Ep. 546: Weird Issues: Planetary Migration

Posted on Nov 1, 2019 in Extrasolar Planets, Planets, podcast | 0 comments


Astronomy Cast 545 & 546 rescheduled for 3 pm/3:30 pm ET Monday, Nov 4

Before we discovered other planets, our Solar System seemed like a perfectly reasonable template for everywhere. But now we see massive planets close to their stars, which leads you to the question, how does it all get there. Do the planets form in place or do they migrate around?

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Astronomy Cast, Episode 546

Weird Issues:  Planetary Migration

Fraser:                         Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela. How you doing?

Pamela:                        I’m doing well. How are you doing?

Fraser:                         Great. For anyone listening, it’s been a week since you’ve heard our dulcet tones, but for us, it’s been scarcely 10 minutes. So, here we are with our next episode on our weird series – weird issue series. And Pamela is traveling, so we had to bunch them up. But I don’t think the science will change dramatically in a week or so, but I could be wrong.

Pamela:                        It’s true.

Fraser:                         Yeah. Now, before we discovered other planets, our solar system seemed like a perfectly reasonable template for everywhere. But now, we seem to have some planets closer to the stars, which leads you to the question how does it all get there? Do the planets form in place or do they migrate around? All right, Pamela. You put this in the docket. You had a much more complicated title when you originally put this in. It was something to the gist of, which I just put as that last sentence, did everything form in place or did it all migrate around?

Pamela:                        I’m not allowed to name things. I am an astronomer, but the one precludes the other. We’ve discussed this before. You were here to name this.

Fraser:                         You are either too poetic or –

Pamela:                        Wordy. Let’s just go with wordy.

Fraser:                         Too wordy. Yeah, yeah. And I’m like “I’m all about the clickbait, man, so keep it short.” But, no. So, again as always, each time we kinda go back and think about the days of yore, and what did we think about planetary formation and the locations of the planets back in old timey land?

Pamela:                        Well, back when we only had one solar system to study in detail –

Fraser:                         Right. There was only one.

Pamela:                        We came up with this beautiful solar nebula model, which the big pieces are still generally thought to be true, but the details, oh my goodness, were wrong. So, the picture that we had before is that solar systems form and the light pressure of the sun pushes most of the gases outward, leaving rocky worlds forming. Internal – the gas gets gathered up, forms usually one giant gas giant. Outer – in the outer solar system, you can end up with some ice giants, maybe a smaller gas giant, and things form in a nice and ordered way. And you may end up with resonances between those massive worlds that push things out further and send rocks cascading in.

But in general, where we see things is where they formed, which means that when we look at things like the asteroid belt, which is located at that place in the solar system where the heat from the sun either baked things dry or left them moist, those asteroids that formed there are still representative of that solar nebula and the distribution on the trail within it. That’s what we thought.

Fraser:                         Right, right. And in fact, the distribution – I forget the exact rules on this, but if you looked at the locations of all the planets in the solar system, there was a really straightforward, almost mathematical position to each one of the planets. As long as you assumed the asteroid belt is a planet –

Pamela:                        It was a beautiful set of ratios that led people to think that that was where they had to be.

Fraser:                         Right. There was a beautiful set of ratios. Yes. And so, we assumed that everywhere we looked, we would see the exact same thing. Did we?

Pamela:                        No.

Fraser:                         What did we find?

Pamela:                        So, one of the first things that we found was gas giants don’t always exist far away from their stars. They tend to be found snuggled up next to their stars periodically. And we’d also assumed that they’d have to be in circular orbits because angular momentum issues. But then, we find them in these highly elliptical orbits. And we assumed that there would only be one gas giant, truly giant, Jupiter-like planet per star, and we sometimes find more than one. And so, basically, all the rules we came up with were wrong.

Fraser:                         Right. And so, now, if we were to look out into the universe and try to make some rules about how planets – where you’ll find planets in other star systems, what are the rules?

Pamela:                        I think that depends on who you talk to and what they ate for breakfast that day.

Fraser:                         So, is it bordering on no rules?

Pamela:                        It’s bordering on new rules appear to come out every week.

Fraser:                         Right. But there are no – there have been star systems found with, as you mentioned, giant planets orbiting close to their stars, multiple giant planets, planets that are small, located close, in between, planets are on red dwarfs – it’s a mess. It’s chaos out there.

Pamela:                        If the solar system could do something, it has done something.

Fraser:                         Right.

Pamela:                        And this is kind of awesome. And what’s fun is watching the theorists play catch up. And one of my favorite examples of this –

Fraser:                         Poor theorists.

Pamela:                        Yes. One of my favorite examples of this is a new paper that came out in the astrophysical journal that had as lead author, graduate student, Renata Ferlich, and this young woman did this amazing work where she ran a myriad of hundreds of simulations of solar systems where she started out each solar system with 10 worlds. And she started out each solar system with a different amount of mass, and let the mass fall as it would to the different planets, and she allowed her models to have multiple, massive gas giants. And this was new. Like this wasn’t a thing people had been doing before because solar systems are not supposed to have multiple, massive gas giants.

Fraser:                         Don’t we have two?

Pamela:                        Well, Saturn’s not that big. So, they’re allowing two Jupiter or larger size.

Fraser:                         Two Jupiters – okay – or more.

Pamela:                        And so, the models that she did were able to explain all of these crazy Jupiters we’re finding as what happens when you allow a solar system to evolve for 20 million years, and notice that worlds collide. And the fact that worlds collide seems to be something that we have to keep re-realizing. And this is one of my favorite things, as someone who does not do this research, to sit back and watch as a spectator. Because first there is this realization of “oh, expletive.

One of the best ways to get at the Earth having a moon is to assume that a Mars-sized object hit the proto-Earth; they collided; they splashed up the light materials; the light materials re-coalesced; we ended up with a moon that’s lower density than the Earth. We even named the prior object Theia.

Fraser:                         Right. And so, I’m just imaging the cognitive dissonance here, right? You’re saying planets don’t move, but also a planet moved and crashed into the Earth. But ultimately, planets don’t move except for that time that they did, which they don’t.

Pamela:                        Right, right. And so, the fact that other collisions may have occurred, people talk about, especially when we look at things like Uranus, which has its pole pointed into the plane of the suns; when we look at Venus, which seems to be flipped all the way over. Well, maybe that could’ve occurred via some sort of a collision, but there’s always that modeler out there trying to figure out how to do this using nothing more than tidal forces so that no collisions are required. Because we keep assuming that collisions have to be rare.

But then, there was a recent paper that was able to explain that the reason Jupiter isn’t as dense in the core as we had previously expected, as measured from its angular moment of inertia, is it appears that Jupiter got smacked by a massive protoplanet many times the size of our Earth when it was young. So, we now have in our solar system alone, Earth moon – result of a collision; Jupiter’s fluffy core, and they did use the word fluffy to describe this explained with a massive collision.

Fraser:                         A super-Earth. 

Pamela:                        A super-Earth. 

Fraser:                         Yeah. We used to have a super-Earth, and then Jupiter ate it.

Pamela:                        Or it committed suicide. I don’t know who was on the elliptical orbit that drove this collision.

Fraser:                         Right. So, then this idea then that a planet crashed into the Earth. A planet was gobbled up by Jupiter, or threw itself into Jupiter. Uranus is flipped over on its side and is in the wrong position in the orbit compared to Neptune in the solar system. It looks like they’ve been flipped. This started to lead astronomers to the possibility that, in fact, maybe planets shift around. Maybe they do shift around in a solar system.

Pamela:                        And once you start thinking about that, you start looking at the data in a completely different way. So, initially when we were trying to figure out how the heck do you get a Jupiter snuggled in next to its sun, everyone was blaming frictional effects, drag effects. The early Jupiter was inside the disk of material, the protoplanetary disk, and it kept consuming things at smaller and smaller orbits until it had migrated in to where there was the hollow spot in the disk, and it stopped there. Well now, by instead looking at this as “oh no, it simply got smacked around through collisions with other objects and got pushed in.”

Perhaps it ended up on a highly elliptical orbit, and highly elliptical orbits do, over time, get tidally made more circular. This is a completely different and, in many ways, easier way to explain what we see, and tidal forces do still play a role. Don’t get me wrong. This is how things like when you look at Mars, it’s inner moon, Deimos, will eventually become it’s inner broken band of rocky rubble, Deimos –

Fraser:                         Phobos.

Pamela:                        Phobos. Yes, sorry.

Fraser:                         Yeah. Phobos is going in. Deimos is going out.

Pamela:                        Deimos is fine. So, it looks like you can also have tidally locked worlds migrate in as well. And so, you have collisions that are driving it, you have frictional effects that are driving I, and the newest result that we’re looking at is planets have now been found near late-type stars that had previously been much bigger red giants.

Fraser:                         Yes. I saw this.

Pamela:                        The planets are being found in locations that used to include where the star was.

Fraser:                         Right. So, we’ve got stars – we’ve got planets that were able to withstand their stars – They were inside their star for a while.

Pamela:                        So, they actually figured out how to explain them without having to have them inside the star, which seemed like a really bad way to be a planet.

Fraser:                         Yeah.

Pamela:                        So, the way they figured out how to explain this is you have massive star and the planets turned out to be in much further out orbits. So, the planets weren’t actually consumed initially. Star expands out. Its rotation rate changes as it expands and contracts. The planets are asymmetric. As the star collapses back down, the same effects that are causing the inner moon of Mars to spiral inwards towards Mars are causing these planets to spiral inwards towards their star. So, they were tidally dragged in towards their star as it shrank.

Fraser:                         Wow. That’s really cool. Yeah. And this is brand new. This is like today, yesterday.

Pamela:                        Last week.

Fraser:                         Yeah. So, then let’s run the clock back now to think about the early solar system. What did it look like?

Pamela:                        It was probably – and here again, this is what we’re thinking today. Tomorrow could be different. There is some consensus, but I wouldn’t say there’s great consensus on our understanding right now. We believe that things started out with all of the outer planets closer into the sun, so our solar system was initially much more compact. The Oort cloud may or may not have been there. The Kuiper belt may or may not have been there. The asteroids may or may not have been there.

Over time, gravitational interactions between the most massive worlds in our solar system redistributed everything leading to the asteroids in the resonant places that they are – and they aren’t in the asteroid belt – leading to the centaurs grouping up where they are and where they’re gonna stay for 1000 years at a time before they get flung in a new direction, leading toward Uranus and Neptune perhaps even switching locations according to some models.

Fraser:                         Yep. And rolling Uranus over on its side, although that could’ve happened with a crash.

Pamela:                        Through an impact. And so, the best that we can say is everything moved, and we luckily ended up on a world that stayed in the habitable zone for the surface of our world.

Fraser:                         Crazy. So, then this idea – how does this help us better explain the other star systems that we see out there in the universe? As we said, we’re now seeing all of this complexity. Why does this knowledge that planets migrate around – how does that help us better understand what we see?

Pamela:                        So, the two big questions that we’re finally able to answer are it’s through this planetary migration that we see so many collisions taking place. And if your initial solar nebula is only able to form one fairly significant Jupiter-sized world, and then things like Saturn, Neptune, and Uranus – if those Saturn, Neptune, and Uranus-type things decide they’re gonna collide together, this is how you start ending up with these truly massive worlds. So, what we’ve been finding is truly massive worlds in highly elliptical orbits, and these new models also explain this highly elliptical orbits. Because the high ellipticity, that’s not natural.

The easy way for things to form is a nice, circular orbits that some resonances can knock things slightly out of the round, but in general, round is how orbits want to be. But when you have collisions and you have separate masses doing it, you end up with a single, much larger object, or moons and a central large object – Earth-moon system. And if it happens just right, you end up with the high ellipiticity systems that we are seeing.

Fraser:                         I mean, I guess when you see a planet – like a hot Jupiter – and you see this planet that is located really close to the star, that shouldn’t be possible.

Pamela:                        Right.

Fraser:                         Like it shouldn’t be there. It couldn’t have formed there, therefore it had to move there. And the fact that it had to move there means that planets move –

Pamela:                        But how?

Fraser:                         But how? And how. So, what are those mechanisms that are causing those planets to shift around?

Pamela:                        So, we’re looking at three main mechanisms for shifting the worlds around. One is when we still have that young solar nebula with a whole bunch of debris in it, you have an object that forms with a gap outside of it relative to its star, and a whole bunch of material internal to it around its star. And it starts gobbling up the material inside of it, and this changes its angular momentum and causes it to migrate slowly inwards depending on how this occurs. And it’s through this consumption from the drag and through the conservation of angular momentum that you get this very complex and hard to math out migration inwards in an early solar system.

Fraser:                         And I’ve heard this process might actually happen very rapidly.

Pamela:                        Yes.

Fraser:                         Like you’ve got these disks of material. The planet is embedded inside of it, and you’ve got these tidal tails that are coming from the disks that are feeding into the planet. And the closest analogy is you can see structures that are kind of like this with the shepherd moons that are orbiting within the rings of Saturn. You can see these lines of material that are coming to these little shepherd moons because of their gravity, and that in fact these movements could have happened in hundreds of thousands of years, the low millions of years at the most. It was not a slow process. It was fast.

Pamela:                        Right. So, primary planetary formation probably only took a few hundred thousand years.

Fraser:                         Wow.

Pamela:                        And it was during that next billion years that all chaos broke out with things moving each other around. And so, you do have this frictional migration.

Fraser:                         You mentioned three mechanisms, right?

Pamela:                        Right. So, the next way that we look at things is you have that next phase of the solar systems – the era of heavy bombardment that we saw within our own solar system. This is where you have massive collisions taking place where you start off with way more worlds than you eventually end up with. And it is through the collision of Earth, Mars, and bigger sized objects that you can grow massive Jupiters. You can grow massive terrestrial worlds.

Fraser:                         So, I kind of imagine this. You’ve got these planets that are on these fast tracks that are moving around inside this disk of material, but potentially at different speeds, and then you’ve got planets crashing into each other as they enter each other’s orbits.

Pamela:                        And what allows this to take place is as things form, they don’t necessarily have the long-term stability that they need. They still have all of these resonances that can put them on short time scales, much more elliptical orbits that cause these collisions to take place. Now over time, the orbits do circularize. But even once they’re circular, you can still have resonances that slowly move things. And if this slow migration ends up with things in just the wrong or right, depending on your perspective, kind of alignment, you can end up with what we had, which is Jupiter and Saturn going around such that for every two times Jupiter goes around the sun, Saturn goes around once.

And gravitationally, anything that hadn’t already been flung somewhere is now gonna get flung somewhere.

Fraser:                         Yeah, and I think you’ve mentioned in the past that there’s still a chance that some of the planets will get thrown out by Jupiter.

Pamela:                        And this is the long-term, slow effects that we model over the course of thousands and millions of years in our computers where we can see how the processing of our own Earth’s orbit, which isn’t a perfect circle, and the procession of all the other not quite circular orbits leads to different alignments over time. We see the Martian pole wandering over time because it doesn’t have a giant moon like we have.

Very few things are in any permanent kind of an alignment, and I don’t think anything in our solar system except for maybe the sun, and even it is only on like millions of years, fairly permanent – nothing has a permanent alignment. Because all of these slight shifts, all of these slight processions can lead to new resonances building up and flinging something hither and yon.

Fraser:                         Is that the end of the story? You talk about there can still be some interactions, some slight instabilities that build up over time and Jupiter throws Mercury out of the solar system, for example.

Pamela:                        That’s not going to happen.

Fraser:                         But could there be – I mean, are there more mechanisms in play? As the sun dies and it loses its mass, will we see another round of this?

Pamela:                        Yes. So, our sun is constantly losing mass. And this means that very gradually over time, everything is migrating outwards a little bit. Now, eventually, it’s going to also expand out as it undergoes a change in the kinds of a nuclear burning that are going on in its core. And this change and this massive expansion coupled with the mass loss that is going to predate this is going to eat some of the planets in the solar system, is going to move some the other planets in the solar system. And this redistribution will, of course, change what resonances are possible as everything moves.

Now, you also have the continued possibility of things, like well there are 60 km objects in the centaur belt which are not entirely stable and could get flung in our direction. And that’s not necessarily going to change our orbit that much, but who knows what else it could hit along the way and change its orbit.

Fraser:                         Right.

Pamela:                        And so, all sorts of collisions are possible, and you never know when we’re going to be affected by another solar system. We orbit our Milky Way.

Fraser:                         So, do you think that we will eventually have a single model? With things like James Webb, they’re gonna be able to see newly forming planetary systems and watch those little gaps moving around. Will we end up eventually with some unified model of being able to calculate how planets moved around and where they are likely to end up, and finally know what a planetary system should look like based on all the factors?

Pamela:                        Well, I think what we’re eventually gonna end up with is a better understanding of all the things that need to go into our computer models, and quantum computers and the ability to say “here is all the stuff that can happen. Universe – go.” And then we’re gonna realize the universe is still finding stuff we didn’t include in our models. I really think that this is going to stay one of those things where the universe remains more creative than we are in all of the diversity that is actually created. There is gonna be multiple ways for the same situation to arise.

And since humans are short-lived, we’re always gonna have to say “well, here are all the different ways you can get that,” and we still may be missing something in our understanding of the universe.

Fraser:                         I can’t wait for us to discover the weirdness and we’ll make another weird issues episode. Pamela, do you have some names for us this week?

Pamela:                        I do. As always, our show is supported through the generous contributions of people on Patreon. If you would like to support our show, this is how we pay for our servers, our software, and Susie, Susie being the most important of those three. You can support all of these things by going to patreon.com/astronomycast. This week, we would like to thank Sciaran Svar, Ed Steven Shewater, Gordon Dewey, Bill Hamilton, Frank Tippen, George Thorwald, Richard Riviera, Alexis Thomas Upstrup, Sylvan Wesby, Jeff Collins, John Drake, Arctic Fox, Marek Videry, Nate Detweiler, James Platt, and Ron Thorson. Thank you.

Fraser:                         Thank you, Pamela, and we’ll see you next week.

Pamela:                        Bye-bye everyone.

Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at information@astronomycast.com, tweet us @astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.

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Duration: 29 minutes

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