Daniel and Jorge Explain the UniverseDaniel and Jorge talk about how instabilities in the early solar system may have ejected an ancient ice giant.
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Hey Daniel, what pets do you have these days?
Oh, we just have our rescue dog, Peppite.
Do haven't you had other pets in the past, like rodents.
We did have rats for a time, and we actually had cats before that.
Oh what happened Pepito ate them?
No, we've never had a pet eat another pet. We've only lost them to old age.
Old age. Huh. All that dark chocolate and big goods in your house just did them in with the heart attack.
We don't feed dark chocolate at the dog, but everybody does eat pretty well at our house.
Unless you like white chocolate, then your starve.
You know, if that's the reason you run away from home, then maybe you never were really a white sun.
Wait, if you don't like white chocolate, you're not a white son. It sounds like a white lie.
I have a dark secret.
Hi.
I am Poor hammy cartoonists and the author of Oliver's Great Big Universe.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I will not waiver in my campaign for dark chocolate.
Dark chocolate, dark matter. You're just a very dark physicist.
I'm trying to bring light to the world at the same time as exposed all the dark secrets of the universe.
Oh, you're trying to expose dark matter. I thought you were all about letting the universe.
Be Absolutely not. I do not believe in universe privacy.
You're like the universe paparazzi.
That's exactly right, except I'm not selling it to the National Inquirer. I'm just publishing papers.
Well, your buyer is the cosmological inquirer, the human inquirer.
Inquiring brains want to know.
Yeah, do you stand outside their home, like, hey, dark matter over here, over here, snapping pictures.
If I knew where dark matter lived, I would definitely go there with my dark matter camera.
I thought dark matter was all around us. It lives in US and within us. It surrounds us and binds the galaxy together.
You're absolutely right, it's everywhere. We just don't know how to take a picture of it.
But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we join our inquiring minds with yours to wonder together about the nature of the universe, To think deeply about how everything comes together to make the cosmos and the night sky that we appreciate, To think about how the tiniest particles and the most massive black holes shape the very world we live in, and whether it has always looked this way.
That's right. We satisfy our curiosity for stars and what they're doing with their lives, and we take pictures and also sound recordings of what's out there in the universe and what's going on to maybe get a clue about how it all works.
We'd like to figure out the fundamental nature of the universal laws that everything follows. But we'd also like to know the story of the universe. What happened, How did we end up where we are? How long have things looked this way for? How long can we rely on things to look this way? Do we live in a momentary blip of the universe or is this a long term trend?
Yeah, Looking at our past is a way to look into our future. We can try to deduce what the rules of the universe are and what they might mean for us in the deep future. What is going to be the future of humanity here in our solar system? Can we call this our home for the next few billion years?
Are you not planning to move out of that house for a few billion years so your kids can always come home and their kids and their kids and their kids.
But we're kind of just squatting in this solar system, right like we just popped in here, started living here. We didn't ask if anyone owned these planets. What if the real owners come back one day they're like, what is going on here?
Call pest control, m Don't we have some sort of like solar B and B contract squatter rights? Maybe exactly after one hundred million years we're officially allowed to call ourselves the owners. But it's a good question how long things have looked this way. When you look by the night guy, you expect to see basically the same stars as you did a year ago, and you know that you're looking at roughly the same stars that Newton looked at, and the Egyptians looked at and the Sumerians looked at thousands of years ago. But the solar system operates on a very different kind of time scale than your life or even human civilization, and in fast forward things don't seem so stable. They seem quite chaotic and dynamic.
Yeah. When we were all kids, we learned in school about the different planets in our Solar system and how many of there are. And that's basically the same story that our kids are learning in school as well, right, Like, it hasn't really changed on much, except maybe for Pluto.
We of course change what we mean by a planet and make up new categories all the time. But you're right, the stuff that's out there that we're seeing, whatever name we give it, hasn't changed in our lifetime or in our grandparents' lifetime.
Yeah, And so I guess you kind of get the sense that maybe it will never change, you know, you sort of memorize these facts and these things and think that maybe it's going to be like that forever. But actually, if you look at the grand scale of the Solar System and the universe in our galaxy, things are rapidly changing. If you look at it from that point of view.
If the Solar System changes, do you think we all have to go back to elementary school to learn a new mnemonic?
Oh, there's a mnemonic. I didn't grow up yours. I don't know what do you use? Obviously the mnemonic didn't work because you don't remember it.
There's a lot of mnemonics to help you memorize the order of the planets. One of them is my very easy method, just speeds up nothing, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Whoa, that is so not kit friendly. How many kids use methods and have methods?
The history of them is actually really funny. There's ones from the fifties that go like, men very easily make jugs, serve useful needs.
Perhaps, Oh, man, I wonder why we'd stopped using that one.
And then a more recent one says, my very energetic mother jumps skateboards under nana's patio.
Oh, there you go. That's a pretty good one, and true as well for some people.
I'm sure my very educated mother just served usnaches. Oh that's an even tastier one, exactly. But the point is that though these things have seemed stable for a long time, it's not necessarily true that they always will be.
Yeah, things are always changing, and in fact, you can ask the question of whether our solar sysm had more planets in the past.
It might be that the planets we know today are not all the planets that have ever orbited our star.
And if we had more, what happened to them? So today on the podcast we'll be tackling the question has our Solar system lost any planets? That just seems kind of irresponsible there. How can you lose a whole planet?
I mean I had it in my hands and then I put my keys down and the last.
Place having that planet was on top of the dog.
Blamed the dog. Huh, that's the first thing.
The dog ate my planet.
Classic excuse, or maybe it just went rogue because it needed to find a white chocolate friendly solar system, you know, maybe it just didn't fit in here.
It just rebelled against your tyranny of trying to dictate what kind of chocolate people should eat or feel good about eating. Doctor Wison, you call it tyranny.
I call it wisdom. Let's call the whole thing off.
Yeah, that's what I'll tire and say at two. But yeah, So it's been an interesting story of the Solar System. You got to wonder if maybe if we had more than nine or eight planets in the past, Well, we definitely had more planets to pay in the past before Pluto got downgraded, but that's a separate story and a separate reason.
Right.
Yeah, Pluto is still there. It's just not called the planet anymore. It's called a dwarf planet.
Right. But we can ask a question of whether our Solar system did really have other giant planets like Jupiter or Mars or Venus. But maybe they decided they didn't like it here.
It's really fun to dig into the history of the Solar System and understand how we got here, how it might have been different, and give us a sense for what other solar systems out there are likely to look like.
So, as usual, we were wondering how many people had thought about the question of whether our Solar system lost any planets, or at least misplaced them temporarily. Maybe, So, as usual, Daniel went out there into the internet to ask people has our Solar system lost any planets?
Thanks very much to our group of volunteers. We greatly appreciate them, but we also would like to add you to their ranks. Please don't be shy write to me to questions at Danielanjorge dot com.
What do people get I know if they sign up?
The satisfaction of hearing their voice on the podcast, and a weekly injection of hard physics questions.
And also a monthly supply of white chocolate that gets kicked out of your house.
I will send you exactly zero grams of white chocolate.
Well, think about it for a second. Do you think our Solar System has lost any planets? Here's what people had to say.
I don't know if we can know for sure, maybe by the orbits of current planets, but I'd have to assume given the five billion years or so that or some spent around, that at least one planet has come in and been kicked out. But maybe it depends on if we consider those objects planets.
So I think that there have been planets knocked out of the Solar System, especially since when the Solar System was first created there would have been loads of rocks flying around to form planets. So then there would have been planets formed and then hit by maybe another planet which knocked them out of the system.
Not that I know of, they're all accounted for. Some of them have lost the designation planet, like Pluto. I think regularly objects get flung out of Solar systems due to gravitational interactions with other.
Lown j objects.
So I can imagine Jupiter getting tired of some planet and flinging it out. Maybe when we're forming all the planets performing them, some of them were close to the Sun and got gabbled up. I'm curious to know if there's any record of planets that we're here in now or not.
Apart from the reclassification of Pluto as a dwarf planet, meaning that we've effectively lost one planet, I have heard rumors about a tenth planet, which would now be a ninth planet, that potentially orbited within our inner Solar System and then could have collided with Earth and created the Moon and then spun off out into an orbit way out in our outer Sodo System. Other than that, I'm unaware of any lost planets.
Interesting answers. It seems to be all over the place. Some people say yes, some people say no, not really, some people say poor Pluto.
There does generally seem to be an appreciation of the fact that the Solar System might not have always been an orderly, stately placed that there might have been primordial chaos.
That's right. It was a big party here in the Solar System, where we're kind of in the after party of the Solar System.
Right, We're waking up the next morning going, man, what happened? And has anybody seen a dog?
Yeah? Well, why am I waking up next to Venus here? How did that happen?
And why is the hot tub filled with white chocolate?
Yeah? So let's start with the basics, Daniel, is it even possible for Solar system to lose the planet? I thought that, you know, once you form, things are kind of stuck to you gravitationally in orbits, or that at least that it's hard to escape the gravitational field of like Sun or all these planets. Wouldn't they either fall in or go into a stable orbit.
I think the key idea is the word you used, form, Like, when do you consider the Solar System to have formed? The Solar system? Formation is a slow and gradual, constant process. It's basically always changing. And so you can go all the way back to the very beginning of the Solar system to understand the chaos of that formation and understand that that formation is a constant process, that things are always potentially bumping into each other and disturbing each other.
Wait, are you saying that if we leave the window open for the fact that maybe the Solar System is still forming. Does that mean we technically haven't lost any planets, like Kenny you said in my real life.
No, it just means that during the formation, planets could form and be lost. There is no final form to the Solar System. It's a constantly evolving thing. It's not like at some point somebody says, Okay, the Solar System is finished, let's package it and ship it and move on to the next project.
I see. It's like a Pokemon, is what you're saying. It's always evolving, it's looking for its final form.
I don't know enough about Pokemon to know whether that analogy holds, so I'm just gonna trust you on that.
I don't know either. To be honest, I just heard final form and it made me think of Pokemon.
Well, it's sort of in the same way that animals never have a final form. Evolution is a constant process. Things are always changing in response to the environment.
Except for crocodiles and sharks, they're pretty settled there in their plateau.
Yeah that's true.
Yeah, but let me take us through some of the early history of the Solar System. How do we get planets in the first place.
So planetary formation is a super fascinating topic and it helps us understand like the formation of the Solar System as a whole. Remember that the Solar System forms from the collapse of a huge cloud of like gas and dust. It's mostly hydrogen, which is made in the Big Bang, and it's also interspersed with a bunch of other heavier stuff that's made from other solar systems where the stars have already fused heavier elements out of that hydrogen. So you have this big cloud of mostly hydrogen with a few heavier bits in it, and it collapses into stars. You don't just get one solar system. You typically get several made at the same time. In one of these stellar nurseries, we have a big blob of this stuff and it collapses and most of the stuff goes into the center to make a star, like ninety nine percent of the stuff goes in to make the star. But you typically have a disk of gas and dust that's orbiting that star. It's spinning too fast to collapse in the way the Moon is orbiting the Earth without falling into the Earth, and so you get this protoplanetary disk around this new star and that disc then coalesces into larger stuff. Gravity is doing the work there to pull the gas and dust in the disc into heavier things. And where you have like little spots of iron or little spots of heavier metals, those things will use their gravity to form larger objects.
But I think the Solar system formed into a disc first, and then the star kind of ignited.
Right. The moment of ignition depends a little bit on the mass of the star. I mean, in some cases you don't even get ignition if there isn't enough mass there. You have like a subcritical brown dwarf, but it's definitely collapsing into a disc as it forms, right, a big amorphous blob is going to collapse, and it's going to collapse into a disc shape because of its angular rotation. So the two things can sort of happen simultaneously. And when ignition happens depends on the mass of the star, right.
And there's also kind of an intermediate step there where the disc kind of turns into rings for a while. Right before you get the planets.
Exactly, you get the seeding of structure and they pull together into larger and larger objects, and rings are basically just clusters of larger objects. So you get these gaps emerging, and then you get those things formed together into planets or not, depending on the tidal forces. A large object can form and sort of gather up a lot of the gas and dust near it, and then it can also distort the other stuff nearby, preventing it from forming. So it's a bit of a chaotic process in the beginning, and it also depends a little bit on your distance from the star. As a point, it's called the snow line, after which water tends to be ice tends to be solid, and before which it tends to be vapor. Like if you're close enough to the star, it's warm enough that the water is vapor and if you're further from that point, the water is frozen. That helps form giant planets. So you tend to have these large planets with ice and rock seeding structure on the outer part of the Solar System past the snow line, and then less ice, so smaller planets before the snow line in the inner Solar System.
Right in the inner Solar System, you get all the rocky planets.
Right exactly because the gas there is blown away by the radiation from the ignition of the Sun in the very beginning of the Solar System, the Sun is pumping out a huge amount of ultraviolet, very high energy photons, which tends to blast the inner planets clean, which is why, like our initial atmosphere on Earth was blown off by this stellar wind in the very early years of the Solar System. So you get the rocky planets in the core, and then you get the gas and ice giants out past the snow line.
Right, And I think the process is like, yep, these rings kind of like Saturn has rings right now, and the rings eventually little by little collapse into planets or first planet tesimals first, right.
Yeah, planet tesimal is a super fun word. They sound like many cute little planets where they're basically like building blocks of planets, and they don't always form, right, which is why you have like the asteroid belt and the Kuiper Belt. It depends on the tidal forces of the nearby stuff, so it's not happening in isolation. This is complicated interplay between all of the objects.
Right, But it's kind of a bit of a runaway process, like once you seed a planet or once more you know, some planetesimals mosh together, then that becomes kind of a center of gravity and more and more stone falls into it, and that's kind of how you get a planet, right.
Yeah, that's kind of how you get a planet exactly. And in this initial picture, everything forms very orderly, like they tend to be mostly in the same plane, and it be mostly circular because you have this big disc as we say, that collapses in the rings and then planeti ismals and then planets. But once you have these large objects formed with their own significant gravity, then they can start to tug on each other pretty hard, and you can get instabilities, you can get chaos, you can get resonances, and that's how planets can migrate, and they can tug on each other and you might even lose.
One, right, because I guess there's no guarantee that your orbit is going to be stable. I mean, it's such a complex and you know, there's so many things moving around that there's no guarantee that you're even if you're orbiting around the Sun, you're going to be there forever, because something else might come around and knock you off your orbit or pull you away from your orbit, right.
And that can be things from outside the Solar system, like a passing star can nudge something and perturb the otherwise stable orbits of the Solar Like even just a little nudge from a star that's coming nearby, it's not like it has to pass right through the Solar system, can cause a cascade effect of instabilities. But also just like the planetismals and the Kuiper Belt or the asteroid belt can tug on stuff, and enough of that happening can cause things to.
Go wonky and wonky they might have gone in our Solar system, perhaps wonk enough to lose a couple of planets here and there. And so let's get into that idea and whether or not we did misplace a couple of planets in our history. But first let's take a quick break.
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All Right, we're talking about whether our solar system has lost any planets. I feel like that sounds very irresponsible of the solar system. Can would you say, like, have any planets escaped our solar system?
Maybe we've grown up and graduated planets. They're like off into the universe living their best lives.
Yeah, there you go. You don't want it to live at home forever.
Exactly when your kid graduates and goes to college, you don't consider that you've lost them.
Yeah, there you go. So have we shepherded planets out into the larger cosmos is the question of the day.
Yeah, in this scenario, they would like come back and visit with their own little moons or something that we could coop over. Oh look, how cute.
Oh yeah, yeah, except we already turned their bedroom into like a workout room or a craft room, and so now there's no room for them.
Sorry about that, your orbits being used?
Yes, sorry, you'll have to airbnb a nearby apartment or something. But yeah, so it's possible in the early chaos of a solar system to lose a planet, right because things aren't quite settled. Even though we're all bound gravitationally to the central star, things can get kind of wonky and maybe wanky enough to actually fling a planet out into space.
Exactly and when this happens is sort of the most recent question people have been struggling with. There's a classic model of the formation of the Solar System and how the planets move around that we'll talk about. It's called the Nie model because it was developed by researchers in Nice, France, that has a bit of an issue with when exactly all this chaos happened. That might be solved by a more recent model with a different picture for how these instabilities might have been triggered.
Well, now, I wonder if some listeners out there might be confused about how you can lose a planet, Like, there isn't that much around us, right in terms of other Solar systems or any large stars or galaxy or you know, very heavy objects. So even if something gets flung into space, wouldn't it eventually come back?
You're right that space near the Solar System is pretty empty. I mean, the closest star is light years away, and so its gravitational pull is pretty weak. But you can still have an escape velocity. If you throw something hard enough off the Earth, it will leave the Earth and never return. If you throw something out of the Solar system with enough velocity enough to escape the gravitational well of the Solar system, then it will not return.
Mmmmm yeah, I think we talked about that in an other episode. It's kind of a weird kind of math, right, Like you need to have enough velocity so that as you get further and further, the pool of gravity pulling your back gets weaker and weaker, and so actually you kind of outrun the pool of gravity exactly.
It seemed confusing because you know that gravity has an infinite extent, like no matter how far away you are, and the Sun is always pulling on you. But just because there are infinite number of contributions doesn't mean it adds up to an infinite force. It's just like any integral or converging series. It can add to a finite amount of energy. So as long as you have more energy, then the sum of all the tugs the Sun will ever pull on you, you can escape the Solar System. So if, for example, you have a planet that gets a push from another planet and gets flung out into the deep dark space, it might never return.
Yeah, I guess sort of like we've done with spacecraft that we sent out into space. Right like it left Earth eventually it was going so fast it left the gravity of Earth and through the gravity of maybe other planets. And now we have some that are going out of the Solar System exactly.
That can be a little bit more complicated because they can have rockets and they can use gravitational assists from other planets, but the principle is the same. Like Voyager and Pioneer, they have enough velocity that they're leaving the Solar System without any more rocket burns or gravitational assists. It's definitely possible to leave home.
All right.
Well, now the question here today is have we actually lost any planets? Did the Solar System have more planets than the eight that we have now, and have we misplaced any or have any left home?
But if you look at the pattern of the planets that we have now and you try to tell a story about how we got there, it's a hard thing to do without another planet. The patterns that we see in the eccentricities of the planets and the structure of the Kuiper Belt and the asteroids is much easier to explain if there was at one point another ice giant like Neptune that was flung out of the Solar System.
I mean, you sort of look at how the planets are moving now and their orbits, and you basically hit the rewind button, kind of like you use math and a computer to backtrack what the Solar System was doing millions and millions of years ago, and you're saying that it doesn't make sense or what it.
Actually usually works in the forward direction, like you start from the Protosolar System and try to evolve forward and see if it matches what we see today. Conceptually, it's the same as backtracking, but the way the simulations actually work is forwards. You know, we model physics equations forwards in time, and we try to see if we can get to the current Solar System and explain everything we see. And what we find is that doesn't really work without another planet.
But are there like a million things that could have happened in between absolutely, how do you make it match what we have now? Like, what do you start with and if you can make it match, how do you know it's not just you're making an error.
Absolutely, it's not definitive, right, it's statistical. It's totally possible that our solar system could have arisen without another planet, but it's just a question of what's more likely. Like, when you run the simulations of our solar system, how many times do you get to something like what we have now with a lost planet and without a lost planet? And so is it just easier to make this arrangement with a lost planet or without. It's totally possible to do it without, but it's just less likely. It happens in fewer of those simulations.
And by like what we have now, you don't mean like exactly what we have now, just kind of like sort of like what we have now.
If you run enough simulations, you can get essentially a sense for what's more likely and what's less likely under various hypotheses. And if you have another planet in your system, then you get more simulations that are similar to ours. Yeah, so more like probability gets clustered in the kind of arrangement that we have now. And the cool thing about that is that it tells the story. You can look at those simulations and you can see, oh, what happened in the inner what happened in the early days of the Solar System? How did this happen?
Okay, So then scientists have been running these simulations and it's kind of hard to get what we have now without some mystery planet that moved away from the Solar System. How did scientists think that happen?
So the original models, called the NIE model, basically blames it on the Kuiper Belt. So in the original Solar system, you have Jupiter, Saturn, Urinus, Neptune, all formed in very nice, neat circular orbits like we talked about, and fairly closely spaced to each other. But then you have these planetismals out in the Kuiper Belt that haven't formed into planets, but they're tugging on Neptune, they're tugging on Urinus, they're tugging on Saturn, and they get pulled into the inner Solar System. And when that happens, these big planets get pushed out a little bit, and then the planetismal falls further into the Solar System until it reaches Jupiter, and then Jupiter actually pushes it back out and Jupiter gets pushed in the effect of these little planetismals, these little tugs is to pull out Neptune Saturn uriness and to push Jupiter in a little bit. So they're disturbing the Solar System. And you might think, well, what can one little rock do? And the key is that there's lots of these rocks, and so over time this can really have an effect on the orbit of the.
Planets because we know that at around that space, that ring of the Solar System, you had a lot of big rocks, and maybe you have a lot of big rocks right now exactly.
The Kuiper Belt is huge. There could be like trillions of objects out there. We think today it's the source of comets that fall into the Solar System, the short period comets. There's an even bigger blob of stuff out in the Oort Cloud, which might be the source of long term comments. But it's an enormous, massive stuff out there, and each of those little bits, as they interact with the Solar System can give a little tug. You have these nice circular orbits that were formed initially, but now they're getting perturbed by these tugs from all these rocks in the Kuiper Belt.
Mmm, okay, so scientists think that maybe these rocks from the Kuyper built maybe cause some planet that we had before to exit the Solar System.
Well, essentially leads to instability because you're pushing Jupiter in, you're pushing Neptune, Saturn, Urinus out, and then those planets start to interact like they used to be in a nice, happy orbit, but now you get instabilities and resonances from those planets themselves. Jupiter starts to drift inwards, and then Saturn pulls on Jupiter, and Jupiter pulls on Saturn. You start to get regular orbits, and Saturn actually pulling on Jupiter is what saves it. Saturn pulls on Jupiter and changes its direction so it migrates back out away from the Sun. Without Saturn there, it might have been that Jupiter would are just like plummeted into the Sun thanks to the influence of the Kyper Belt.
You know, it all sounds kind of complicated, So I wonder why do scientists think that making it more complicated by adding another planet makes it easier to understand? Like it's all really complex dynamics, right, Like, so then how does adding another planet make it easier to predict, like, what's the missing thing that we currently have that a missing planet would help with.
Some of the features of our Solar system that we see today are difficult to explain without adding another planet, you know, like the irregular orbits of Jupiter and Saturn. They're not perfect circles. There are sort of ellipses. They have like a five percent eccentricity, and there's this structure in the material of the Kuiper Belt. A lot of it seems to have been lost, and a lot of the rest of it is in resonance with Neptune. And these things aren't like smoking guns that say like, oh, look, there has to be another planet here. But when you run the simulations, you get these kind of features more often when you add another planet. If you put another ice giant in around the size of Neptune between Saturn and Urinus, then the story you get when you run these simulations more closely resembles the Solar system we have today.
And I guess scientists just throw this mystery planet in and all kinds of velocities in all kinds of sizes, and you sort of see overall like Hey, it does kind of shape the Solar System more into what we have now.
Yeah. Another way to say it is like they were running the simulations with just the current planets and they were noticing that they pretty rarely ended up describing the situation that we see today, Like, it was very unlikely to get Jupiter and Saturn to have these eccentricities, and to have these asteroid belts along Jupiter's orbit, the Trojans and the Greek camp of asteroid belts, those things were pretty rare to get in a Solar system without this additional planet. But when you put that new planet in, it started to be less unlikely. It started to be like, oh, this kind of thing happens pretty frequently, and so it's just a question of like, how do you explain it. It's not the only possible story, right, Maybe there are two other planets. Maybe something else happened that could explain this, but it does make the story more likely.
Maybe the planet had a big fight with Jupiter, stormed out of the house with all their bags, left to go live with their aunt or their niece. This is the nice model, right.
This is the Niese model exactly. They went to live with their aunt in France.
So then that's one model you're seeing. One model says, and maybe it was all these big rocks from the Kuiper Belt that maybe caused a lot of instability out there in the icy planets, and then maybe it caused a planet that we used to have to fly away exactly.
And one of the nice things about this model until recently was that it lined up with other pieces of evidence for when this instability happened. And in the nice model, this happens like about a billion years after the Solar System is formed, so you get like the ignition of the Sun, you get the formation, and the gas planets and the rocky planets. Things cycle around for a little while, and it takes time for the Kuiper Belt to sort of drive this because these are tiny little rocks. This sort of lines up with another piece of evidence from looking at our moon. Our moon is a great record for impacts in the Solar System, like when rocks have been raining down in the inner Solar System. And when the astronauts in the Apollo mission went to the Moon, they gathered a bunch of samples to study, like the craters and the impacts to try to get a sense for like, what is the history of the Solar System, one of them been sort of more or less impacts when it's been like bad weather and good weather. And for a long time, there was this evidence for what we call a late heavy bombardment, that there's this period of billion years after the Solar system formed when a lot of rocks were raining down in the inner Solar System, and that kind of lines up with the story of the nice model that like, all these rocks from the Kuiper Belt were coming in and making trouble and maybe also some of them were landing on the Moon. So that was sort of a nice story for a while.
Meaning like there's a lot of activity from the Kuyper Bell which may have contributed to us kicking out an icy planet exactly.
And until a few years ago that all seemed to kind of hang together. But then there was a reanalysis of this data from the Moon and it turns out that it may have been a mistake.
Well, we lost the theory the dog a theory.
It turns out of the way the astronauts gathered the data, they may have basically only collected data from one big impact. So what we thought was a bunch of impacts that all happened at the same time, like three and a half billion years ago, might have actually just been one big impact that the astronauts gathered from. So it could have been like essentially just a statistical anomaly in the data that made it look like there was really bad weather for like a few hundred million years three and a half billion years ago, But it was really just one bad day that the astronauts happened to collect data from.
Wait, what, so we didn't we just had one sample. We didn't take some samples from all over the moon or analyze the creds visually through telescopes, so.
We don't have samples from all over the moon. And they definitely collected a bunch of samples from different locations, and that's why they thought maybe this was like a fair sample from everywhere on the Moon. But a reanalysis of it suggests that a single impact site, Imbrium might be responsible for basically all of the evidence that the astronauts gathered. I don't know if the astronauts are being lazy and not following instructions or if it was not a well organized study, but more recent analysis suggests there may have been no late heavy bombarkment. There may just be like a gradual decline in the number of impacts over time.
And so maybe this idea that kyper Berl maybe caused us to lose an icy planet maybe didn't really happen, or could it still have happened without this late heavy bombardment.
This really causes us to doubt that model. And there's been another lingering problem with this Nie model that has never really been answered, which is why the terrestrial planets kind of survived it. If you have these gas giants doing this dance a billion years after the Solar System is formed, when Earth and Mars are all already formed, then how did the Earth and Mars and Venus survive all these gravitational tugs. If Jupiter comes into the inner Solar System basically turns around at the asteroid belt, how does Mars stay in orbit? How does Earth stay where it is? So one concerned about the Nice model has always been how did the terrestrial planets not get disrupted by the giants. So now there's a news story about when this instability happened and what the cause was that doesn't rely on this late heavy bombardment and places the blame on the instabilities somewhere else.
But I guess why do we assume that there was an instability because we think that maybe we did lose a planet.
Because we can't explain the orbits and the eccentricities and the structure of the Kuiper Belt without some kind of motion of these planets. We know the planets moved around, we know there was interaction. We know that they did not form in the order that we see them today.
All right, So then what's this new model? Not so nice, the less nice model exactly, not the nice model, the nephew model.
The nibbling model. So this model is called the rebound model, and it suggests that this instability happened much much earlier, actually while the rocky planets were forming, or maybe even before they formed, that it was basically an early instability.
Well, that's a big difference in timescale. But don't your simulations as the Solar system sort of help you pinpoint when it happened.
It turns out that the simulation is can of commemon to either an early instability or a later instability like the instability for in the Nice model, like a billion years after the formation of the Solar system can explain the orbits that we got if there, in fact was a bunch of interactions from the Kuiper Belt. But you could also have an instability much earlier on that could reproduce the orbits and the eccentricities that we see today.
All right, so then what does this model say? What happened according to this model?
So this is called the rebound model, and essentially the instability trigger here. The thing that kicked off all this bouncing around was the interaction of the planets with this gas. Imagine the formation of the Solar System, as we talked about earlier. You have these planets forming and they're pulling their stuff together, but you still have something of a protoplanetary disk. You still have a bunch of gas sort of in between the planets. Now we don't have that today, and the reason is that the Sun has effectively blown all that out. As the Sun triggered and the ignited and its radiation grew and grew, it blew out all that gas from the Solar System. So in the first ten million years or so, that gas disc is sort of moving out through the Solar System and it affects the orbits of those planets. If the planets are passing through the gas, it slows them down, and if that gas is getting pushed out by the star, it actually carries those planets with them a little bit. So as this gas disc is getting pushed out of the Solar System, it passes through all of these orbits and it gives them all a little tweak. So this rebound model suggests that the interaction of the planets with this gas disc as it's getting blown out of the Solar System can trigger these same instabilities and can explain all the features we see in the Solar System today.
But couldn't you kind of make the same argument as before with the other model, like Wooden huck, How is it then that we'd the Earth and Mars and Venus have such a nice even orbits if we were disturbed and blown out.
Yeah, great question. It's because this happened much earlier, and so essentially this happened before those terrestrial planets even form the terrestrial planets we think formed after the gas giants come. The gas giants have a lot of advantages over the rocky planets in the inner Solar systems. Number One, there's ice out there, which is like a solid that can help seed the formation of planets to there's a lot more gas out there because the Sun hasn't gobbled it up and you don't have this proto star messing everything up and heating things. So it's much colder, which makes it easier for gravity to pull things together. So the outer Solar System is a much easier place to form planets. So we think the gas planets formed before the gas disc actually evaporated, sometimes in like the two to ten million year range, But rocky planets take longer because the inner Solar System is much hotter and messier, is less gas available to form planets and no ice whatsoever, So those take like thirty to one hundred million years to form.
Okay, So I think what you're saying is that this new model, this rebound model, is saying that we kicked off a gassy icy planet a long time ago, before we even had Earth and Venus and Mars and the rocky planets inside, and that it was due to a lot of this gas being blown out of the center.
Exactly, and as the sort of inner radius of that gas passes through these early ice giants and gas planets, it triggered that instability, They did their crazy dance with Jupiter moving in and the other planets moving out, and ejected an ice giant planet that left the Solar System. And all that happened even before the Earth and Mars were formed, so they didn't mess up the formation of the Earth and Mars because it was already done by then.
All right, well, those are both great stories. Now the question is can we see the planet that we kicked out? Are there lonely dejected planets floating out there in space that we can see and maybe identify and track to perhaps our Solar System? So let's stick into that. But first, let's take another quick break.
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All right, we're asking the question, did our Solar system lose a planet or I guess shepherd it out peacefully into the cosmos.
That's the NIS model. If anything happened, it sounds like we can blame it on the gas giants. Like we weren't even around when all of this went down. It is just the drama we heard about when we showed up.
Oh I see, it's like, yeah, it's like you're the younger sibling and there's all this draft before you were even.
Born exactly, Like why is everybody so mad? And who is this missing sibling everybody's talking about you never met?
Oh wow, this is just got This is just turn into a Korean drama. I feel like, super complicated.
But it is sort of the story. I mean, what we learn from this is that these unstable giant planets, the ice giants and the gas giants, basically sculpted the inner Solar System. I mean it didn't disrupt the already formed Earth in Mars and Venus, but it created the gravitational context for them to form and probably changed how they did form. The way younger siblings arrive and family dramas that have existed for years.
But again, I guess these are this is just kind of a model, right, or sort of a guess to maybe explain some of what we see. We don't quite know for sure.
Right, We definitely don't know for sure. We've quibbled before about what a guess means. Scientifically. I think we have a model, we have some evidence for it, We're never exactly sure. I mean, we have not like identified a planet and said that's our lost planet. It's just easier to ex lane what we see in the universe when you add this to the story. But that happens for lots of things, Like we don't witness the early years of the Earth's formation, but we have a pretty detailed story about the formation of the Earth based on the patterns of evidence that we see in the rocks beneath our feet. And so that's a big part of science, is developing a story to explain the data, even if you don't directly witness all of those events, right, Right.
But I guess what I'm trying to say is that we had a pretty good story that seemed to check out and make sense before, but then it turned out to be not quite correct.
Yeah, that's true. These stories are always evolving in they're getting better. Like we like the nice model, but there were some dangling questions about how the Earth and Mars survived it, and now we like this new model, the rebound model. But there's always going to be dangling questions, and somebody's going to come along with a better model and maybe tell a different story in five years. It's a constantly evolving story.
I guess. But I wonder if maybe like a smoking gun or something to definitely be able to say, like, hey, there used to be a planet here in the Solar System that we don't have anymore, is to actually maybe see this planet that we kicked out or that left Liane its own out there in space. Isn't it possible that we could see a planet and track it to our Solar system out there beyond our Solar System?
I suppose it's possible, but we're talking about events that happened four billion or more years ago, so that planet is pretty far gone by now. If it did leave, we can do sort of more indirect discovery, so we can look out and say, are there any rogue planets? If this is happening in our Solar System, it should be happening in other solar systems. Shouldn't space be filled with these ejected ice giants when it happened in other families, not just ours? And we can go and look for those.
Yeah, we had a whole episode on rogue planets. There might be a billion of them out.
There right exactly. We can actually spot some of these using what we call micro lensing. If one of these planets out there floating between stars, passes in front of a star, like a little eclipse, then it'll blink out and actually change the way that light bends around the planet. So we can use these micro lensing techniques to try to spot them. Have infrared telescopes like the Wise telescope that can try to directly image them. These planets don't glow in the visible light, but they do have some temperature, and everything with the temperature glows in some frequency. These would low in infrared, and so it's possible to see them. So we have seen a bunch of these rogue planets, and so we can estimate that there's like one of these things for every star in the galaxy.
Well, I mean, there's a one hundred billion of them right in our galaxy.
Exactly, and we've only spotted a few of them, and so the calculation of like how many there are is very uncertain. There's a huge extrapolation there, which means a huge uncertainty, but we know it's a pretty big number. We know it's not just like ten in the galaxy. There's lots of these things out there, and that leads credence to this story that, like when solar systems form, there's a period of instability when big planets can get thrown out.
So we've actually seen these, like if you look at a picture of the Nice Skuy in the infrared, you can see these thoughts moving across the sky.
We can actually see these planets, and we have seen with direct imaging some of these rogue planets. Again, not very many, and so we're extrapolating from a handful up to a big number. We've definitely seen non zero and very recently James Webb saw some really crazy stuff out there. They found these Jupiter mass binary objects. They call them jumbos. These are pairs of planets floating out there in the galaxy with no star nearby.
Wait what, well, first of all, James Webb, do you mean the telescope. Right, Yes, not James web from Erie, Pennsylvania.
We did not exhume the previous NASA administrator and asked him to look at the night sky and then write down with what he said. Though that would make a pretty cool graphic novel.
Yeah.
Yeah, Well, I just don't want to assume everyone knows what James Webb is.
No, you're exactly right. The James web Space telescope a very powerful device that we launched a couple of years ago and is an infrared telescope capable of seeing things that are pretty cold. Spotted forty two pairs of jumbos.
WHOA and so is there? Is it weird that they're in pairs? Or does it feel normal that they're in pairs? Meaning does that mean that they were ejective from Solar system in pairs like they left together.
It's a great question. We don't know the answer to that. Simulations suggest that it's unlikely for big planets to leave the Solar System together, right. They would have to be like bound together and then leave together, which means that their fragile orbits around each other would have survived very chaotic period. Seems very unlikely. So these are rogue planets that are out there without their star, but they don't seem to have been ejected from solar systems. So it just sort of like adds to the murkiness of what's going on with rogue planets.
Whoa wait, wait, so maybe they were ejected and then they met up with another jumbo out there in space.
We actually don't have a great story to explain how these even exist like that seems very unlikely for all these jupiters to like start dancing around each other just in space.
Well, maybe they have like a planet dating app or something.
Maybe they're speed dating there, or they're square dancing or something. They're all changing partners.
That's right, they have a jumpler on their phones.
Some people suggested maybe they just formed independently, like you had a solar system and it didn't have enough stuff to actually have a star. You just got a couple of Jupiter's. But we think that there's like a minimum amount of mass you need to get like your own solar system, otherwise you just get slurped up in like a neighboring solar system. When that huge stellar nurseries breaking up into chunks that form solar systems, So we think that these things are probably too small to like have seated their own structure and be their own solar system. We don't think they can have been ejected from other solar systems. So it's something of a question of where these came from, you know, just to paint the picture that like, there's a lot we still don't know. There's a lot of guessing going on.
But I guess if they didn't come from a solar system, then that doesn't really tell us anything about this idea of how often solar systems kick out planets exactly.
But it adds doubt to the argument that because we see a bunch of rogue planets out there that suggest that planets are lost from solar systems, because there are planets out there whose formations we just don't understand and we think don't come from having been lost by a solar system.
Right, Well, I guess to answer the question of the episode, has our solar system lost any planets? The answer is maybe, we guess. So we guess. Maybe we used to have a brother, an older brother, but now nobody likes to talk about him or her, and it's very awkward. It makes all the models break all the pets are uncomfortable.
Almost all of the models we use to explain the Solar System do have an additional planet. It's not absolutely required. It's possible to explain the Solar System without an additional planet that got ejected during one of these early instabilities. But it just makes the story come together more crisply. It makes our Solar System seem less unlikely.
Now, how does this matchup with I know there are scientists that think that we have maybe a ninth planet in our Solar sysm we just can't see it planet X right.
Yeah, there are some people who look at like gravitational aberrations in the orbits of our planet to see if there's something else out there tugging on it. But there's no conclusive evidence for that. It's like very very gentle, and there's a lot of disagree about whether it's just noise or has other explanations.
It's almost sort of the same, right, the use simulations and try to figure out what would best explain what we have now.
Yeah, exactly, but the data there are not conclusive.
All right. Well, another interesting lesson to keep track of your planets. Don't lose them, because once they're gone, They're gone forever.
And try to understand where you came from and what your context is, what happened before you showed up on the scene.
Yeah, well, not that we have much influence over what happens, but it's interesting to think about what might happen in the future, Like, is it possible that the Earth might get kicked out of the Solar System right.
In the future exactly? And the difference between these models tells a very different story. If it really is lots of gentle tugs from Planetesimals, well, that could still happen in the future. We have the Orc Cloud, we have the Kuyper Belt. There's still tugging going on. But if it was something that only happened in the very early formation of the Solar System itself, as this gas was pushed out, that's not something that's likely to be reproduced, and so that level of instability is probably not going to happen again.
Now, Daniel, if we did have an icy gas planet but it was filled with white chocolate, are you happy that it's gone or are you sad that we don't have it anymore?
No, it's bittersweet. I wish it's the best.
No, it's not bitter, it's sweet. It's white chocolate. That's the whole point of white chocolate.
It's oversweetened. That's the problem.
But maybe it'd be good for you because it would make all the white chocolate lovers go to this planet and leave ours exactly.
Let's arrange transit to the frozen planet of white chocolate.
Well, call it white sun, no son of mine. All right, Well, we hope you enjoyed that. Thanks for joining us. See you next time.
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