Dr. Konstantin Batygin, professor of Planetary Science at Caltech, made a joke that turned out to be true. In 2014, another Caltech professor, Mike Brown, brought an intriguing research paper to Batygin's attention. The paper described a peculiar planetary alignment in the outer solar system. While developing theoretical models to explain the paper's assertions, Brown and Batygin had a running gag, "We were like, okay, as a joke, let's put a planet in the outer solar system just to see what would happen." In this episode of Looking Up, Dean Regas chats with Dr. Batygin and gets the full story.
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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. This transcript may include additional material from the conversation, not featured in the audio.
Dean Regas: Pluto’s not a planet, right? I think we are all in agreement about that…
So that leaves us with eight planets in our solar system. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Thanks for not turning off the episode, by the way. Stay with me because what, what if I were to tell you that there was another planet out there - lurking in the farthest reaches of our solar system, way beyond Pluto?
It’s so distant and so dark that no telescopes on Earth are yet powerful enough to detect it. Yet. A ninth planet. Let’s call it Planet Nine. Sure, you say. Actually, I say the same thing. If we can’t see it, how do we know it’s there? Math! And math found a planet before. Can it do it again?
From the studios of Cincinnati Public Radio, I’m your host Dean Regas and this is Looking Up! The show 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. My guest today is astronomer and mathematician Konstantin Batygin, Professor of Planetary Sciences at Caltech.
Dean Regas: I’m excited to talk with Konstantin Batygin about Planet Nine. He is one of the main people who have predicted its existence, and I got him to almost bet it will be found soon. And many Planet 9 believers cite precedence. Neptune, or as I may call Planet 8, was discovered in 1846 when it was first spotted in a telescope. But two mathematicians found Neptune on paper long before then.
Astronomers were watching Uranus, discovered in 1781, and it moved in a funny way. It was AS IF an unseen object was perturbing (or affecting) its orbit ever so slightly. When John Couch Adams in England and Urbain Le Verrier in France looked at the problem, they computed a mass that would cause such perturbations, unseen to that point, and its possible location in space. Le Verrier went so far as to write to astronomers and in essence claim, “point your telescope there, and you’ll find it.”
On the night of September 23, 1846, an astronomer at the Berlin Observatory, Johann Gottfried Galle, following Le Verrier’s instructions, aimed the telescope. And within 1 degree, 1 degree of where Le Verrier predicted, there it was. A deep blue disc. It was Neptune.
Galle must’ve been shocked and awed when he wrote to Le Verrier later: "the planet whose place you have computed really exists!" This was math’s day in astronomy. Could history repeat itself? Could the mathematicians be right about Planet 9?
Dean Regas: Well, Konstantin, thanks so much for joining me today.
Dr. Konstantin Batygin: Thanks so much, Dean. Pleasure to be here.
Dean Regas: So, your work in modeling the early solar system, it really shows it to be quite different now than it was way back then.
So, what does your research say about how many planets there were orbiting, like a newborn son?
Dr. Konstantin Batygin: Well, at least the ones that we see around the sun now, but, but possibly more, you know, dating back to work that. You know, we did, started doing really more than a decade ago, like 15 years ago, you know, we considered the question of could the outer solar system have had, could, could there have been more ice giants than, than there are today?
Because the picture had been crystallizing that the solar system kind of underwent a transient instability, right? The planets were all kind of closer. Or rather the outer planets, Jupiter, Saturn, Uranus, and Neptune. And we're all kind of closer together, when the solar system was just born and have since expanded through a violent, period of, of scattering.
And during this period, you can eject planets. Some planets can collide with one another. Some planets can attain very large orbits and get trapped very, very far away. All kinds of crazy things can happen. So that was kind of the motivation and indeed. The emerging picture is that the solar system evolution scenario works better if there is one more kind of sacrificial eyes, giants, so to speak, that you, start out with. It's not, it's not a complete, you know, deal breaker if there isn't one, but it helps a little bit.
Dean Regas: So, the order of the planets we see today, they could have been much different in the past, or is the lineup we see today kind of how it's always been? Has one of them shifted anywhere?
Dr. Konstantin Batygin: Well, they, their orbits have expanded. So, I mean, Jupiter. Being the kind of big anchor of the solar system, being the most massive planet has more or less stayed where it's at. It's kind of moved in a little bit, but it's not too, it's not something to write home about. But conversely, we think, for example, the orbit of Neptune has expanded by about a factor of two, right?
So, Neptune started out in about 15 times the distance between the earth and the sun, and is now at 30 Now during this process of orbital expansion. There could have been instabilities where Uranus and Neptune switch places, but we really can't tell definitively. And the reason we can't tell definitively is that instabilities are chaotic things.
And it's a little bit like asking, you know, the air in a room, like a particular air molecule. Where did it come from? And you, you can say statistical things about it, you can tell statistically it's moving at some speed, but you can't really say much about its origin. So, a lot of information about the early solar system gets scrambled during these, the first kind of a hundred million years of evolution.
Dean Regas: And do we know what like the main driver or drivers were that kind of changed this, this setup?
Dr. Konstantin Batygin: Yeah, we almost, we, we, I would say almost certainly believe that it was the population of, planetesimals, kind of icy asteroids that were present in the original kind of early solar system, roughly between, you know, 15 and 30 astronomical units.
So, kind of, you can think of. As being sort of the distance currently occupied by Uranus and Neptune. We think there was, it was quite a massive belt, about 20 earth masses of material that was there. And through scattering these planetesimals, these asteroids around, the planets began to move basically by playing gravitational soccer.
Right Now, of course, if you play gravitational soccer, as I'm sure you do, you know, on the weekends and stuff. You know that momentum must be conserved. So if you scatter something out, you know an object must move in. If you scatter something in, an object must move out. So through the process of grinding through this belt of, of asteroids and scattering them in for Jupiter to scatter out of the solar system, the, the, planets, planets expanded.
But the configuration is probably not too different from what it was originally.
Dean Regas: Back in 2016, you worked with Astronomer Mike Brown to, try to mathematically prove the existence of a ninth planet, an unseen planet far beyond the, orbits of Detune and Pluto. So, first question. Mike, of course, is famous for discovering many dwarf planets that. Contributed to Pluto's getting kicked out the Planet Club.
Did Mike come to you because he felt bad about what he did to Pluto, and he wanted to make up for it?
Dr. Konstantin Batygin: Well, that's, that's sort of like a joke story that he, he likes to tell, but the answer is no, actually. The answer, the, the real story is more interesting. He came to me because. Well, I had just started, you know, I had just returned to Caltech, just started my faculty job and Mike was like, listen, there's this remarkable paper by, by these guys, Trujillo and Shepherd, Chad Trujillo and Scott Shepherd, and they are talking about.
The, this like alignment of the arguments of perihelion and like, what the hell is that? And like, we should kind of look into this. It looks kind of interesting. And then we sort of, you know, over the, the course of a year, I mean, I started kind of developing theoretical models to try and explain some of this.
And we would routinely make fun of the idea. That, you know what, like, you know, it would be hilarious is if we wrote a paper about how this is caused by an extra planet in the outer solar system. And, you know, as we kind of work through the various sort of theoretical, the theoretical explanations and like our takeout blooper reel is full of ideas that didn't work.
We were like. Okay, as a joke, let's, let's try and put a planet in the outer solar system just to see what would happen. Like the worst thing is that's gonna happen is it's gonna be a ridiculous, and, and then like suddenly all of these patterns that we were seeing in the outer solar system started to, kind of crystallize and, and come together.
And, you know, even going beyond. What the original motivation was like. There was, there's this alignment that I mentioned in this peculiar parameter called argument of perihelion, but along the way we discovered there were other patterns as well. And, and so it was this slow crystallization where we went from an idea that we would just make fun of and joke around about to being like, wait, like this is the only thing that works.
And so, yeah, it was a really fun first, you know, kind of. Year and a half when we kind of led up to the, to that first paper. And of course we've, you know, over the last decade worked quite a bit on improving the theory and, and kind of understanding the mechanisms through which these patterns are shaped much better and discovering new ones along the way.
Dean Regas: And so this, a hypothetical planet is called Planet Nine, or at least that's the name that, that kind of got somewhere along the lines. and is it, what were some of the, like, the main kind of like warning signs? I always read that like CNAs orbit was something kind of like a mm-hmm. Pointing to it, but were there other objects that were found that kind of pointed you in that direction?
Dr. Konstantin Batygin: Absolutely. Yeah. So, Sedna is part of the story, but it's by no means the only object. So the first, the first couple lines of evidence really were that if you look at the most distant orbits of the hyper belt, right, these are things that take 10,000 years to go around the sun once and have highly, highly elliptic orbits.
Then you find that preferentially, they all point in the same direction. And that's kind of weird, just without doing any. Physics, you can just ask and answer the question of should the solar system be like that or not? And there's something strange about that, right? And then if you further examine these objects, their, their orbital plane is preferentially tilted into a common direction as well, which is about 20 degrees off the plane of the solar system.
So it's another weird thing. It's like something is aligning the angular momentum vectors. Those were the, the first couple lines of evidence. But since then, we've found a lot more. So, for example, in the calculations. So if you put in Planet nine into the solar system and do synthetic, create kind of a synthetic solar system, and ask yourself, what should you expect to see?
What you should expect to see is a large swath, a large population of objects of, of little, you know. KY belt objects, I see planetesimals that orbit on super highly inclined orbits, right? Some can be not just retrograde, but in the plane of the solar system, but orbiting the wrong way. Of course, that's not by any means, a natural outcome of planet formation theory, but if you actually look at the data, we see them.
So, that the, the kind of existence of these retrograde bodies is naturally tied to the existence of Planet Nine as a, as a mechanism to create them. And then you can also look at the population of objects that are highly unstable. The objects that cross the orbit of Neptune and will all get ejected if you leave them alone in 10 million years.
And yet there's a ton of them in the Outer solar system. Why are they there? It's because Planet Nine is sending them in. And kind of perpetually resupplying this population. So there's, there's all this stuff about the solar system that's slowly coming together into a coherent picture that's all pointing to the same planet.
Dean Regas: Well, I gotta, I gotta be the, the skeptic here partially, you know, because it's been nine years. Go for it. Since you proposed Planet Nine and it's still remaining undetected. So, from your point of view, is it just like we're waiting for a technology to advance far enough? Or are you having any doubts, you know, as a mathematician, what percentage of certainty are you that Planet Nine exists? Oh, if you wanted to be a betting person. Which, you know, mathematicians bet pretty well, so Yeah,
Dr. Konstantin Batygin: So let me ask answer maybe the first question first of like, yeah, it's been nine years. Where the hell is Planet Nine? Well, first of all, in 2016, I, I said this is gonna take at least a decade because, and, I'm on record all over the place saying that because you're looking for one thing.
This is a very strange, astronomical search. Usually in astronomy you look for a population of bodies and you hope to capture some fraction of it. Here there's one, one little dim star and it's dim as hell. I mean, it's visual magnitude, probably 24. It's right at the edge of what can be done with sort of the best telescopes like Subaru telescope.
And critically, we don't know where to look for it. Like we, we can compute the orbit and the mass. But if you ask what does the or an orbit look like on the night sky, it looks like a, a line, right? And so there's this huge swath of the night sky that we have to cover. We gave up looking for it, you know, using Subaru a few years ago because we found our search to be so inefficient.
Dr. Konstantin Batygin: Because the way it works is, you get a few nights a year—say three or four—to observe. And in order to detect Planet Nine, you need three consecutive nights of observation, and your detection limits are set by the worst of those nights.
Being a theorist, I really didn’t appreciate the extent to which you are not in control when you're observing. When you're doing theory and the weather’s bad—great! You sit at home, drink some wine, do some math, relax.
But when you're observing, you're completely at the mercy of the environment. Suddenly the atmosphere becomes turbulent, and your seeing is way down. Or suddenly the telescope breaks. Or the fog rolls in. Or someone goes hiking nearby, gets hurt, and now there's an ambulance at the observatory—strobing red lights obscuring all the observations.
All of this stuff meant that our search efficiency was probably around 20%, maybe 25%. So, we said, “Alright, we’re just going to wait for LSST—the Vera C. Rubin Observatory—to come online.” And it just did, last month.
As they start collecting real data, I think that will be a real turning point for this whole business. I think the observational story has been on hold for four or five years now—and that’s about to change.
Dean Regas: Whew. Good thing you said a decade.Dr. Konstantin Batygin: That’s right. I’ve got one more year. After next year, I’m in trouble!Dean Regas: Yeah. Well, being a student of astronomy history, I can’t help but think about the discovery of Neptune. That was all done on paper, right? These mathematicians got together and said, “Okay—point the telescope there,” and boom—they found Neptune.
You’re making me think: wow, that was child’s play compared to finding Planet Nine. Was that discovery actually a piece of cake in comparison?
Dr. Konstantin Batygin: Well, the amazing thing is—it was in exactly the opposite parameter regime.
So, the way Le Verrier knew Neptune was there was by observing deviations in the orbit of Uranus from what was predicted. And you can actually look at the data—it’s beautiful. You can see, aha, here’s the phase of the orbit where it’s not doing what it's supposed to do. And in an almost impulsive way, you can understand where to point the telescope—just look at the part of the orbit where things are going wrong.
There’s a remarkable component to all this, which is that Uranus and Neptune happened to be near conjunction in 1820–1830, when Le Verrier was doing his calculations. Neptune was discovered in 1846, and around that time was the right moment to be making those predictions.
Now, Le Verrier couldn’t figure out Neptune’s orbit or mass very well—he got the mass wrong by about a factor of two, and the semimajor axis was too big. But—and I’m not criticizing his calculations; he was an amazing mathematician—he was still able to pinpoint the direction of where the gravitational acceleration was coming from on the sky exceptionally well.
With Planet Nine, we’re in exactly the opposite situation.
We’re inferring its existence based on orbital properties—like the alignment of distant Kuiper Belt Objects (KBOs). So, we can say things about Planet Nine’s orbit, but not where it is right now.
If we could observe those KBOs orbiting the Sun for the next 10,000 years, then I could tell you exactly where Planet Nine is.
So, I plan to live on as some kind of LLM, you know—some kind of superintelligent AI. I’ll still be doing that calculation. So look out for it in the year... I don’t know... 20,000 or something. I’ve already written the introduction to that draft!
Dean Regas: That’s great. The AI version of you has already got the paper written. Once the discovery of Planet Konstantin comes in, we’ll be all set.Dr. Konstantin Batygin: That’s right.Dean Regas: Well, I want to go a little farther afield—Dr. Konstantin Batygin: Go for it.Dean Regas: We’ve had some very interesting interstellar visitors come through our solar system lately. I think the object ‘Oumuamua made headlines, and now we’ve got 3I/ATLAS passing through our solar system.
How exciting are these for someone in your field? I mean, these interstellar interlopers—how big a deal are they?
Dr. Konstantin Batygin: Well, look—first and foremost, their existence is not at all a surprise.
We talked earlier about how the solar system started out with 20 Earth masses of icy debris that the giant planets scattered out. But the solar system isn’t special. All the other stars do this too—maybe not as efficiently—but to some extent, you should expect that there will be Earth masses worth of random junk floating around the galaxy per star.
So from that point of view, should we expect occasional interstellar asteroids? Absolutely.
Now, what was quite strange about ‘Oumuamua was the absence of a coma and the presence of non-gravitational acceleration. That’s what really drove all the excitement—trying to figure that out.
Usually, comets have non-gravitational acceleration galore, but they also show a coma. 3I/ATLAS, for example, has a coma. In fact, the early idea that it was a 20-kilometer object was almost certainly incorrect. It’s probably a 20-kilometer coma with a tiny nucleus. The nucleus must have been maybe 50 meters or something like that—that's the latest I’ve heard.
The same was true for 2I/Borisov—Borisov looked like the lamest, most everyday solar system comet ever. Nothing interesting about it.
But ‘Oumuamua was different.
And for me, the interesting part was watching how that sparked so much speculation—about aliens, and the intense debates around it. “Aliens vs. not aliens.” I mean, I’ve got zero stake in that game, but it’s been fun to observe from the sidelines.
Dean Regas: I was going to ask you about it—off the record, probably. So yeah, when you heard some of your—well, I was going to say colleagues—say definitively that ‘Oumuamua is definitely an alien spacecraft passing through the solar system… what was your immediate thought, and then what was your secondary thought?
Dr. Konstantin Batygin: Oh, I didn’t have any thoughts. I was just... too stunned with information. It was too exciting.
One of my thoughts was: imagine this was a different field. Like solid-state physics or quantum information or chemistry. And you found some material with strange properties—what would be the threshold to ask, “Is this alien technology?”
I’d argue the threshold is much higher in other fields than in astronomy.
In astronomy, we’re fundamentally kind of romantics. There’s this pull toward meaning—the possibility of life elsewhere, aliens… and it’s such a strong pull that you really have to doubly check yourself—Dean Regas: Before you wreck yourself.Dr. Konstantin Batygin: Exactly.Dean Regas: That would’ve been good advice for that individual. My personal thought was: if ‘Oumuamua is an alien spacecraft, it’s the most poorly designed, out-of-control, slow-as-can-be alien spacecraft ever. So it’s the perfect cover! Because no one would ever design a spacecraft that behaves like that. It’s so bad that it has to be fake—it’s pretending to be a rock.
Dr. Konstantin Batygin: Exactly. That’s what the aliens want you to think.Dean Regas: Yep. There we go. We solved it.Dr. Konstantin Batygin: That’s right.Dean Regas: So yeah, I think we can both say—definitely aliens on that one.Dr. Konstantin Batygin: A hundred percent. In fact, everything I see is more or less aliens.Dean Regas: Perfect. That’s going in the podcast—no doubt about it. You’re never going to live that one down.
Dean Regas: Konstantin mentioned 3 Interstellar visitors we have had so far to come to our solar system. These are objects whose journey originated from distant stars and have only recently wandered into our neighborhood.
They are 1 - i - Oumuamua. That’s the object that sparked curiosity not only because it was first but because it behaved so strangely. And… sparked alien ideas. Ugh, don’t worry, we’ll do a whole episode on that soon.
The second one was 2 - i - Borisov. This looked to be a typical but was in fact a rogue comet - on a trajectory that will just pass through. Some astronomers believe Borisov was ejected from a star system about 13 light years away.
And that leaves 3 - i - Atlas, the third of its kind passing through. The thing with interstellar interlopers is that we only recently have the technology to detect them. And the telescope that can help, the telescope I may have a crush on, the Vera Rubin Telescope could unlock and discover so many objects that it will keep people like Konstantin busy for the rest of our lives. But come on, let’s find the 9th planet already… Again!
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