Ep. 613: Pluto’s Demotion: 15 Years Later

It’s been 15 years since Pluto was kicked out of the planet club. It also happens to be the topic of our very first episode of Astronomy Cast more than 600 episodes ago. Are there any updates? Does Pluto have a chance of regaining planethood again?

Show Notes | Transcript

Show Notes

Pluto Demoted: No Longer a Planet in Highly Controversial Definition (Space.com)

Astronomy Cast: Ep. 1: Pluto’s Planetary Identity Crisis

David Joseph Wesley

Noel Ruppenthal (IMdB)

March 13, 1930: Clyde Tombaugh’s discovery of Pluto announced (APS)

Percival Lowell Biography (Space.com)

Lowell Observatory

Clyde Tombaugh: Astronomer Who Discovered Pluto (Space.com)

International Astronomical Union

Eris (NASA)

Ceres (NASA)

Alan Stern

New Horizons (JHUAPL)

Pluto Moons (NASA)

A fifth moon for Pluto, and a possible hazard for New Horizons (Planetary Society)

Detail: Pluto “chaos region” (Planetary Society)

New Horizons Spacecraft Displays Pluto’s Big Heart (JHUAPL)

Pluto and the Developing Landscape of Our Solar System (IAU)

Pluto’s Wispy Atmosphere May Be Surprisingly Robust (Scientific American)

Europlanet Science Congress (EPSC)

Dawn (NASA)

4 Vesta (NASA)

Hydrostatic Equilibrium (Swinburne University)


Titan (NASA)


Mike Brown (Caltech)

Rubin Observatory (LSST)

Kuiper Belt (NASA)

Caltech Researchers Find Evidence of a Real Ninth Planet (Caltech)

Konstantin Batygin

Evidence Indicates There’s Another Planet the Size of Mars in Our Solar System (Interesting Engineering)

Daily Space (CosmoQuest)

Apparent Magnitude (Swinburne University)

Thirty Meter Telescope

Hubble Space Telescope

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Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast, Episode 613. Pluto’s Demotion: 15 Years Later. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today, and with me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of Cosmo Quest.

Hey, Pamela, how are you doing?

Dr. Gay:                      I’m doing well. I can’t believe that the 15th anniversary of Pluto’s demotion means we’ve been doing this for 15 years. Our very first episode was the week after that event.

Fraser:                         Now you’ve just stolen my introduction. But that’s fine, that’s fine. It’s a little different. Yeah, 15 years, that’s amazing; 613 episodes. Man, my children we born just before we started doing this show, and now they’ve all moved out of the house.

Dr. Gay:                      Yeah, it’s wild, and the technological revolutions that we’ve watched. We started out talking to each other on Skype and recording locally. We went to Google Hangouts. We’ve jumped to Slack, to Discord, to Twitch.

Fraser:                         Yeah. Zoom.

Dr. Gay:                      We are bringing you a whole new layout today.

Fraser:                         Yes. It only took us half an hour to get all of the technology to finally settle down and serve our bidding, but it’s cool. If you haven’t got a chance, go to YouTube, watch the livestream, and then get to about the halfway mark, and you’ll see the new intro. It’s pretty cool. That’s awesome. So, where did that new intro come from?

Dr. Gay:                      That came from Noel Ruppenthal. He’s one of our committee members –

Fraser:                         Oh, awesome, amazing.

Dr. Gay:                      – and he took the audio from David Joseph Wesley, and we have an amazing intro now.

Fraser:                         That’s amazing. That’s so great. All right, so, it’s been 15 years since Pluto was kicked out of the planet club. It also happens to be the topic of our very first episode of Astronomy Cast, more than 600 episodes ago. Are there any updates? Does Pluto have a chance of regaining planet-hood status again?

                                    All right, so, I guess we should kind of catch – I mean, for people who don’t know what happened to poor Pluto, why don’t we catch people up briefly on what happened.

Dr. Gay:                      So, poor Pluto. It was officially discovered in 1930. It had appeared in a few images prior to that, but hadn’t been identified as Pluto prior to 1930. And when it was first found, it was the culmination of Percival Lowell funding Lowell observatory and saying, “There is another world out there, and we’re going to find it.” And when Clyde Tombaugh found it, all of the astronomers were like, “That’s really not enough light for what we were looking for, and maybe we don’t actually need another large world out there. We think we can explain Neptune now.”

                                    And nevertheless, Percival Lowell and everyone else involved in making his dream come true had decided a priori they were finding a planet. So, Pluto became our ninth planet, until 2006, when during a meeting of the International Astronomical Union, folks were like, “Look, Pluto is no longer the biggest thing out in the Kuiper Belt. There’s this thing called Iris. And Ceres lost its planet-hood when we found the rest of the Asteroid Belt. So, let’s make Pluto something else.”

                                    It was more complicated than that. There were more details to that, but that is the TLDR version of what happened. And in 2006, an entire roomful of professional astronomers from around the world raised their colored pieces of paper that designate, “Yes, I am a member of this organization,” and holding those high, they – in a kind of close vote – down-voted Pluto’s status, and it’s determined we’re now going to have dwarf planets, and Pluto would be the lead object in a new group called “Plutinos,” and I don’t think anyone’s really used that word since.

Fraser:                         No, no. It’s definitely fallen out of favor. Were you there for that vote?

Dr. Gay:                      No. No, I actually – so, 2006, I was that August moving from Boston to Edwardsville to start my job as a professor. The very next IAU, I would become a member, but that particular IAU, I was coming up with this show, enjoying just married life, and realizing, “Wow, home ownership is a lot.”

Fraser:                         Right, right. But normally, you do attend the IAU meetings, and have been to others since.

Dr. Gay:                      Yes. And every year, the question comes up, “Is this the year we’re going to reopen that can of worms?” and every year, we’re like, “Nope, this is not.”

Fraser:                         Okay, so, then, that was how Pluto lost its planet-hood status, and of course the very first episode, if you want to go all the way back to Episode 1, you can hear us discuss in great detail exactly what happened during that whole back-and-forth. But you just mentioned briefly that every four years or so, astronomers go, “Should we talk about it? Nah.” What has happened since?

Dr. Gay:                      So, poor Pluto, and poor Alan Stern. Prior to 2006 and that great demotion, the New Horizons spacecraft had taken off on a journey of exploration that would launch it as the fastest thing to leave the earth and head towards the outer solar system. That New Horizons spacecraft was the culmination of decades of effort to finally get a mission that would go to the world that the Voyagers hadn’t made it to.

And in the launch footage, they refer to Pluto as a planet. And prior to us actually getting New Horizons to Pluto, the best images we had came from the Hubble Space Telescope. And those Hubble Space Telescopes were enough to show us that Pluto and its moon, Charon, were pretty similar in size; that there were these additional little moonlets like Hydra that hung out; and it was a very dissatisfying image.

And researchers were literally researching the noise and the data to figure out what was noise, and what was moonlet. We found moons, and as the New Horizons spacecraft went on its way, there was this realization of, “Wait. Pluto and Charon are fairly much the same size. There’s all of these little moons. What if all of those moons actually occupy a disc of debris?”

Fraser:                         Yeah, yeah. That was a bit of nervousness leading up to the fly-by.

Dr. Gay:                      So, there was a whole lot of research done to try and figure out, from the earth – or at least Earth’s orbit – if there was a debris disc. So, we eventually realized: No, there’s not. And as New Horizons approached that summer holiday encounter with Pluto, we got to see more and more of the planet. And at an unfortunately large distance, we realized that Pluto has on one of its sides a series of tiger stripes; not identical to what we see on Enceladus, but nevertheless quite marked.

But the mission was too far away to really make sense of this incredibly dynamic feature set, and as we got closer and closer – or rather, as New Horizons got closer and closer – Pluto rotated, and the side that was closest as we got there was the side that has Heart, Sputnik Planitia.

And it was really this incredible experience to go from seeing these crazy tiger stripes chaotic terrain until the heart started to rotate into view. We watched the heart rotate across view, and just as the heart started to leave, we had our closest approach.

Fraser:                         So, you mentioned that they called Pluto a planet as the spacecraft was launching, but principal investigator, Alan Stern, will call Pluto a planet to this day.

Dr. Gay:                      It’s true.

Fraser:                         Yeah, so, he still burns a candle for Pluto.

Dr. Gay:                      And to be fair, the definition they came up with isn’t one that allows exoplanets to be planets.

Fraser:                         Yes.

Dr. Gay:                      And that isn’t rooted in geology or geophysics. It was astronomers who defined things for the planetary community.

Fraser:                         The observations made by New Horizons of Pluto were stunning like nothing we’ve ever seen. It showed us how different Charon was from Pluto. It showed that Pluto has an atmosphere that had just been theorized up until this point. It showed the landscape, the glaciers, the ice mountains. It had much more geography than anyone was ever expecting. Did any of this crack the hearts, at all, of the stern astronomers who had voted Pluto out of the planet club in the first place?

Dr. Gay:                      I love the pun. So, I think a lot of our hearts cracked, but then they cracked again when the Dawn mission got to Ceres and Vesta. And now we have a whole new view, and a whole new argument to open.

Fraser:                         All right. So, we were just about to open up another argument about who gets to be a planet, and why, and why not.

Dr. Gay:                      So, one of my favorite moments was sitting in the audience at a European Planetary Sciences conference in Madrid, and listening to the Dawn team refer to Vesta, an asteroid, in the Asteroid Belt – that is not round, because it got hit with something really big that made it go squish. As they discussed Vesta, they called it a planet.

And to be fair to Vesta, one of the rules about what makes a world a planet is, “Has it collapsed down so that gravitationally it has become a sphere, because hydrostatic equilibrium has been reached?” And Vesta did that, and it got squished. And then I hear people, again, on the Dawn mission, talking about Ceres, which is round, which is the OG former planet, as a planet, because it has  geysers; it has geology; it’s differentiated.

And so, when we look at Pluto and we see a world that’s surface is constantly changing – we believe that in areas, its surface has been around for less time than bees have been on the planet Earth. When we look at the largest asteroids and we see differentiated worlds that are geologically active, the question becomes, “Okay, where do we draw the line, people? How do you define planet, not planet?”

And a lot of people – and I’m one of them – is just like, “Okay, we need a geology argument. Can we please promote Ceres and Vesta?” – and it’s the fact that that “and” ends us up with dozens of objects that causes a lot of folks to go, “We can’t do that to the children. They can’t memorize that many planets.”

Fraser:                         Right. Yeah, it’s funny. The proposal that Alan Stern and others have made is, “Let’s just make anything that has hydrostatic equilibrium be a planet. Anything that is a sphere, or trying really hard to be a sphere, let’s just call it a planet. Even the moons.” And when you think about it that way – like imagine, we were using James Webb and we were observing some distant exoplanetary system, and we saw a bunch of objects in orbit around it. We would just count up the dots of light and say, “There are five planets,” even if one was smaller than Pluto. We’d call it a planet. So, I think it’s perfectly reasonable for us to just call them all planets. It’s also perfectly reasonable just to call the big ones planets. It also just doesn’t matter.

Dr. Gay:                      And this is where I’ve quite purposely tried to eradicate the word planet, largely, from my vocabulary.

Fraser:                         It’s fine. Me too. “World” is the one I use.

Dr. Gay:                      Yes, yes. I want to make it clear, Titan is a world with an amazing thick atmosphere; that we have things Io, that showcase geology like nothing else in the solar system. These orbit big old planets, but they’re worlds. And if Endor is a moon and can support ewoks, let’s call it a world, and use that definition, and be done with it.

Fraser:                         So, have we gotten even close at this point for the international astronomical union bringing back the topic, and potentially holding another vote? Because I’ve definitely heard rumors.

Dr. Gay:                      So, there is always a group that is like, “We need to put together a resolution,” but the resolution never makes it all the way to the point of being voted on. And this is because – well, on one hand, a lot of the folks who are upset don’t actually want to work with the IAU anymore. They had their heart broken. They’ve moved on. And so, they’re trying to make the change through the literature, through books, through active engagement, but not necessarily by working it through the system.

And within the system there’s a lot of inertia, and there’s kind of been a pandemic, so we haven’t really met when we were supposed to. And honestly, I think a lot of us are holding our breath to see just what Michael Brown finds once the Large Synoptic Survey Telescope – which is now Vera Rubin Observatory – starts looking at the outer solar system.

Fraser:                         Right. So, I guess one of the big things that’s happened in the 15 years of Pluto’s demotion is just the vast number of Kuiper Belt objects that have been found out in that region. There are now many, many objects that are roughly the size of Pluto. And so, would you say that that argument is starting to prove out to be the right decision?

Dr. Gay:                      So, where it gets confusing is we either need to say, “Everything that is round, and then hydrostatic equilibrium, and doesn’t orbit something that isn’t a star, is a planet.” I’m willing to have the planet/moon separation; I’m down with that – and that would give us Ceres. That would give us Eris. That would give us so many of these large, icy bodies in the outer solar system.

Or, when we start finding other ginormous things out there and it’s thought that there’s another potential ice giant or gas giant out there, the so-called “Planet 9,” and there are new hints of a Mars-sized object, a potential Planet 10 out there, as well – if we can find these other Mars and gas or ice-giant-sized worlds, I can see the dichotomy between the ten massive planets and all the tiny things being maintained, because there is a fairly significant size gap. But we’re still stuck in the fact that Mercury is tiny, and it really becomes hard to justify Mercury as a planet.

So, I’m sort of like, “We even need to ditch Mercury – because you know whatever’s in the Kuiper Belt won’t have cleared its orbit out – or let’s just accept all of this stuff.” So, it’s either, “Give me Eris, give me Pluto, give me Ceres, give me all of these, or take away Mercury when we find those two additional worlds. Pick one, please.”

Fraser:                         Now, you mentioned briefly that there’s potentially a Planet 9, something fairly large; maybe a Mars. How have we discovered these?

Dr. Gay:                      As we’ve gotten better and better, and done more and more surveys of the sky, we’ve uncovered giant blocks of ice and rock – Kuiper Belt objects; some round, some not – that have orbits that aren’t exactly random, aren’t at all circular, and numerically point to the influence of some other large body out there in a very specific and calculatable orbit. What it doesn’t tell us is where on that orbit that so-called Planet 9 might be.

And this is where Konstantin Batygin and Michael Brown – they’re the observer and mathematician pair – an astrophysicist, but the one running all of the numerical models – who’ve gone through and figured out, “Okay, at what percentage can what we see for orbits out there be created randomly?” And while it’s possible that it’s random, it’s possible at such a low level that I feel like there’s got to be something else out there influencing our solar system.

Now, the evidence for a 10th object, a Mars-sized object, is much, much less. We’re going to be doing an interview on the Daily Space hopefully next week, with one of the people who published about that.

But it’s important to realize: Sometimes you can see invisible forces in the motions of others. When you’re looking at a human battling the wind, leaning 30 degrees forward into that wind, you know there’s something there preventing them from falling down. When we look at the orbits, we know there’s something preventing them from circulizing.

Fraser:                         And you mentioned that the Large Synoptic Survey, the Vera Rubin Observatory, this is going to be the machine that will find them. So, give people a bit of an update on what’s going to happen with that.

Dr. Gay:                      So, like so many things, this telescope’s completion has been delayed by the current pandemic. When it does finally come online, hopefully within the next 12 months, it’s going to be surveying the entire southern viewable sky every few nights, and it’s going to be surveying it down to mid-20th magnitude.

And this means that if there’s a Planet 9 or Planet 10 out there, and it’s in the Southern Hemisphere sky or the southern part of the northern sky, it’s going to be able to find it over the course of its first year of observations. That constant, repeating cadence means that anything that’s moving, however slowly, that motion is going to show up. It’s possible that we already have images, but because of the spacing of the observations, we can’t see the motion of the object. It’s possible.

Now, unfortunately, there is the potential, but it’s actually in the Northern Hemisphere right now. And if it is, we either need to wait for it to move, which could take decades, centuries, or we need to wait for a Northern Hemisphere massive observatory to be built, and that would be the TMT, which currently isn’t being built.

Fraser:                         And people are always surprised to hear – like if we knew exactly where Planet 9 was, it would be resolvable easily in most of the world’s big observatories. They could all resolve it as a dot. It would be easy to spot in the Hubble Space Telescope. James Webb would reveal all kinds of details about it.

But the trick is to know where to look, and the sky is really big, and so, people don’t realize just how many tiny little pinpricks you could be looking at across the night sky. And the key with the width of the Vera Rubin observatory is that it’s going to be looking at all of them simultaneously, waiting to catch those little movements, to find those new objects.

So, here we are, I guess, in 2021. It’s been 15 years since Pluto lost its eminent status as one of the planets of the solar system. Let’s imagine a world that Planet 9 is found, Planet 10 is found. So, we know that there’s a Neptune-sized object out there and a Mars-sized object, and maybe even another one; maybe an Earth-sized object even farther out. Does Pluto somehow make its way back in, or will that just make the case against it even worse?

Dr. Gay:                      I think it will make the case against it even worse. And I suspect that this will be one of those things that astronomers argue the definition, and at some point, people are going to be like, “Okay, you have to use adjectives. You have to use adjectives. We will count it as a planet, but it’s a dwarf, and you’d better say dwarf. Earth is a terrestrial planet. You’d better say terrestrial planet.” I kind of see a future where adjectives are required.

Fraser:                         Right. Well, maybe in 15 years from now, after we do Episode 1200, we will have an update and an answer for people. Thank you, Pamela.

Dr. Gay:                      Thank you, Fraser.

Fraser:                         And do you have some names for us?

Dr. Gay:                      Our show is here thanks to the generous contributions of so many of you out there.

Your patronage on Patreon.com/astrononomycast makes this possible. And this week, I would like to thank Bill Nash, Janelle Duncan, Helge Bjørkhaug, Richard, Kevin Parker, David Truog, Janelle Duncan. And I just want to say, it’s good to have you back, Fraser.

Fraser:                         It’s good to be back. Thanks, everyone, and we’ll see you all next week.

Dr. Gay:                      Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Comments Attribution license. So, love it, share it, and remix it, but please credit it to our hosts Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, AstronomyCast.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at Patreon.com/AstronomyCast. 

Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

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