Science Friday - Should Pluto be a planet again?

Episode Date: April 2, 2026

In 2006, a vote by the International Astronomical Union determined that Pluto was no longer a planet. The decision sparked a heated public debate, and many planetary scientists disagreed with kicking Pluto out of the planet club.  Twenty years later, Pluto is back in the news: NASA administrator Jared Isaacman said he wants to make Pluto great again by declaring it… a planet again. And he’s urging President Trump to do so by executive order. Why does this Plutonian debate keep rearing its head? And does the president have the power to do that? To answer those questions and more, Host Ira Flatow talks with planetary scientists and Pluto champions Amanda Bosh and Alan Stern.  Guests: Dr. Amanda Bosh is the executive director of the Lowell Observatory in Arizona, where Pluto was first discovered. Dr. Alan Stern is the vice president at the Southwest Research Institute and principal investigator of the New Horizons mission to Pluto. Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

This is Ira Flato, 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 the heated public debate? Well, that decision was hardly the final word on Pluto status. Many planetary scientists immediately disagreed with kicking Pluto out of the planet club. And now Pluto is bad. 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 Bosch, Executive Director of the Lowell Observatory where Pluto was first discovered, Dr. Bosch 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. Thanks, Ira. Thanks, Ira. Looking forward to speaking with you.

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. Sure. Well, I'll say that we appreciate the administrators' thoughts on this. And, you know, a previous NASA administrator, Jim Bridenstein, 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, you know, ultimately scientists make up their minds one at a time based on facts, not based upon, you know, politics. or even public sentiment.

Fortunately, the sciences 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. Amanda, what do you think about that? 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. Amanda, just to get this out of the way, does Trump even have the power to declare Pluto a planet? The naming of bodies in the, in our universe is currently handled by the International Astronomical Union, the official naming. So as Alan said, that, you know, planetary scientists call it a planet by enlarge, but the International Astronomical Union is the body that makes that official decision.

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? What? What happened? Well, what happened was 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 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 populace. 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, work very well in science, and we're never going to hear the end of it. You know, it just goes on and on. Amanda, what was the part of their definition that excluded Pluto in particular? 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, you know, that was the thing that made Pluto not a planet. But there are, you know, there are many ways of being a planet. Well, it became a dwarf planet, right? What does it mean? What's the difference between a dwarf planet,

a man versus a regular planet, besides just its carving out a spot in its orbit. 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 Loll Observatory, we say dwarf planets are people too, or dwarf planets are planets too. And, you know, 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, you know, is questionable as to whether or not it makes sense scientifically.

Sorry, Amanda. It's okay. 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. You know, 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 Astronomically Union fouled it up, was simply meant to be in parallel to, you know, giant stars, giant planets, dwarf stars, dwarf planets. And that's all it was meant to me was just

the descriptor term about size. After the break, what Pluto's planetary status tells us about how the scientific process works or doesn't work. 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? 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's 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 IEU 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, the 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. Yeah.

So in your mind, then, what's a better definition for determining what is or is not a planet? The definition for planet that most planetary scientists, and the very great majority, 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. 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, you know, 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. How similar, then, is Pluto to the other small planets and the Kuiper Belt at the far reaches of the solar system? Is there anything that makes it especially unique, Alan? 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, you know, the four rocky planets near the sun. Mars, Venus, Earth, and Mercury are a pretty different lot themselves. And in that respect, the Corporate 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 that are large enough to be rounded by self-concels. 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. I mean, you came on the 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? They really were. It turned up 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. Yeah, 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. 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, re-ignited 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. And I think that that really, you know, 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? You know, 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 as a planet. People really, sometimes people just really root for that underdog.

And they like Pluto as a planet. Yeah, and of course you must have a special, relationship with it because you're at the Lowell Observatory where Pluto was first discovered. Absolutely. That's right. It was discovered in 1930 and it created a big stir back then because, you know, 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.

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? I think that you're onto something that because Pluto was so far away and so mysterious for so long. And somewhat smaller, you know, it's the size of kind of the United States instead of the Earth. And I think they can spell 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, you know, really, as scientists, we're just trying to categorize objects as to what they're, you know, what things are alike 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, or planetary scientist standpoint, it's really a decided matter that the dwarf planets are planets too. 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? 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. And Alan, you know, 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?

Well, you're right. Many thousands of planets. 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 is that there are lots of kinds of planets out there that we don't have in our solar system. For example, the planets call 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.

There are planets that are very underdense. They have the density of balsa wood, which no one predicted. There are planets with pulsars that no one predicted, and yet we have them. And many more types like that, even some call 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 of 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. As we wrap up, I want to end on some Pluto joy, if we can. Can you give me your favorite Pluto fact or, you know, trivia? 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, you know, 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. Wow. We'll be here with you waiting, Amanda. So thank you both for taking time to be with us today. Thanks, Ira. Thanks. We really, fun. Great talking with you, Ira. Dr. Amanda Bosch, 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 Shoshana Bucksbaum. Do you have any planetary piccadillos you want us to look into? Well, give us a call. 8774 SciFRI. Our sci-fry listener line is always open. That's 877, the number four, SciFRI. I'm I Refleito.

We'll catch you next time.