Konstantin Batygin: Planet 9 and the Edge of Our Solar System | Lex Fridman Podcast #201

The following is a conversation with Konstantin Batygin, planetary astrophysicist at Caltech, interested in, among other things, the search for the distant, the mysterious planet 9 in the outer regions of our solar system. Quick mention of our sponsors, Squarespace, Literati on it and ni. Check them out in the description to support this podcast. As a side note, let me say that our little sun is orbited by the not just a few planets in the planetary region, but trillions of objects in the Kuiper Belt and the Oort Cloud that extends over three light years out. This, to me is amazing, since Proxima Centauri, the closest star to our sun, is only 4.2 light years away and all of it is mostly covered in darkness. When I get a chance to go out swimming in the ocean far from the shore, I'm sometimes overcome by by the terrifying and the exciting feeling of not knowing what's there in the deep darkness. That's how I feel about the edge of our solar system. One day, I hope humans will travel there, or at the very least, AI systems that carry the flame of human consciousness. This is the Lex Friedman podcast and here's my conversation with Konstantin Batygin. What is Planet nine?

Planet nine is an object that we believe lives in the solar system beyond the orbit of Neptune. It orbits the sun with a period of about 10,000 years and is about 5 earth masses.

So that's a hypothesized object. There's some evidence for this kind of object. There's a bunch of different explanations. Can you give like an overview of the the planets in our solar system? How many are there? What do we know and not know about them at a high level?

All right, that sounds like a good plan. So look, the solar system basically is comprised of two parts, the inner and the outer solar system. The inner solar system has the planets Mercury, Venus, Earth and Mars. Now, Mercury is about 40% of the orbital separation of where the Earth is, is closer to the Sun. Venus about 70%. Then Mars is about 160% further away from the sun than is the Earth. These planets that we, one of them we occupy, right, are pretty small. Okay? They're two leading order, sort of heavily overgrown asteroids, if you will. And this becomes evident when you move out further in the solar system and encounter Jupiter, which is 316 Earth masses, 10 times the size. Saturn is another huge one. 90 Earth masses at about 10 times the separation from the sun, as is the Earth. And then you have Uranus and Neptune at 20 and 30 respectively for a long Time, that is where the kind of massive part of the solar system ended. But what we've learned in the last 30 years is that beyond Neptune, there is this expansive field of icy debris, a second icy asteroid belt in the solar system. A lot of people have heard of the asteroid belt, which lives between Mars and Jupiter, right? Like that's a pretty common thing that people like to imagine and draw on lunchboxes and stuff. But beyond Neptune, there's a much more massive and much more radially expansive field of debris. Pluto, by the way, it belongs to that second, you know, icy asteroid belt, which we call the Kuiper Belt. It's just a big object within that population of bodies.

Well, Pluto the planet.

Pluto the dwarf planet, the former planet.

You know, why is Pluto not a planet anymore?

I mean, it's tiny. We used to size matters when it comes to planets. 100%. 100%. It's actually a fascinating story. When Pluto was discovered in 1930, the reason it was discovered in the first place is because astronomers at the time were looking for a seven Earth mass planet somewhere beyond Neptune. It was hypothesized that such an object exists. When they found something, they interpreted that as a seven Earth mass planet and immediately revised its mass downward because they couldn't resolve the object with the telescope. So it looked like just a point mass star rather than a physical disk. They said, well, maybe it's not seven, maybe it's one. And then sort of over the next, I guess, 40 years, Pluto's mass kept getting revised down, downwards, downwards, downwards, until it was realized that's like 500 times less massive than the Earth. I mean, like Pluto's surface area is almost perfectly equal to the surface area of Russia, actually. And, you know, Russia is big, but it's not a planet. Well, I mean, actually we can, we can touch more on that.

That's another discussion. So in some sense, earlier in the century, Pluto represented kind of our ignorance about the edges of the solar system. And perhaps planet nine is the thing that represents our ignorance about. Now the modern set of ignorances about the edges of our solar system.

That's a good way to put it.

By the way, just imagining this belt of asteroid of debris at the edge of our solar system is incredible.

Yeah.

Can you talk about it a little bit? What is the Kuiper Belt and what, what is the Oort cloud?

Yeah. Okay, so look, the simple way to think about it is that if you imagine, you know, Neptune's orbit like a circle, right. Kind of, maybe a factor of one and a half, 1.3 times bigger. On a radius of 1.3 times bigger, you've got a whole collection of icy objects. Most of these objects are sort of the size of Austin, you know, maybe a little bit smaller. If you then zoom out, right, and explore the orbits of the most long period Kuiper Belt object, these are the things that have the biggest orbits and take the longest time to go around the Sun. Then what you find is that beyond Earth, a critical orbit size, beyond a critical orbit period, which is about 4,000 years, you start to see weird structure like all the orbits sort of point into one direction. And all the orbits are kind of tilted in the same way by about 20 degrees with respect to Sun. This is particularly pronounced in orbits that are not heavily affected by Neptune. So there you start to see this weird dichotomy where, where there are objects which are stable, which are, which Neptune does not mess with gravitationally and unstable objects. The unstable objects are basically all over the place because they're being, you know, kicked around by Neptune. The stable orbits show this remarkable pattern of clustering. We back, I guess five years ago, interpreted this pattern of clustering as a gravitational one way sign, the existence of a planet in a distant planet, something that is shepherding and confining these orbits together. Of course, you have to have some skepticism when you're talking about these things. You have to ask the question of, okay, how statistically significant is this clustering? And there are many authors that have indeed called that into question. We have done our own analyses and basically, just like with all statistics where there's kind of like know multiple ways to do the exercise, you can either ask the question of, if I have a telescope that has, you know, surveyed this part of the sky, what are the chances that I would discover this clustering? That basically tells you that you have zero confidence, right? Like, that's not. That does not give you a confident answer one way or another. Another way to do the statistics, which is what we prefer to do, is to say we have a whole night sky of discoveries in the Kuiper Belt. And if we have some object over there which has right ascension and declination, which is a way to say it's there on the sky and it has some brightness, that means somebody looked over there and discovered an object, was able to discover an object of, of that brightness or brighter through that analysis, you can construct a whole map on the sky of kind of where all of the surveys that have ever been done have collectively looked. So if you do the exercise, this Way the false alarm probability of the clustering on which the Planet Nine hypothesis is built is about 0.4%.

Wow. Okay, so there's a million questions here. One, when you say bright objects, why are they bright? Are we talking about actual objects within the Kuiper Belt or the stuff we see through the Kuiper Belt?

This is the actual stuff we see in the Kuiper Belt. The way you go about discovering Kuiper Belt objects, pretty easy. I mean, it's easy in theory, right? Hard in practice. All you do is you take snapshots of the sky, right? Choose that direction, say, and take the high exposure snapshot. Then you wait a night and you do it again. And then you wait another night and you do it again. Objects that are just random stars in the galaxy don't move on the sky, whereas objects in the solar system will slowly move. This is no different than if you're driving down the freeway. It looks like, you know, trees are going by you faster than the clouds, right? This is parallax. That's it. It's just they're reflecting light off of the sun and it's going back and hitting this.

There's a little bit of a glimmer from the different objects that you can see based on the reflection from the sun. So like there's actual light, but it's not darkness.

That's right. These are just big icicles basically that are just reflecting sunlight back at you. It's then easy to understand why it's so hard to discover them. Because light has to travel to, you know, something like 40 times the distance between the Earth and the sun and then get reflected back.

Was that like an hour travel or.

Yeah, that's right. That's something like that. Because the Earth to the sun is eight minutes, I believe.

And so something, you know. Yeah, in that. In that order of magnitude. So that's interesting. So you have to like, account for all of that. And then there's this huge amount of data, pixels that are coming from the pictures. And you have to integrate all of that together to paint a sort of like a high estimate of the different objects. Can you track them? Can you be like, that's Bob. Can you like.

Yes, exactly. In fact, one of them is. Is named Joe Biden. I'm not like, this is not even a joke, right?

Is there a trump one or no.

No, no, I don't know. I haven't checked for that. But the way it works is if you discover one, you right away get a license plate for it. So like the first four numbers is the first year that this object has appeared on in the data set, if you will. And then there's this code that follows it which basically tells you where in the sky it is. So one of the really interesting Kuiper Belt objects, which is very much part of the Planet Nine story, is called VP113 because Joe Biden was vice president at the time. Got nicknamed Biden VP113.

Yeah, he got nicknamed Biden. Beautiful. What's the fingerprint for any particular object? Like how do you know it's the same one? Are you just kind of like. Yeah, from night to night you take a picture. How do you know it's the same object?

Yeah, so the way you know is it appears in almost exactly the same part of the sky, except it move, but it moves by. And this is why actually you need at least three nights. Because oftentimes asteroids which are much closer to the Earth will appear to move only slightly, but then on the third night will move away. So that third night is really there to detect acceleration. Now the thing that I didn't really realize until I started observing together with my partner in crime and all this, Mike Brown, is just the fact that for the first year when you make these detections, the only thing you really know with confidence is where it is on the night sky and how far away it is. Okay, that's it. You don't know anything about the orbit because over three days the object just moves so little. Right. That whole motion on the sky is entirely coming from motion of the Earth. So the Earth is kind of the car, the object is the tree, and you see it move. So then to get some confident information about what its orbit looks like, you have to come back a year later and then measure it again.

Oh, interesting. So do three nights, then come back a year later and do another three nights. So you get the velocity, the acceleration from the three nights and then you have the, maybe the additional, the additional information.

Because an orbit is basically described by six parameters, so you at least need six independent points, but in reality you need six many more observations to really pin down the orbit well, and from.

That you're able to construct for that one particular object in orbit. And then there's of course, like how many objects are there?

There's like 4ish thousand now, but like.

The, in the future there could be like millions.

Oh sure. Oh sure. So in fact these things are hard to predict, but there's a new observatory called the Vera Rubin Observatory, which is coming online maybe next year. I mean, with COVID these things are a little bit more uncertain, but they've actually been making great progress with construction. And so that telescope is just going to sort of scan the night sky every day automatically. And just. It's such an efficient survey that it might increase the census of the distant Kuiper Belt, the things that I'm interested in, by a factor of 100. I mean, that would be. That would be really cool.

And. Yeah, that's a. That's an incredible name.

I mean, they might just find Planet nine.

I mean, that's like almost like literally pictures, like, visually.

I mean, sure, yeah. Like the first detection you make, all you know is where it is in the sky and how far away it is. If something is 500 times as far away from the sun as is the Earth, you know, that's Planet nine. That's when the story concludes. Then you can study it now.

You can study it. Yeah. By the way, I'm going to use that as like, I don't know, a pickup line or a dating strategy. Like see the person for three days and then don't see them at all and then see them again in a year to determine the orbit. And over time, you figure out if sort of from a cosmic perspective this, this whole thing works out.

I have no dating advice to give.

I was gonna. I was gonna use this as a metaphor to somehow to map it onto the human condition. Okay, you mentioned the Kuiper Belt. What's the Oort cloud? If you look at the neptune orbit as 1, then the Kuiper bell is like 1.3 out there. And then we get farther and farther into the darkness. What?

So, okay, you've got the kind of main Kuiper belt, which is about, say, 1.3, 1.5. Then you have something called the scattered disk, which is kind of an extension of the Kuiper Belt. It's a bunch of these long, very elliptical orbits that hug the orbit of Neptune but come out very far. So the scattered disk with the current senses, like the. Some of the longest orbits we know of have a semi major axis, so half the orbit length, roughly speaking of about a thousand, thousand times the distance between the Earth and the Sun.

Wow.

Now, if you keep moving out, okay, eventually once you're at sort of, you know, 10,000 to 100,000, roughly, that's where the Oort cloud is. Now, the Oort cloud is a distinct population of icy bodies, and it's distinct from the Kuiper Belt. In fact, it's so expansive that it ends roughly Halfway between us and the next star. Its edge is just dictated by. To what extent does the solar gravity reach?

Solar gravity reaches that far?

Yeah.

So it has to. Wow.

Yeah.

Imagining this is a little bit overwhelming. So there's like a giant, like vast icy rock thingy.

It's like a sphere. It's like, you know, it's an almost spherical structure that engulfs, that encircles the Sun. And all the long period comets come from the Oort Cloud. They come the way that they appear. I mean, for already, I don't know, hundreds of years, we've been detecting occasionally like a comet will come in and it seemingly comes out of nowhere. The reason these long period comets appear that on very, very long timescales, right. These Oort Cloud objects that are sitting 30,000 times as far away from the sun as is the Earth actually interact with the gravity of the galaxy, the tide, effectively the tide that the galaxy exerts upon them. And their orbits slowly change and they elongate to the point where once they. Their closest approach to the sun starts to reach a critical distance where ice starts to sublimate, then we discover them as comets because then the ice comes off of them. They look beautiful on the night sky, et cetera. But they're all coming from really, really far away.

So is there. Are any of them coming our way from collisions? Like, how many collisions are there? Or is there a bunch of space for them to move around?

Yeah, there's completely collisionless out there. The physical radii of objects are so small compared to the distance between them. Right. It's just. It is truly a collisionless environment. I don't know. I think that probably in the age of the solar system, there have literally been zero collisions in the Oort Cloud.

Wow. When you, like, draw a picture of the solar system, everything's really close together. So everything, I guess here is spaced far apart. Do rogue planets, like, fly in every once in a while and join not rogue planets, but rogue objects from out there?

Oh, sure. Oh, sure. Yeah.

Join the party.

Yeah, absolutely. We've seen a couple of them in the last three or so years. Maybe four years now. The first one was the one called Uamuamua. It's been all over the news. The second one was Comet Borisov, discovered by a guy named Borisov. Yeah. So the way, you know they're coming from elsewhere is unlikely. Solar system objects which travel on elliptical paths around the sun, these guys travel on hyperbolic paths. So they come in, say hello, and then they're gone. And the fact that they exist is totally, like, not surprising. Right. The Neptune is constantly ejecting Kuiper Belt objects into interstellar space. Our solar system itself is sort of leaking icy debris and ejecting it. So presumably every planetary systems around other stars do exactly the same thing.

Let me ask you about the millions of objects that are part of the Kuiper Belt and part of the Oort Cloud. Do you think some of them have primitive life? Kind of makes you sad if there's like primitive life there and they're just kind of like lonely out there in space. Like, how many of them do you think have life? Like bacterial life?

Probably a negligible amount. Zero. You know, like zero with like a plus on top. Right? Yeah, if so, you know, if you and I took a little trip to the interstellar medium, I think we would develop cancer and die real fast. Right?

It's rough.

Yeah. It's a pretty hostile radiation environment. You don't actually have to go to the interstellar medium. You just have to leave the Earth's magnetic field to. And then you're not doing so well suddenly. So, you know, this, this idea of, you know, life kind of traveling between places, it's not, it's not entirely implausible, but you, you really have to twist, I think, a lot of parameters. One of the problems we have is we don't actually know how life originates, right? So it's kind of a second order question of survival in the interstellar medium and how resilient it. We think you require water, but. And that's certainly the case for the Earth, but, you know, we really don't know for sure. That said, I will argue that the question of, like, are there aliens out there? Is a very boring question because the answer is, of course there are. Right. I mean, like, we know that there are planets around almost every star. Of course there, of course there are other life form. Life is not some specific thing that happened on the Earth and that's it. Right. That's a statistical impossibility.

Yeah. But the, the difficult question is before even the fact that we don't know how life originates, I don't think we even know what life is like, definitionally. Yeah, like formalizing a kind of picture of, in terms of the mechanism we would use to, to search for life out there or even when we're on a planet to say, is this life? Is this rock that just moved from where it was yesterday? Life? Or maybe not even rock, something else.

I got to tell you, I want to know what life is okay. And I want you to show me.

I think there's a song to basically accompany every single we talk about today. And probably half of them are love songs. And somehow we'll integrate George Michael into the whole thing. Okay, so your intuition is there's life everywhere in our universe. Do you think there's intelligent life out there?

I think it's entirely plausible. I mean, it's entirely plausible. I think there's intelligent life on Earth.

And so, yeah, taking that, like, say whatever this thing we got on Earth, whether it's dolphins or humans, say that's intelligent.

Definitely dolphins. I mean, have you seen the dolphins?

Well, they do some cruel stuff to each other. So if cruelty is a definition of intelligence, they're pretty good. And then humans are pretty good in that regard. Then there's like, pigs are very intelligent. I got actually a chance to hang out with pigs recently. And they're. Aside from the fact they're trying to eat me, they're very. They love food. They love food. But there's an intelligence to their eyes that was kind of like haunts me because I also love to eat meat and then to meet the thing I later ate. And that was very intelligent and almost charismatic with the way it was expressing himself, herself, itself was, was, was quite incredible. So all that to say is, if we have intelligent life here on Earth, if we take dolphins, pigs, humans, from the perspective of like planetary science, how unique is Earth?

Okay, so Earth is not a common outcome of the planet formation process. It's probably a something on the order of maybe a 1% effect. And by Earth, I mean, not just an Earth mass planet, okay? I mean, the architecture of the solar system that allows the Earth to exist in its kind of very temperate way. One thing to understand, and this is pretty crucial, right, is that the Earth itself formed well after the gas disk that formed the giant planets had already dissipated. You see, stars start out with the star and then a disk of gas and dust that encircles it. From this disk of gas and dust, big planets can emerge. And we have, over the last two, three decades, discovered thousands of extrasolar planets in orbit of other stars. What we see is that many of them have these expansive hydrogen helium atmospheres. The fact that the Earth doesn't is deeply connected to the fact that Earth took about 100 million years to form. So we missed that train, so to speak, to get that hydrogen helium atmosphere. That's why actually we can see the sky, right? That's why the sky is. Well, at least in most places, that's why the atmosphere is not completely opaque. With that kind of thinking in mind, I would argue that we're getting the kind of emergent pictures that the Earth is not everywhere. Right. There's sort of the sci fi view of things where we go to some other star and we just land on random planets and they're all Earth, like that's totally not true. But the even a low probability event, even if you imagine that Earth is a one in a million or one in a, you know, one in 10 million occurrence, there are 10 to the 12th stars in the galaxy, right. So you just, you always win by large numbers. That's right. By supply, they save you.

Well, you've hypothesized that our solar system once possessed a population of short period planets that were destroyed by the evil Jupiter migrating through the solar nebula. Can you explain, if I was to.

Say, what was the key outcome of searches for extrasolar planets, it is that most stars are encircled by short period planets that are a few Earth masses, so a few times bigger than the Earth and have orbital periods that kind of range from days to weeks. Now if you go and ask the solar system what's in our region, in that region, it's completely empty, it's astonishingly hollow. And think, you know, from the sun is not some special star that decided that it was going to form the solar system. So I think the natural thing to assume is, is that the same processes of planet formation that occurred everywhere else also occurred in the solar system. Following this logic, it's not implausible to imagine that the solar system once possessed a system of intra Mercurian like compact system of planets. So then we asked ourselves, would such a system survive to this day? And the answer is no. At least our calculations suggest that it's highly unlikely because of the formation of Jupiter. And Jupiter's primordial kind of wandering through the solar system would have sent this collisional field of debris that would have pushed that system of planets onto the sun.

So was Jupiter this primordial wandering? What did Jupiter look like? Why was it wandering? It didn't have the orbit it has today.

We're pretty certain that giant planets like Jupiter, when they form, they migrate. The reason they migrate is on a detailed level, perhaps difficult to explain, but just in a qualitative sense. They form in this fluid disk of gas and dust. So it's kind of like if I plop down a raft somewhere in the ocean, will it stay where you plop it down or will it kind of get carried around? It's not really a good analogy, because it's not like Jupiter is being advected by the currents of gas and dust. But the way it migrates is it carves out a hole in the disk, and then by interacting with the disk gravitationally, it can change its orbit. The fact that the solar system has both Jupiter and Saturn here complicates things a lot because you have to solve the problem of the evolution of the gas disk. The evolution of Jupiter's orbit in the gas disk plus evolution of Saturn's and their mutual interaction. The common outcome of solving that problem, though, is pretty easy to explain. Jupiter forms, its orbit shrinks, and then once Saturn forms, its orbit catches up basically to the orbit of Jupiter and they both come out. So there's this inward outward pattern of Jupiter's early motion that happens sort of within the last million years of the lifetime of, of the solar system's primordial disk. So while this is happening, if our calculations are correct, which I think they are, you can destroy this inner system of few Earth mass planets. And then in the aftermath of all this violence, you form the terrestrial planets.

Where would they come from in that case? So Jupiter clears out the space, and then there's a few terrestrial planets that come in, and those come in from the, from the disk somewhere.

Like one of the larger. Yeah. What actually happens in these calculations, you leave behind a rather mass depleted, like remnant, remnant disk, only a couple Earth masses. So then from that remnant population, annulus of material over 100, over 100 million years, by just collisions, you grow the Earth and the moon and everything else you said.

Amulus.

Annulus.

Annulus.

Annulus. Yeah.

That's a beautiful word. What does that mean?

Well, it's like, it's like a disc that's kind of thin. It's like a. Yeah, it's something that is, you know, a disc that's so thin it's almost flirting with being a ring.

Like I was gonna say, this reminds me of Lord of the Rings. So, like this, the word just feels like it belongs in a Tolkien level. Okay, so that's incredible. And so that in your senses you said like 1%. That's a rare. The way Jupiter and Saturn danced and cleared out and, you know, cleared out the short period debris and then changed the gravitational landscape. That's a pretty rare thing too.

It's rare. And moreover, like, you don't even have to go to our calculations. You can just ask the night sky, how many stars have Jupiter and Saturn analogs? The answer is Jupiter and Saturn analogs. Are found around only 10% of sun like stars. So they themselves, like, you kind of have to score an A minus or better on the test to, you know, on the planet formation test to become a solar system analog even in that basic sense. And moreover, you know, low lower mass stars which are very numerous in the galaxy, so called M dwarves think like 0% of them, well, maybe like some negligible fraction of them have giant planets. Giant planets are a rare outcome of planet formation. One of the really big problems that remain unanswered is why we don't actually understand why they're so rare.

How hard is it to simulate all of the things that we've been talking about? Each of the things we've been talking about, and maybe one day all of the things we've been talking about and beyond meaning, like from the initial primordial solar system, you know, a bunch of disks with, I don't know, billions, trillions of objects in them. Like simulate that such that you eventually get a Jupiter and a Saturn and then eventually you get the Jupiter and the Saturn that clear out a disk, change the gravitational landscape, Then Earth pops up like that whole thing and then be able to do that for every other system in the. Every other star in the galaxy and then be able to do that for other galaxies as well.

Yeah.

So look, maybe start from the smallest simulation, like what is actually being done today. I mean even the smallest simulation is probably super, super difficult. Even just like one object in the Kuiper Bell is probably super difficult to simulate.

I mean, I think it's super easy. I mean like it's, it's just not that hard. But you know, let's, let's ask the most kind of basic problem. Okay, so the problem of having a star and something in orbit of it that you don't need a simulation for. Like you can just write that down on a piece of paper.

This gravity, like. Yeah, I guess, I guess it's important to try to. One way to simulate objects in our solar system is to build the universe from scratch.

Okay, we'll get to building the universe from scratch in a sec, but let me just kind of go through the hierarchy of what, you know, what we do.

Two objects.

Two objects analytically solvable. We can figure it out very easily if you just. I don't think you. Yeah, you don't need to know calculus. It helps to know calculus, but you don't necessarily need to know calculus. Three objects that are gravitationally interacting. The solution is chaotic. Doesn't matter how many simulations you do the answer loses meaning after. After some time.

I feel like that is a metaphor for dating as well, but go on. Apologies.

Yeah. So the fact that you go from analytically solvable to unpredictable when your simulation goes from two bodies to three bodies should immediately tell you that the exercise of trying to engineer a calculation where you form the entire solar system from scratch and hope to have some predictive answer is a futile one. We will never succeed at such a simulation.

So, like I said, just to clarify, you mean explicitly having a clear equation that generalizes the whole process enough to be able to make a prediction? Or do you mean actually, like, literally simulating the objects as a hopeless pursuit? Once it increases beyond three, the simulating.

Them is not a hopeless pursuit, but the outcome becomes a statistical one. What's actually quite interesting is I think we have all the equations figured out right? Like, you know, in order to really understand this, the formation of the solar system, it suffices to know gravity and magnetohydrodynamics. I mean, like the combination of Maxwell's equations and, you know, Navier Stokes equations for the fluids. You need to know quantum mechanics to understand opacities and so on. But we have those equations in hand. It's not that we don't have that understanding. It's that putting it all together is, A, very, very difficult. And B, if you were to run the same evolution twice, changing, you know, the initial conditions by some infinitesimal amount, some, you know, minor change in your calculation to start with, you would get the. You'd get a different answer. This is one. This is part of the reason why planetary systems are so diverse. You don't have, like, a very predictive path for you start with a disk of this mass, and it's around this star. Therefore, you're going to form the solar system, right? You start with this, and therefore you will form this huge outcome, huge set of outcomes, and some percentage of it will resemble the solar system.

You mentioned quantum mechanics, and we're talking about cosmic scale objects. You've talked about that the evolution of astrophysical disks can be modeled with Schrodinger's equation.

I sure did.

Why? How does quantum mechanics become relevant when you consider the evolution of objects in the solar system?

Yeah. Well, let me take a step back and just say I remember being utterly confused by quantum mechanics when I first learned it. And the Schrodinger equation, which is kind of the parent equation of that whole field, seems to come out of nowhere. The way that I was sort of explaining it, I remember asking my professor, I was like, but where does it come from? Well, just don't worry about it and just calculate the hydrogen, you know, energy levels, right? So it's like I could do all the problems. I just did not have any intuition for where this parent, you know, super important equation came from. Now, down the line, I was, remember, I was preparing for my own lecture and I was trying to understand how waves travel in self gravitating disks. So, you know, again, there's a very broad theory that's already developed, but I was looking for some simpler way to explain it really for the purposes of teaching class. And so I thought, okay, what if I just imagine a disk as an infinite number of concentric circles, right. That interact with each other gravitationally. That's a problem in some sense that I can solve using methods from like the late 1700s. I can write down Hamiltonian. Well, I can write down the energy function basically of their, their interactions. And what I found is that when you take the continuum limit, when you go from discrete circles that are talking to each other gravitationally to a continuum disk, suddenly this gravitational interaction among them, the governing equation becomes the Schrodinger equation. I had to think about that for a little bit.

Did you just unify quantum mechanics and gravity?

No, this is not the same thing as fusing relativity and quantum mechanics, but it did get me thinking a little bit. So the fact that waves in astrophysical disks behave just like wave functions of particles, kind of like an interesting analogy, because for me, it's easier to imagine waves traveling through astrophysical disks or really just sheets of paper. And the reason that analogy exists is because there's actually nothing quantum about the Schrodinger equation. The Schrodinger equation is just a wave equation, and, and all of the interpretation that comes from it is quantum. But the equation itself is not a quantum being.

So you can use it to model waves. It's not turtles, it's waves all the way down. You can pick which level you pick the wave at. So it could be at the solar system level that you can use.

Right. And also it actually provides a pretty neat calculational tool because it's difficult. So we just talked about simulations, but it's difficult to simulate the behavior of astrophysical disks on timescales that are in between a few orbits and their entire evolution. So it's over a timescale of a few orbits. You do a hydrodynamic simulation. Basically, that's something that you can do on a modern computer on a timescale of, say, a week. When it comes to their evolution over their entire lifetime. You don't hope to resolve the orbits, you just kind of hope to understand how the system behaves in between to get access to that. As it turns out, it's pretty cute. You can use the Schrodinger equation to get the answer rapidly. So it's a calculational tool.

That's fascinating. By the way, astrophysical disks, what are they? How broad is this definition?

Okay, so astrophysical disks span a huge, huge amount of ranges. They start maybe at the smallest scale. They start with actually Kuiper Belt objects. Some Kuiper belt objects have rings. So that's maybe the smallest example of an astrophysical disk. We've got this little potato shaped asteroid which is sort of the size of la or something. And around it are some rings of icy matters. That object is a small astrophysical disk. Then you have Saturn, the rings of Saturn. You have the next set of scale. You have the solar system itself when it was forming. You have a disk. Then you have black hole disks, you have galaxies. Disks are super common in the universe. The reason is that stuff rotates, gravity works.

And those rings could be the material that composes those rings. Could be. It could be gas, it could be solid, it could be anything.

That's right. So the disk that made from which the planets emerged was predominantly hydrogen helium gas. On the other hand, the rings of Saturn are made up of icicle little like ice cubes this big, about a centimeter across.

That sounds refreshing. So that's incredible. Hydrogen, helium gas. So in the beginning it was just hydrogen and helium around the sun. How does that lead to the first formations of solid objects in terms of simulation?

Okay, here's the story. So you're like, have you ever been to the desert?

Yes, I've been to the Death Valley. And actually it was terrifying. Just total tangent. I'm distracting you. But I was driving through it and I was really surprised because it was at first hot. And then as it was getting into the evening, there's this huge thunderstorm. Like it was raining and it got freezing cold. Like, what the hell? It was the apocalypse. I had to like just sit there listening to Bruce Springsteen. I remember and just thinking, I'm probably going to die. And I was okay with it because Bruce Springsteen was on the radio.

Look, when you've got the Boss, you know you're ready. You're ready to meet the Boss. Yeah, so look, I mean, that's a good line anyway.

Sorry, the dust.

Yes, that's true. Yeah. By the way, to continue on this tangent I absolutely love the Southwest for this reason. Just during the pandemic, I drove from LA to New Mexico a bunch of times.

The madness of weather.

Yeah, the chaos of weather. The fact that it'll be blazing hot one minute and then it's just like we'll decide to have a little thunderstorm. Maybe we'll decide to go back momentarily to 1000 degrees and then go back to the thunderstorm. It's amazing. That, by the way, is chaos theory in action. But let's get back to talking about the desert. So in the desert, tumbleweeds have a tendency to roll because the wind rolls them. And if you're careful, you'll occasionally see this family of tumbleweeds where there's a big one and then a bunch of little ones that kind of hide in its wake and are all rolling together and still almost looks like, you know, a family of ducks crossing a street or something. Or, for example, you know, if you watch Tour de France, right, you've got a whole bunch of cyclists and they're like cycling, you know, within 10 centimeters of each other. They're not BFFs, right? They're not trying to be. Trying to ride together. They are riding together to minimize the collective, you know, air resistance, if you will, that they experience. Turns out, solids in the protoplanetary disk do just this. There's an instability wherein solid particles, right, things that are a centimeter across, will start to hide behind one another and form these clouds. Why? Because cumulatively, that minimizes the solid component of the disk aerodynamic interaction with the gas. Now these clouds, because they're kind of a favorable energetic condition for the dust to live in, they grow, grow, grow, grow, grow until they become so massive that they collapse under their own weight. That's how the first building blocks of planets form. That's how the big asteroids got there.

That's incredible.

Yeah.

So that. Is that simulatable or is it not useful to simulate?

No, no, that's simulatable. And people do these types of calculations. It's really cool. That's one of the many fields of planet formation theory that is really, really active right now. People are trying to understand all kinds of aspects of that process because, of course, I've explained it as if there's one thing that happens. Turns out it's a beautifully rich dynamic. But qualitatively, formation of the first building blocks actually follows the same sequence as formation of stars, right? Stars are just clouds of gas, hydrogen, helium gas that sit in space and slowly cool. And at some point they contract to A point where their gravity overtakes the thermal pressure support, if you will, and they collapse under their own weight and you get a little baby solar system.

That's amazing. So do you think one day it'll be possible to simulate the full history that took our solar system to what it is today?

Yes, and it'll be useless.

Okay, so you don't think your story, many of the ideas that you have about Jupiter clearing the space, like retelling that story in high resolution, is not that important.

I actually think it's important. But at every stage you have to design your experiments, your numerical computer experiments, so that they test some specific aspect of that evolution. I am not a proponent of doing huge simulations because even if we forget the information theory aspect of not being able to, to simulate in full detail the universe, because if you do, then you have made an actual universe, it's not a simulation. Right. Simulation is in some sense a compression of information. So therefore you must lose detail. But that point aside, if we are able to simulate the entire history of the solar system in excruciating details, I mean, it'll be cool, but it's not going to be any different from observing it. Right? Because theoretical understanding, which is what ultimately I'm interested in, comes from taking complex things and reducing them down to something that, you know, some mechanism that you can actually quantify. That's the fun part of astrophysics, just kind of simulating things in extreme detail is we'll make cool visualizations, but that doesn't get you to any better understanding than you had before you did the simulation.

If you ask very specific questions, then you'll be able to create very highly compressed, nice beautiful theories about how things evolved. And then you can use those to then generalize to other solar systems, to other stars and other galaxies, and say something generalizable about the entire universe. How difficult would it be to simulate our solar system such that we would not know the difference? Meaning if we are living in a simulation, is there a nice, think of it as a video game, Is there a nice compressible way of doing that? Or just kind of like you intuited with a three body situation is just a giant mess, that you cannot create a video game that will seem realistic without actually building universe from scratch?

I'm speculating, but one of the. Yeah, I know you have a deep understanding of this, but for me I'm just going to speculate that at least in the types of simulations that we can do today, inevitably you run into the problem of resolution. Right? Doesn't matter what you're doing, it is discrete. Now, the way you would go about asking what we're observing, is that a simulation or is that some real continuous thing? Is you zoom in. You zoom in and try and find the grid scale, if you will. Yeah, I mean, It's a really interesting question. And because the solar system itself and really, you know, the double pendulum is chaotic, right? Pendulum sitting on another pendulum moves unpredictably once you let them go. You really don't need to inject any randomness into a simulation for it to give you stochastic and unpredictable answers. Weather is a great example of this. Weather has a Lapunov time of typical weather systems have a lap enough time of a few days. There's a fundamental reason why the forecast always sucks two weeks in advance. It's not that we don't know the equations that govern the atmosphere. We know them well. Their solutions are meaningless, though, after a few days.

The zooming in thing is very interesting. I, I think about this a lot, whether there'll be a time soon where we would want to stay in video game worlds, whether it's virtual reality or just playing video games.

I mean, I think that time like came in like the 90s and it's been that time.

Well, it's not just, it's not just game. It's. I mean, it's accelerated. I just recently saw that wow and Fortnite were played 140 billion hours. And those are just video games. And that's like increasing very, very quickly, especially with the people coming up now being born now and become, you know, becoming teenagers and so on. Let's have a thought experiment where it's just you and a video game character inside a room. Well, you remove the simulation. They need to simulate sort of a lot of objects. If it's just you and that character, how far do you need to simulate in terms of zooming in for it to be very real to you? As real as reality. So like, first of all, you kind of mentioned zooming in, which is fascinating because we have these tools of science that allow us to zoom in, quote, unquote, in all kinds of ways in the world around us. But our cognitive abilities, like our perception system as humans is very limited in terms of simulation. So we might be very easily fooled.

Some of the video games, like on the PS4 look pretty real to me. Right. I think you would really have to interrogate. I mean, I think even with what we have today, I don't know. Ace Combat 7 is a great example. Right. Like I mean, the way that the clouds are rendered, it's, I mean, looks just like when you're flying, you know, on a real airplane. The, the kind of transparency. I think that the, you know, our perception is limited enough already to not be able to tell some of the, you know, some of the differences.

There's a game called Skyrim. It's an Elder Scrolls role playing game. And I just, I played it for quite a bit and I, I think I played very different than others. Like, there'll be long stretches of time where I would just walk around and look at nature in a game. It's incredible.

Oh, sure.

It's just like the graphics is like, wow, I want to stay there. It was better. I went hiking recently. It was like as good as hiking.

So. Look, I know what you mean. Not to go on a huge video game, you know, tangent, but like the, the third, like Witcher game was astonishingly beautiful, right? Especially like playing on, on a good hardware machine. It's like, this is pretty, this is pretty legit. That said, you know, I, I don't resonate with the. I want to stay here. You know, like, one of the things that I love to do is to go to my, like boxing gym and, and box with a guy, right? Like that's, there's, there's nothing quite like that physical, you know, experience, like, that's fascinating.

That might be simply an artifact of the year you were born. Maybe because if you're born today, it almost seems like stupid to go to a gym. Like you're going to a gym to box with a guy. Why not box with Mike Tyson when you yourself is like in his prime, when you yourself are also an incredible boxer in the video game world?

For me, there's a multitude of reasons why I don't want to box with Mike Tyson.

No, no, no, no.

I enjoy teeth and I want to have an ear.

No, but your skills in this meat space, in this physical realm is very limited and takes a lot of work. And you're a musician, you're incredible scientist, you only have so much time in the day. But in the video game world, you can expand your capabilities in all kinds of dimensions that you can never have possibly have time in the physical world. And so that it doesn't make sense to be existing, to be working your ass off in the physical world when you can just be super successful in the video game world.

But I still.

You enjoy sucking at stuff.

Yeah, I, I really struggling to get better. I sure do. I mean, I think like these days with music Music is a great example. Right. We just started, you know, practicing live with my band again, you know, after not playing for a year. And, you know, it's just. It was terrible. Like, we were just kind of. A lot of the nuance, you know, a lot of the detail is just that detail that takes, you know, years of collective practice to develop is just lost. But it was just an incredible amount of fun. Way more fun than all the, like, studio, you know, sitting around and playing that I did, you know, throughout the entire year. So I think there's something. There's something intangible or maybe. Maybe tangible about being. Being in person. I sure hope you're wrong and that. That's not something that will get lost, because I think there's, like, such a large part of the human condition is to hang out. If we were doing this interview on Zoom, right? I mean, I'd already be. I'd already be bored out of my mind.

Exactly. I mean, there's something to that. I mean, I'm almost playing devil's advocate, but at the same time, you know, I'm sure people talk about the same way at the beginning of the 20th century about horses, where they are much more efficient, they're much easier to maintain than cars. It doesn't make sense to have, you know, all the ways that cars break down, and there's not enough infrastructure in terms of roads for cars. It doesn't make any sense. Like horses and like nature. You could do the nature, like, where, you know, you should be living more natural life. Those are real. You don't want machines in your life. They're going to pollute your mind in the minds of young people. But then eventually, just cars took over. So in that same way, it just.

Seems going back to horses just, you know, well, you can be.

You can play. What is it? Red Dead.

Red Dead. Redemption.

Redemption. And you can ride horses in the video game world.

That's true.

So let me return us back to.

Planet nine, always a good place to come back to.

So now that we did a big historical overview of our solar system. What is Planet nine?

Okay. Planet nine is a hypothetical object that orbits the solar system at an orbital period of about 10,000 years and an orbit which is slightly tilted with respect to the plane of the solar system, slightly eccentric, and the object itself, we think, is five times more massive than the Earth. We have never seen Planet nine in a telescope, but we have gravitational evidence for it.

And so this is where all the stuff we've been talking about, this clustering, ideas, maybe you can speak to the approximate location that we suspect. And also the question I wanted to ask is what are we supposed to be imagining here? Because you said there are certain objects in the Kuiper Belt that kind of have a direction to them, that they're all like, like flocking in some kind of way. So that's a sense that there's some kind of gravitational object not changing their.

Orbit, but kind of confining them. Right. Finding like grouping their orbits together. See, what would happen if planet nine were not there is these orbits that roughly share a common orientation. They would just disperse, Right. They would just become as a mutually symmetric point everywhere. Planet nine's gravity makes it such that these objects stay in a state that's basically anti aligned with respect to the orbit of planet nine and sort of hang out there and kind of oscillate on timescale of about a billion years. That's one of the lines of evidence for the existence of planet nine. There are others. That's the one that's easiest to maybe visualize just because it's fun to think about orbits that all point into the same direction. But I should emphasize that, for example, the existence of objects, again, Kuiper Belt objects that are heavily out of the plane of the solar system, Things that are tilted by, say, 90 degrees. We don't expect that as an outcome of planet formation. Indeed, planet formation simulations have never produced such objects without some extrinsic gravitational force. Planet nine, on the other hand, generates them very readily. So that provides kind of an alternative, you know, population of small bodies in the solar system that also get produced by planet nine through an independent kind of gravitational effect. So they're kind of. There's basically five different things that planet nine does individually that are like kind of maybe a one sigma effect where you'd say, yeah, okay, if that's all it was, maybe it's no reason to jump up and down. But because it's a multitude of these puzzles that all are explained by one hypothesis that's really the magnetism, the attraction of the Planet 9 model.

So can you just clarify? So most orbit, most planets in the solar system orbit at approximately the same. So it's flat.

Yeah, it's like one degree. The difference between them is about one degree.

But nevertheless, if we looked at our solar system, it would look and could see every single object. It would look like a sphere.

The inner part where the planets are would look flat. Right. The Kuiper belt and the asteroid belt have a larger.

It gets fatter and fatter it's kind of like a sphere.

That's right. And if you look at the very outside, it's polluted by this quasi spheroidal thing. Nobody's, of course, ever seen the Oort cloud. Right. We've only seen comets that come from the Oort cloud. So the Oort cloud, which is this population of distant debris, its existence is also inferred. You could say, alternatively, there's a big cosmic creature that occasionally sitting at 20,000 AU and occasionally throws an icy rock towards the sun.

Spaghetti Monster, I think it's called. Okay, so it's a mystery in many ways, but you can kind of infer a bunch of things about it. By the way, both terrifying and exciting that there's this vast darkness all around us that's full of objects that are just throwing.

Just there. Yeah. It's actually kind of astonishing, right, that we have only explored a small fraction of the solar system. Right. That, that really kind of baffles me because, remember, as a student, you know, studying physics, you know, you do the problem where you put the Earth around the sun, you solve that and like it's one line of math and you say, okay, well, that surely was figured out by Newton. So, like, all the interesting stuff is not in the solar system, but that it's just plainly not true. There are, there are mysteries in the solar system that are remarkable that we are only now starting to just kind of scratch the surface of.

And some of those objects probably have some information about the history of our solar system.

Absolutely, absolutely. Like a great example is, you know, small meteorites. Right. Small meteorites are melted. Right. They have, they're differentiated, meaning some of the iron sinks. You say, how can that be? Because they're so small that they wouldn't have melted just from the heat of their accretion. Turns out the fact that the solar nebula, the disk that made the planets was polluted by Aluminum 26, is in itself a remarkable thing. It means the solar system did not form in isolation. It formed in a giant cloud of thousands of other stars that were also forming, some of which were undergoing, going through supernova explosions and releasing these unstable isotopes of which we now see kind of the traces of. It's so cool.

Do you think it's possible that life from other solar systems was injected and that was what was the origin of life on Earth?

Yeah, the panspermia idea.

That's seen as a low probability event by people who studied the origin of life. But that's because then they would be out of a job.

Well, I don't think they'd be out of the job because you just then have to figure out how life started there.

But then you have to go there. We can study life on Earth much easier. We could study it in the lab much easier because we could replicate conditions that are from an early life Earth much easier from a chemistry perspective, from a biology perspective. You can intuit a bunch of stuff. You can look at different parts of.

Earth and just to an extent, I mean, the early Earth was completely unlike the current Earth. Right. There was no oxygen. So one of my colleagues at Caltech, Joe Kirchnik, is certain, right. Something like 100% certainty that life started on Mars and came to Earth on Martian meteorites. This is not a problem that I like to kind of think about too much, like the origin of life. It's a fascinating problem, but, you know, it's not physics. And I just, like, I just don't, don't love it.

It's the same reason you love. I thought you're a musician. So music is not physics either. So why are you showing 100% physics?

Okay, no, no, look, in all seriousness, though, there are a few things that I really, really enjoy. I genuinely enjoy physics, I genuinely enjoy music, I genuinely enjoy martial arts, and I genuinely enjoy my family. I should have said that all in a reverse order or something. But I like to focus on these things and not worry too, about everything else. You know what I mean?

Yes.

Just because there is a. Like you said earlier, there's a time constraint. You can't do it all.

There's many mysteries all around us, and they're all beautiful in different ways. To me, that thing I love is artificial intelligence, that perhaps I love it because eventually I'm trying to suck up to our future overlords. The question of. You said there's a lot of kind of little pieces of evidence for this thing. That's Planet nine. If we were to try to collect more evidence or be certain, like a paper that says, like, you drop it, clear, we're done. What does that require? Does that require us sending probes out, or do you think we can do it from telescopes here on Earth? What are the different ideas for conclusive evidence for Planet Nine?

The moment Planet nine gets imaged from a telescope on Earth, it's done. I mean, it's just there.

Can you clarify? Because you mentioned that before. From an image, would you be able to tell.

Yes. So from an image, the moment you see something, something that is reflecting sunlight back at you and you know that it's hundreds of times as far away from the sun as, as is the Earth. You're done.

So you're thinking, so basically if you have a really far away thing that's big.

Yeah.

You know, five times the size of Earth, that means.

That is planet.

Could there be multiple objects like that?

I guess in principle, yeah. I mean there's no, there's no law of physics that that doesn't allow you to have multiple. There's also no evidence at present for there being multiple.

I wonder if it's possible. So just like we're finding exoplanets, whether given the size of the Oort cloud, there's basically, it's rare and rarer, but they're sprinkled planet 9, 10, 11, 12. Like these.

Some got 13.

Yeah, it goes after that. I can just keep counting. So like just something about the dynamic system, like it becomes lower and lower probability event, but they gather up like they become larger and larger. Maybe something like that. I wonder if discovering planet nine will just be almost like a springboard. It's like, well, what's beyond that?

It's entirely plausible. The Oort cloud itself probably holds about 5 earth masses or 7 earth masses of material. Right. So it's not nothing. And the, it all ultimately comes down to at what point will the observational surveys sample enough of the solar system to kind of reveal interesting things? There's a great analogy here with Neptune and the story of how Neptune was discovered. Neptune was not discovered by looking at the sky. Right. It was discovered by. It was discovered mathematically. Right. So yeah, the orbit of Uranus when Uranus was found, this was 1781. It's the kind of tracking of both the tracking of the orbit of Uranus as well as the reconstruction of the orbit of Uranus immediately revealed that it was not following the orbit that it was supposed to. Right. The, the predicted orbit deviated away from where it actually was. So in the mid-1800s. Right. A French mathematician by the name of Orban Le Verrier did a beautifully sophisticated calculation which said if this is due to gravity of a more distant planet, then that planet is there. Okay. And then they found it. But the point is, the understanding of where to look for Neptune came entirely out of celestial mechanics. The case with planet nine is a little bit different because what we can do I think relatively well is predict the orbit and mass of planet nine. We cannot tell you where it is on its orbit. The reason is we haven't seen the Kuiper Belt objects complete an orbit their own orbit even once. Because it takes 4,000 years. But I plan to live on as an AI being, and I'll be tracking those orbits as all it takes four.

Or five thousand years. I mean, it doesn't have to be AI, it could be longevity. There's a lot of really exciting genetic engineering research. So you'll just be a brain waiting, your brain waiting for the orbit to complete for the basic Kuiper Belt objects.

That's. That's right. That's like kind of the worst reason to want to live a long time, right? Just like, can the brain, like, smoke a cigarette?

I know.

Can you just like, light one up while you're waiting or.

But you're making me actually realize that the one way to explore the. The galaxy is by just sitting here on Earth and waiting. So if we can just get really good at waiting. It's like a Muamua or these interstellar objects that fly in. You can just wait for them to come to you. Same with the aliens. You can wait for them to come to you. If you get really good at waiting, then that's one way to do the exploration. Because eventually the thing will come to you. Maybe that's the entire. Maybe the intelligent alien civilizations get much better at waiting. And so they all decide so game theoretically to start waiting. And it's just a bunch of like, ancient intelligent civilizations of aliens all throughout the universe. They're just sitting there waiting for each other.

Look, you can't just be good at waiting. You got to know how to chill, okay? Like, like you can't just like, sit around and do nothing. You got to be. You got. You got to know how to chill.

I honestly think that as we progress, if the aliens are anything like us, we enjoy loving things. We do. And it's, it's very possible that we just figure out mechanisms here on Earth to enjoy our life and we just stay here on Earth forever. Forever. That exploration becomes less and less of an interesting thing to do. And so you basically, yes, wait and chill. You get really optimally good at chilling and thereby exploring is not that interesting. So in terms of 4,000 years would be nothing for scientists. We'll be chilling and just all kinds of scientific explorations will become possible because we'll just be so chill. You have a paper out recently because you already mentioned some of these ideas, but I'd love it if you could dig into it a little bit. The injection of inner Oort cloud objects into the distant Kuiper Belt by Planet Nine. What is this idea of Planet nine injecting objects into the Kuiper Belt.

Okay, let me take a brief step back and say when we do calculations of planet nine, when we do the simulations, as far as our simulations are concerned, sort of the Neptune like kind of the trans Neptunian solar system is entirely sourced from the inside, namely the Kuiper Belt gets scattered by Neptune and then planet nine does things to it and aligns the orbits and, and so on. And then we calculate what happens on the lifetime of the solar system, yada, yada yada. During the pandemic, one of the kind of questions we asked ourselves, and this is indeed something we, Mike and I, Mike Brown, who's a partner in crime on this, and I do regularly, is we say how can we A disprove ourselves and B, how can we improve our simulations? Like what's missing? One idea that maybe should have been obvious in retrospect is that all of our simulations treated the solar system as some isolated creature. But the solar system did not form in isolation. It formed in this cluster of stars. And during that phase of forming, together with thousands of other stars, we believe the solar system formed this almost spherical population of icy debris that sits maybe at few thousand times the separation between the Earth and the sun, maybe even a little bit closer. If planet nine is not there, that population is completely dormant. And these objects just slowly orbit the sun. Nothing interesting happens to them ever. But what we realize that if planet nine is there, Planet nine can actually grab some of those objects and gravitationally reinject them into the distant solar system. So we thought, okay, let's look into this with numerical experiments. Do our simulations, does this process work? And if it works, what are its consequences? So it turns out indeed, not only does planet 9 inject these distant inner Oort cloud objects into the Kuiper Belt, they follow roughly the same pathway as the objects that are being scattered out. So there's this kind of two way river of material, some of it is coming out from.