar system again. It's either eight, 13, 36, thousands, or millions. Take your pick. I guess I like eight. Unless we find an entirely new planet. One that has escaped detection because it's too far away and too faint until now. Find Planet Nine now. And then we can argue about that for another 20 years.
I don't know. This debate is great for my book sales, I do admit. How to Teach Grownups About Pluto, still available wherever you get books.
Looking Up with Dean Regas is a production of Cincinnati Public Radio. Kevin Reynolds and I created the podcast in 2017. Ella Rowen and Carlos LopezCornu produce and edit our show and orbit the sun, dominate their neighborhood, but are luckily not round enough to be considered planets. Sorry, folks. Jenell Walton is our vice president of content and Ronny Salerno is our digital platforms manager. Our theme song is "Possible Light" by Ziv Moran. Our social media coordinator is Hannah Pflum and our cover art is by Nicole Tiffany. I'm Dean Regas. Keep looking up.
