Ep. 301: Planetary Migration

We’re so familiar with the current configuration of the planets in the Solar System, but did the planets always orbit in this way? Did they form further out and then migrate inward to their current positions? And what about other star systems out there?

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    Fraser: Astronomy Cast episode 301 for Monday, April 8, 2013 – Planetary Migration
    Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
    My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest.
    Hi Pamela, how are you doing?
    Pamela: I’m doing well how are you doing Fraser?
    Fraser: Great. You have an announcement to make?
    Pamela: As many of you have heard, CosmoQuest which is our citizen science virtual research facility for the public is facing some financial crises brought on by US sequestration. We are trying to keep ourselves from going into the future by asking for your help in exchange for our silliness Nicole Gugliucci and I are starting with a 24 hr, but may go out to a 36 hour, online hangout-a-thon. Think Jerry Lewis telethon except you’ll be saving science and using Google hangouts because we lack money for mainstream television.
    Fraser: And the technology would rock that, it’ll work just great.
    Pamela: That’s true. We’re going to do this starting 10am CST on June 15th going into June 16th and we’re going to start getting all of the events posted for that in the next couple of days so if you’re listening to this on the pod cast it should already be announced on the CosmoQuest website. The more you donate the longer we will stay online bringing you science and bringing you silly.
    Fraser: Fantastic. I think I’ll be making an appearance with many of our other friends and we’ll get to watch you and Nicole go crazy from 24 to 36 hours?
    Pamela: Yeah, if we get enough money in.
    Fraser: EPIC! Okay well we’ll have more information on Astronomy Cast and on CosmoQuest and I’m sure we’ll mention it in Universe Today.
    Fraser: We’re so familiar with the current configuration of the planets in the Solar System, but did the planets always orbit in this way? Did they form further out and then migrate inward to their current positions? And what about other star systems out there? This is actually one of those those topics that has changed a little bit since we’ve started recording Astronomy Cast. I think with all of the discoveries of all of these extra solar planets and especially these gigantic Jupiter that orbit so close into their parent stars has made astronomers puzzle out how the systems are getting into their configuration. Before current ideas, what did astronomers think on how the planets got where they are?
    Pamela: Honestly they basically said that with the solar nebula model things formed from rocks to gas to ice and it was just that simple. We were lame.
    Fraser: So small particles of dust clumped together and they got bigger and bigger and whichever gravity well this material well could fall into it formed a planet.
    Pamela: Right and that part of the story is still what we have. The Solar Nebula model hasn’t been completely been thrown out. It’s been matured from starting with a disk of material and the sun’s radiation dries out the inner part, things gravitationally clump together and they get bigger and bigger and clear their orbits, and the planets form in place; to the planet forming and it may migrate over time. This idea really started to come about when we found 51 Pegasus back in 1998 which was a hot Jupiter where no hot Jupiter belonged. Over the years we had to try and figure out how the planets migrate. The model we’ll be discussing today actually came out before our show in 2005 but it’s really started to gain acceptance and prominence throughout the course of our recording. Fraser and I often come across topics where Fraser is like “Lets talk about it!” and I’m like “No, not yet, it’s not mature yet.” Well this is one of those topics that has matured and is ready to be discussed.
    Fraser: I get very enthusiastic about the shiny new stuff, I have to admit.
    Pamela: It’s kind of like wine. Sometimes you just have to wait because it will be better later.
    Fraser: I think you just glossed over it but that discovery of 51 Peg was amazing. So what is this object that they discovered and how was it surprising?
    Pamela: It’s an object that significantly larger than Jupiter, orbiting a star not too different from the sun, but its orbit is smaller than mercury’s orbit. When you start with a model: planets form, rocks near the sun, gas giants out from the sun… and then you find a gas giant on an orbit that’s smaller than Mercury’s. That says something is very wrong with the picture you’ve mathematically painted.
    Fraser: So they find this object and wonder how does this work then? how does this work then? I guess that’s when the investigation really started was wondering how these things got to these places.
    Pamela: That wasn’t the only piece of data that we had that had us confused. The other thing that we were dealing with was looking at the moon and other bodies throughout our solar system and it appeared that there was this ancient period in time where things were just getting hammered with rocks from space. It looked like for a brief period, the moon in particular, was going through what is often to referred to as a lunar catastrophe where there was a vast influx of objects from the outer solar system bombarding the inner solar system. We found evidence for this also happening on Mars and Mercury. It’s a very brief window of hundreds of millions of years, if that long. So we needed to somehow figure out how to get this random isolated period in time when they cream the inner solar system for no particularly good reason.
    Fraser: I think the formation of the moon itself could be considered a catastrophe, perhaps an Earth catastrophe when it actually happened. I think what you’re saying is this period is quite short.
    Pamela: Right, and when then the moon formed that was just one giant impact. The late heavy bombardment or the lunar catastrophe was object after object hurling into the inner solar system and just clobbering the surfaces of all the worlds.
    Fraser: Assuming that if the moon was so badly beaten up you can just imagine what must have happened to the earth.
    Pamela: Yeah we weren’t entirely solid there for a while.
    Fraser: (Laughs) Just great big rocks splashing into molten magma because the whole surface of the earth was kept in this liquid state from all the heat. It was not a place you’d want to live on.
    Pamela: It wasn’t entirely liquid but it wasn’t pretty.
    Fraser: So then what is the explanation for this late heavy bombardment then?
    Pamela: Well what we looking at now explains both of these problems of how you get planets to move and what caused the heavy bombardment. What were looking at is something called the Nice model. This model says that when the solar system initially formed we had Uranus and Neptune at, maybe, ten or more astronomical units; ten or more times the distance between the earth and the sun which is closer to the sun than they are now. We had Jupiter perhaps a little further out from where it is and Saturn further in from where it is. Over time they orbited the icy bodies in Kuiper belt which now is tenths of an earth in amount of total mass. Maybe it was tens, maybe it was hundreds of earth masses worth of icy stuff early in the solar system. As that material came tumbling into the inner solar system it gravitationally interacted with these planets and it interacted in such a way that the ice got hurled into the inner solar system. Most of the planets slowly over time were moved outward. As they moved what ended up happening was Jupiter and Saturn ended up in this resonance such that for every two times Jupiter went around, Saturn would go around once. This was causing them to keep lining up over and over and basically they pumped all of this gravitational energy into all the stuff around them. They sent rocks left and right, creaming into the inner solar system; they caused Uranus and Neptune to end up on highly elliptical orbits. Some computer models actually have Uranus and Neptune reversing location in the solar system.
    Fraser: We see Uranus tilted over on its side so something hit it.
    Pamela: Yeah or torqued it. During what we think was probably just a brief period in the history of our solar system, everything got flung all over the place. In this process Jupiter came in, Saturn went out, Neptune and Uranus went way out and rocks from space bombarded the inner solar system depleting the Kuiper Belt depleting its mass.
    Fraser: Would that also explain where the water on Earth might have come from?
    Pamela: Yes. When we start trying to figure out volatiles that’s one of the source of volatiles. We’re looking at a solar system that is 5-5.5 billion years old and it looks like all of this happened in a period of roughly 3.8 billion years ago.
    Fraser: Wow
    Pamela: Yeah, the universe is humming along and the solar system is humming along and suddenly WAM!
    Fraser: So how much of that then depends on there being Jupiter and Saturn working together. If there was just Jupiter or just Saturn would we have had the same outcome?
    Pamela: Well it’s hard to tell what magnitude it would have been because we still had Neptune and Uranus. Depending on how they lined up over time it could be that you ended up with a smaller version of this, with two different worlds in resonance causing all of this to happen through a different form of gravitational interaction. Both Uranus and Neptune are smaller than Saturn so the effect would have been smaller. It’s hard without running the models just how bad it would have been but there could have been something similar happening through a different form of resonance.
    Fraser: So Jupiter, good for the Earth or bad?
    Pamela: Early in the solar system quite bad but we got over it.
    Fraser: Is it gobbling up all of the debris in the solar system? We talked about last week how it’s still getting hit by a surprisingly large amount of material.
    Pamela: Right and this is where “good for the earth or bad for the earth”, we have to ask why period in time you’re looking at. It’s like Jupiter has a bad boy past where it had to beat up the entire solar system for a while there. When you had this resonance taking place the gravitational interactions between Jupiter and Saturn grabbed all of the rocks that, were at the time, orbits that were in resonance with the sun and Jupiter or the sun and Saturn, flung those into the inner solar system. The epic in time when the Kuiper belt was being depleted, it was clear that having these four giant worlds in the outer solar system was really bad for the inner solar system. Except for maybe the whole bringing water but that could have happened in a less traumatic fashion. Over time as we look at it now, it’s clearly eating rocks periodically and what we’re trying to still figure out is whether it’s protecting us or potentially flinging stuff back out and ultimately we’re not sure if it’s gotten past its bad boy habits.
    Fraser: Now we look out into the universe into the galaxy and we’ve now seen more than a thousand extra solar planets. What are astronomers starting to see with the other star systems out there.
    Pamela: As we look at these hundreds of star systems, many of which have multiple planetary systems, what we’re seeing is a young system refining the planetesimal disks; the disks of material that are still forming into planets that fits that solar nebula model we had before. What we’re finding as we look across more and more solar systems is that there is an over abundance of gas giants that are snuggled up right next to their host star. Occasionally we can find evidence for what we think are stars that have eaten some of their inner planets. Where we are still left trying to figure out is what starts the migration, what stops the inward migration, why is it everything didn’t fall into the sun, how is it that we ended up with these nice circular orbits? Well we can answer that with our solar system; we think that over time the constant interactions with what’s left in the asteroid belt and the Kuiper belt had the effect of taking Neptune’s and Uranus’ orbits and circularizing them and probably all of these interactions over time have worked to circularize most of the orbits in our solar system… except for poor Pluto.
    Fraser: Is that the trend that planets want to go on with their gravitational interactions, especially with all of these smaller objects is to circularize their orbit?
    Pamela: Yeah as long as there is other stuff out there to interact with the larger bodies through all of these little tiny interactions adds up over time to a tug that averages out to circular. That’s one of the neat things about having this distribution of debris because every single interaction is very small. Over time all of these interactions add up to a massive effect that initially migrated the planets outward and then went on to orbit and stabilize our solar system into the configuration we have today. There is still this mystery that while things were migrating inwards, why did they stop? What was it about the distribution of material in our solar system that made things such that we don’t have Jupiter in an orbit that is smaller than Mercury’s? Whereas in so many other solar systems we are seeing that. We are also now finding rocky solar systems that look more like ours so we still have a lot of really big question marks in our understanding.
    Fraser: Now do we know if those stars like the 51 Peg or that hot Jupiter that is orbiting it are in a stable orbit or is it just spiraling inward and we’re just catching it at a very terrifying time?
    Pamela: In some cases it’s pretty clear that we are catching them at a very special time. In a few cases we’ve found planets that have material that is getting sucked off of them so basically they are getting gravitationally sucked dry and they’ll eventually spiral in and they will die rather badly. We see some planets that have tails from material that’s getting blasted off by the radiation pressure from the stars that their orbiting. There are lots of nasty things that are going on that are going to cause several of the planets that we can see to spiral into their doom. It’s not a friendly galaxy. Getting stable solar systems isn’t always straight forward.
    Fraser: So you mentioned that one of the big mysteries here is this idea that we don’t really know why the planets stopped this motion. Why did they settle into this final location? There must be some theories on it.
    Pamela: Well people put forward such as: Some stars maybe they blast empty their inner-most solar system and the act of flinging material that may happen through other solar systems heavy bombardment doesn’t fill in that particular area of the solar system allowing thing to keep getting interactions that cause them to spiral in from various drag or other frictional effects. Here the idea is if a star can blast out a region in the center or planetary formation can clear out a region in the center. An outer planet doesn’t have a reason to keep migrating but if that doesn’t happen, if the inner part of the solar system isn’t able to form planets or doesn’t get blasted free of material through the stars radiation pressure. Maybe that gas giant as it goes around, it’s constantly interacting with the inner materials and there is this drag-like effect where it keeps nomming the stuff and it keeps spiraling in further and further. If you have an empty gap, that feeding process stops and migration inward stops. If you don’t have that gap it just keeps going in; that’s one of the theories that people are working on. It’s not finalized yet, we don’t know enough yet.
    Fraser: Is it possible that there were a lot more planets and there were a lot more material and it did migrate just into its doom?
    Pamela: In our solar system we don’t think so. In our solar system it seems like what we have makes sense. We can explain the mass of pretty much everything but the Kuiper belt so while it’s possible, there is no reason to believe that’s what happened. This is based on seeing formations in terms of composition that make sense for things forming in certain parts of the solar system and there not being a whole bunch of material that we can’t account for that got eaten in the past.
    Fraser: So there was a migration that happened in the early days of the solar system but there are probably other migrations that are going to happen in the end of days in the solar system as the sun bloats up into a red giant and start to give off material. It’s going to bring that whole cycle around again right?
    Pamela: Here’s where it starts to get interesting because that is a slow and gradual process. Over time the sun is continually mass and as it loses mass the gravitational pull on the rest of the planets is decreasing. This allows everything to slowly migrate outwards. It’s a constant effect. There doesn’t seem to be any trigger points in this effect that will cause a sudden change in the dynamics of the solar system. It look like things will just expand outwards as the gravity changes. Where it gets interesting is where you have the sun’s radiation pressure changing as it transitions from being the nice hot star that it is now to a much cooler but significantly brighter red giant in the future. We’re going to have the solar surface pretty much creep up on top of us. It’s going to be this interesting combination of the light being right on top of us and blasting us but the gravity from the sun although it’s physically larger in radius, it’s gravitational pull will be less because of the mass loss that just happened. We don’t see any reason for anything bad to happen as a result of this except Mercury and Venus will be consumed.
    Fraser: Will they? They’ll be inside the atmosphere of the sun…
    Pamela: Yes… they will be eaten.
    Fraser: And there will be just so much drag that they will spiral inward at that point and…
    Pamela: I think it’s more a matter of if the sun is going to just bloat out and eat them. They’re not going to have a choice.
    Fraser: But what will actually happen to them inside the sun?
    Pamela: It’s hot and dense and they will become a happy little plasma lock of planets.
    Fraser: And they will just migrate into the core because they’re heavier so they’ll just sink into the core of the sun?
    Pamela: The thing is, it doesn’t quite work that way.
    Fraser: Please explain to me exactly how it does work.
    Pamela: (Laughs) The way to think of it is taking a rock and throwing into a vat of boiling metals or boiling plasmas (that’s not how you think of plasmas), it’s so hot that the rock is just going to turn into its component atoms rather quickly. Some of those component atoms are going to be light and will stay towards the surface. Some of the component atoms are heavy and those, depending on how they get mixed, sink somewhat. Our sun’s atmosphere has titanium in it; it has all sorts of other heavy metals in it that we see in the spectral line so there is no need to think it’s like throwing a rock into a pond. It’s more like throwing a glob of butter into boiling soup; the glob of butter stays at a higher density than the soup and it doesn’t sink to the bottom but instead it stars to sink and then becomes a part of the boiling soup in the process.
    Fraser: So we’ll just see a smear of Mercury in the atmosphere of the sun for a few rotations and then it will be one with the sun.
    Pamela: We’re getting to see how this happens actually and we talked about this in the Weekly Space Hangout last week. We’ve observed some white dwarfs that are consuming planetary bits in their solar system, out in the high 80’s cluster. We can see in the compositions of the white dwarf atmospheres the remnance of these shredded dead, no-longer, planet bits that used to exist and are now consumed. What we’re now seeing with these white dwarfs, the signature of the planets in the atmosphere of the white dwarfs is the kind of signature when Mercury gets consumed. We’ll be dead…
    Fraser: Right.
    Pamela: Someone else will see in another solar system.
    Fraser: As we’ve said, astronomers are still on the fence with whether or not the earth will be consumed or just scorched.
    Pamela: I think more and more of the evidence is piling up that mass loss will be significant enough for us to perhaps be safe. It’s really a question though of how big will the red giant be.
    Fraser: One last series of questions here and these are about the interactions between either binary systems or other star systems that may come close or come to us. Would these have any impact? For example, when we first formed we were in a solar nebula with a lot of other stars and there was a certain amount of gravitational interaction between these objects. Would that have any impact on the planets that formed and how they migrated?
    Pamela: When we first formed, luckily, we seemed to have been in a tight enough ball in this star forming region that we were fairly safe. We did form with this extensive Kuiper belt with the Oort cloud surrounding us. While we have probably gone through periods where different external objects have influenced how many Kuiper belt objects or Oort cloud objects have been disturbed and thrown into the inner solar system. It doesn’t seem to, with our solar system, effect the planets. We do think that in other solar systems there are rogue planets that have been stripped out of other solar systems and wonder somewhat lost between the stars. We just seemed to have escaped that fate.
    Fraser: So far…
    Pamela: Luckily stellar interactions are so rare that statistically it’s unlikely that something of a significance to remove a planet would ever occur.
    Fraser: Even when we collide with Andromeda the chances are nothing is going to happen.
    Pamela: What’s kind of neat is that I’ve seen people run mathematical computations on a black hole passing quickly through our solar system. If a black hole passes through our solar system quickly, it may not wreck too much havoc, it all depends on the crossing times. It’s kind of neat to realize that it’s really hard to destroy a solar system.
    Fraser: Is this the point where we plug Phil’s book again?
    Pamela: Eh, we can, yeah.
    Fraser: Death From the Skies and it has a whole chapter just on a black hole moving through the solar system and it’s an awesome book.
    Pamela: If it goes slowly we’re all toast, luckily black holes are rare.
    Fraser: Exactly but it would be the same as a very massive star moving slowly through our solar system, also a very bad day.
    Pamela: Yes
    Fraser: Well thank you very much Pamela
    Pamela: My pleasure, thank you.
    This transcript is not an exact match to the audio file. It has been edited for clarity.

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