Should Pluto be a planet again?

[MUSIC PLAYING] IRA FLATOW: This is Ira Flatow. And you’re listening to Science Friday. Remember when members of the International Astronomical Union voted to strip Pluto of its planet designation 20 years ago and how it immediately sparked a heated public debate? Well, that decision was hardly the final word on Pluto’s status. Many planetary scientists immediately disagreed with kicking Pluto out of the planet club. And now Pluto is back in the headlines again. NASA administrator Jared Isaacman said that he wants to make Pluto great again by declaring it a planet again. And he’s urging President Trump to decree Pluto a planet by executive order.

So why does this Plutonian debate seem never-ending? And does the president have the power to reinstate Pluto as a planet? Two planetary scientists and Pluto enthusiasts are here to explain. Dr. Amanda Bosh, Executive Director of the Lowell Observatory, where Pluto was first discovered. Dr. Bosh is in Flagstaff, Arizona. And Dr. Alan Stern, Vice President at the Southwest Research Institute and principal investigator of the New Horizons mission to Pluto. Both of you, welcome back to Science Friday.

AMANDA BOSH: Thanks, Ira.

ALAN STERN: Thanks, Ira. Looking forward to speaking with you.

IRA FLATOW: Let me ask both of you first, what do you make of Jared Isaacman’s campaign to make Pluto a planet again? Let me begin with you, Alan.

ALAN STERN: Sure. Well, I’ll say that we appreciate the administrator’s thoughts on this. And a previous NASA administrator, Jim Bridenstine, did virtually the same thing six or so years ago. So that makes two NASA administrators that agree with the majority of planetary scientists. But ultimately, scientists make up their minds one at a time based on facts, not based upon politics or even public sentiment. Fortunately, the scientists have pretty much made up their mind and walked away from that IAU decision a long time ago in favor of small planets like Pluto being planets.

IRA FLATOW: Amanda, what do you think about that?

AMANDA BOSH: I think that anytime that Pluto is in the news, I think that this is a good thing. Pluto is– it’s our planet. It was sort of claimed by a large number of people just because of its discovery circumstances here in the United States in 1930. It is an amazing world in its own right. And we saw that in great detail when New Horizons flew past in 2015. And just having Pluto be visible, having people be talking about it, these are all great things.

IRA FLATOW: Hmm. Amanda, just to get this out of the way, does Trump even have the power to declare Pluto a planet?

AMANDA BOSH: The naming of bodies in our universe is currently handled by the International Astronomical Union, the official naming. So as Alan said that planetary scientists call it a planet, by and large. But the International Astronomical Union is the body that makes that official decision.

IRA FLATOW: And just as a refresher, because it’s been a while since we talked about this, what led to Pluto being stripped of its planet status in 2006? What happened?

ALAN STERN: Well, what happened was, is that we started to discover, because our technology and our telescopes got better, that Pluto was only the first of many small planets that orbit way out far in the solar system beyond the orbit of Neptune. And the International Astronomical Union held a meeting in 2006 in which a small number, maybe 5% of their members, were there, and they decided that we shouldn’t have too many planets, else little school children couldn’t remember their names. So they created a definition that excluded Pluto and small planets because they were becoming too populous.

Personally, I found this to be scientifically objectionable. After all, we don’t legislate the number of elements in the periodic table, just to keep them to a memorizable number, or anything else in science. And really, it’s been controversy ever since they made that vote. Votes don’t work very well in science. And we’re never going to hear the end of it. It just goes on and on. [LAUGHS]

IRA FLATOW: Amanda, what was the part of their definition that excluded Pluto in particular?

AMANDA BOSH: So the definition that was adopted was that it had to orbit our sun, and it had to be massive enough to pull itself into a sphere. And then the piece of it that kicked Pluto out was that it needed to have cleared its orbit. And Pluto exists in an area in our solar system where there are other bodies. Part of that is because it’s actually in a resonance with Neptune. So these bodies are kept there. But that was the thing that made Pluto not a planet. But there are many ways of being a planet.

[LAUGHTER]

IRA FLATOW: Well, it became a dwarf planet. What does it mean? What’s the difference between a dwarf planet, Amanda, versus a regular planet, besides just it’s carving out a spot in its orbit?

AMANDA BOSH: Right, so I think that the idea then was that with Pluto being not massive enough to have cleared its orbit, then it got this title of dwarf planet. Here at Lowell Observatory, we say dwarf planets are people, too, or dwarf planets are planets, too, and that it’s like a giant planet is a planet, and a dwarf planet is a planet. But it was just a way to, I guess, to put Pluto in a different category because of this definition, which is questionable as to whether or not it makes sense scientifically.

ALAN STERN: Sorry, Amanda.

AMANDA BOSH: It’s OK, Alan.

ALAN STERN: I just want to jump in on this for one minute, because I am the person who coined the term “dwarf planet” in 1991, in the scientific literature. And it was meant to describe small planets that we expected to discover in large numbers. The sun is a dwarf star. It doesn’t make it not a star. It’s just a smaller star than giant stars. And this terminology, dwarf planet, which was in the literature long before the International Astronomical Union fouled it up, was simply meant to be in parallel to giant stars, giant planets, dwarf stars, dwarf planets.

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And that’s all it was meant to be, was just a descriptor term about size.

IRA FLATOW: After the break, what Pluto’s planetary status tells us about how the scientific process works or doesn’t work.

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IRA FLATOW: So you are, as we’re hearing, among the planetary scientists who rejected the definition. And it was done by voting and you say a minority of the IAU members, right?

ALAN STERN: Yeah, something like 5% of their membership. But more importantly, we don’t take votes in science. That’s not how science is done. You could gather together 100 Nobel Laureates, and if they all voted the sky is green, it wouldn’t make it so, would it? We don’t vote on quantum mechanics. We don’t vote on the theory of relativity. We don’t vote on evolution or climate change or anything in science.

And this process that the IAU adopted is quite antithetical to science. Science is normally done by individual experts, scientists making up their minds one at a time to reach a consensus, but not through some sort of ballot process. It was really, I think, one of the worst moments for science in my lifetime because it’s taught a lot of people in the public that unfortunate lesson that somehow science is arbitrary, instead of actually fact-based or theory-based, that it’s just based on voting. And that’s had bad implications for science policy ever since.

IRA FLATOW: Yeah, so in your mind, then, what’s a better definition for determining what is or is not a planet?

ALAN STERN: The definition for planet that most planetary scientists and the very great majority of planetary scientists use is simple. It’s an object in space that’s large enough to be rounded by self-gravity, but not so large and massive that it ignites a nuclear fusion, in which case we call it a star.

AMANDA BOSH: And then the one thing that I want to point out here as well is that there are lots of planets around other stars as well. And so the IAU definition specifies the sun as– that the planets only exist around the sun. So we need to acknowledge that there are just a variety of planets in our solar system, in the universe, that follow this kind of a definition that Alan just put forth, and so that we can study them as a whole and see what that variety of planet-type bodies is and how can planets be, what can they look like. The more we study, the more we find, the more we know.

IRA FLATOW: How similar, then, is Pluto to the other small planets in the Kuiper Belt at the far reaches of the solar system? Is there anything that makes it especially unique, Alan?

ALAN STERN: That’s a great question, Ira. The planets of the Kuiper Belt are a diverse group. They’re diverse in terms of their sizes, their colors, their surface compositions, the number of moons they have, whether or not they have an atmosphere. But that’s no different than the four rocky planets near the sun. Mars, Venus, Earth, and Mercury are a pretty different lot themselves.

And in that respect, the Kuiper Belt planets and the Earth-like planets in our solar system both show a lot of variety. And they have a lot in common with each other as solid bodies, so they are large enough to be rounded by self-gravity. In fact, they have a lot more in common than they do with the big gas giant planets that are really a completely different kind of beast.

IRA FLATOW: I mean, you came on this show back in 2018 to talk about the New Horizons mission to Pluto. And the photos of Pluto were just surprisingly breathtaking, weren’t they?

ALAN STERN: They really were. It turned out to be a really active and complicated world that exceeded our imaginations and just showed us that Mother Nature is just spectacular, and that even out far, far from the sun, where temperatures are so cold, that Mother Nature can produce objects like this that have mountain ranges and glaciers and atmospheres and moons and all those other attributes that we think of as part of being a planet.

IRA FLATOW: Yeah, and Amanda, I would think that after the public saw that, they would have the same ideas that, gee, this has got to be a planet.

AMANDA BOSH: I think that that is exactly what happened. And may I also say the thing that the New Horizons spacecraft also taught us is really, we don’t know everything. And the surprises that were found from that particular mission were just amazing. And it really ignited the public’s interest in Pluto, reignited the public’s interest in Pluto. And just to be able to see what is on the surface of this world– and of course, it was so helpful that it had this lovely heart shape on its belly, so to speak.

[LAUGHTER]

And I think that that really– people can be amazed at just trying to understand what could produce that. What could be happening on this body that is so far away from the sun? But why is it no longer considered a planet? Why did it get downgraded? Why did it get demoted? We actually had a little voting box at our old visitor center where people could vote is Pluto a planet, is it a dwarf planet, is it something else? And the overwhelming majority of the votes went to Pluto is a planet. Sometimes people just really root for that underdog. And they like Pluto as a planet.

IRA FLATOW: Yeah, and of course, you must have a special relationship with it because you’re at the Lowell Observatory, where Pluto was first discovered.

AMANDA BOSH: Absolutely.

IRA FLATOW: Right?

AMANDA BOSH: That’s right. It was discovered in 1930. And it created a big stir back then because another planet in our solar system that was discovered here and the story of the discovery and all of that. And then, as you may not know, but in 2024, Pluto was declared to be Arizona’s state planet as well. And just as a whole, I think that the connection that people have to Pluto as a planet is really strong.

IRA FLATOW: Yeah. What makes it so special, Alan, do you think that people always look at it? What is there about that planet? Is it because it’s so far away?

ALAN STERN: I think, Ira, that you’re on to something, that because Pluto was so far away and so mysterious for so long and somewhat smaller– it’s the size of the United States, instead of the Earth, and I think they can smell a rotten egg when they saw one with the astronomers’ vote– that kind of made Pluto an underdog back then in 2006, and it just gained a lot of sympathy.

But really, as scientists, we’re just trying to categorize objects as to what things are like and what things are different. And planets like Pluto clearly look a lot more like planets than they look like stars or look like asteroids or anything else. And really, for planetary scientists, it’s really a decided matter that the dwarf planets are planets, too.

IRA FLATOW: And as planetary scientists keep discovering small planets, how do you think we should think about them in relation to bigger ones we all know the names of?

AMANDA BOSH: Well, we’re learning a lot about the different types of planets that can exist and the small, rocky planets versus the large gas giants, how far away they are from their star. I think that there’s going to be– there are already a lot of different types of things that are important about planets– how large they are, do they have atmospheres, how many satellites do they have.

Those, I think, are the more important things that we will be, and we already are, focusing on. Not what makes a planet a planet, but what makes this type of planet different from that type of planet. What is the type of planet that is most likely to support life as we know it? What other kinds of interesting things are going on, on these planets? Those are the questions that are really capturing people’s attention right now in the science.

IRA FLATOW: And Alan, we keep discovering, what, thousands of these exoplanets that are very, very far away. What can they help tell us about Pluto or the other planets in our solar system?

ALAN STERN: Well, you’re right. Many thousands of planets around other stars are now known. And in fact, it’s well appreciated from the data that, essentially, all stars have planets, with very rare exceptions. So planets are more common than stars are in our galaxy or in the universe, according to what we know.

But what the exoplanet discoveries have also taught us– it’s very important– is that there are lots of kinds of planets out there that we don’t have in our solar system. For example, the planets called hot Jupiters that are basically gas giant-sized planets, but that orbit right down next to their star, kind of like Mercury does in our solar system. And there are planets that are very underdense. They have the density of balsa wood, which no one predicted. There are planets–

IRA FLATOW: Wow.

ALAN STERN: –called pulsars that no one predicted, and yet we have them and many more types like that, even some called super-Earths that are more massive, rocky planets than the Earth, none of which were predicted. But that’s what nature does, is it doesn’t read our textbooks and really care what we predict.

Nature is out there producing all kinds of variety in terms of planets, from the little dwarf planets in the Kuiper Belt to the massive, even bigger than Jupiter-sized planets that orbit some stars, and everything in between. And we probably have a lot more types of planets to still discover with better and better telescopes and space missions that can explore this.

IRA FLATOW: As we wrap up, I want to end on some Pluto joy if we can. Can you give me your favorite Pluto fact or trivia?

AMANDA BOSH: Absolutely. The thing that got me interested in Pluto in the first place and that I still find really fascinating is the fact that this small planet out on the edge of the solar system has an atmosphere, and that that atmosphere is changing, depending upon its distance from the sun. And I find that to be endlessly fascinating. I did when I started studying Pluto. And just seeing all of the new studies that have come out and what people have learned about this atmosphere, and also, what we are going to learn as Pluto gets even further away from the sun, I’m just waiting with bated breath.

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IRA FLATOW: Wow. We’ll be here with you waiting, Amanda. So thank you both for taking time to be with us today.

AMANDA BOSH: Thanks, Ira.

ALAN STERN: Thanks. It was really fun. Great talking with you, Ira.

IRA FLATOW: Dr. Amanda Bosh, Executive Director of the Lowell Observatory, where Pluto was first discovered. She’s based in Flagstaff, Arizona. Dr. Alan Stern, Vice President at the Southwest Research Institute and principal investigator of the New Horizons mission to Pluto.

This episode was produced by Shoshannah Buxbaum. Do you have any planetary peccadilloes you want us to look into? Well, give us a call, 8774-SCIFRI. Our SciFri listener line is always open. That’s 877, the number 4, SCIFRI. I’m Ira Flatow. We’ll catch you next time.

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