Ep. 678: World Building: Planet Formation, Growth, & Ejection

Okay sci-fi writers, today we’re going to give you a guided tour of building planets. How they form, how they grow, and how things can go horribly horribly wrong.


(This is an automatically generated transcript)

Fraser Cain [00:01:19] Astronomy Cast. Episode 678. World Building. Planet formation, growth and ejection. Welcome to Astronomy Cast for weekly facts based on 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, the publisher of Universe Today. With me is Doctor Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Causal Quest. Hey, how you doing? 

Pamela Gay [00:01:41] I am doing well, but sleepy like you. And not even quite as extreme as you. We got up to attempt to watch a giant rocket. Yeah, go up and hopefully come back down. And it did not. But we we are still paying the sleep penalty for it. 

Fraser Cain [00:02:02] So we’ve like even though the weather is just horrible here, we’ve been getting a lot of really interesting wildlife things. The swallows just showed up for us for the first time this year. The daffodils have opened up for the first time this year where I live. And sandhill cranes. So I’m on the flight path for sandhill cranes that go from their summer to their winter homes. And it’s just amazing. Like you hear them when they’re over the horizon, and then you go outside and you wait, and then and then it’s just this giant formation of sandhill cranes that say overhead hundreds and hundreds and hundreds of them giant birds, like, as big as anything. Any other bird that you’ll ever see and like, think like pink flamingos size like the big. Yeah, yeah. And they, they fly overhead and they make this great sound. And we have these ponds in like a little marshy area. You know, I’m out there and I’m like, wave them. Come on down here. Like spend the night like I will, I will offer you great facilities. But they just keep flying overhead. So so so far we have yet to convince a, a a wing of sandhill cranes to come and land our property and enjoy an evening on our palatial estate. 

Pamela Gay [00:03:28] That would be amazing. 

Fraser Cain [00:03:29] It would really be amazing. Yeah. Yeah, I’m really excited. Hopefully. Apparently around here, they do. So. So various places. They’ll stop for the night. And it’s just like, cacophony just sounds wonderful. 

Pamela Gay [00:03:43] That’s. Yeah, that’s. We just have lake barred owls being very loud this time of year. Yeah. And they sound like monkeys. And it’s really disturbing to wake up at two in the morning to what sounds like monkeys going between the trees, but it’s just the owls. 

Fraser Cain [00:04:02] Yeah. All right, sci fi writers, today we’re going to give you a guided tour of building planets, how they form, how they grow and how things can go horribly, horribly wrong. All right. So how do you want to approach this? I mean, we’ve got. How do you get this is how you get planets, I guess. How do you get planets? 

Pamela Gay [00:04:24] Well, when when a large enough, stellar nebula gets knocked by a shockwave of some kind, it might collapse. And while collapsing, sometimes, if you’re very lucky, you can end up with a blob of matter glob ING on gravitationally to other blobs of matter until they form a planet. 

Fraser Cain [00:04:52] All right. That’s too simplified. I even use a more complicated word. Like I get the analogy you’re trying to run with here. When have Mommy Planet lost? Anyway. How so? Like, what is the. I mean, how do we get these planets? 

Pamela Gay [00:05:13] So. So basically when when you have a star forming, you have a cloud that is majority hydrogen, helium. And then in addition to that are going to be heavier elements in a variety of different proportions depending on how many different supernovae and other things like stellar mass loss through solar winds have occurred in that vicinity. 

Fraser Cain [00:05:39] So like how polluted you’re still in tabular is by previous generations of stars. 

Pamela Gay [00:05:44] Exactly. And if you have those heavier materials as everything comes in. Angular momentum is is the great deterrent of so many things in this universe, as a material tries to fall in towards that forming star. It ends up forming a disk. And within that disk you have the potential for these atoms to form molecules, to, form dust grains for dust grains to then collide together and through electrostatic forces, begin to build larger and larger things until they start to get big enough that gravitationally, they start pulling in the material around them. 

Fraser Cain [00:06:31] On top of that, for one, for one second. Sorry. Yeah. So, I mean, and this is something that always sort of confused me. Was that you if you have these little pieces of dust? 

Pamela Gay [00:06:40] Yeah. 

Fraser Cain [00:06:41] They’re so small and have so little gravity that they’re not going to cling to each other gravitationally when the interactions and perturbations from the rest of the source are just going to keep tearing them apart. And I’m assuming this was a mystery in astronomy for the longest time to. 

Pamela Gay [00:06:59] I don’t know how long it was a mystery. I have to admit. That’s that’s one of those things that I don’t know the history of. But as professors all deal with chalk dust, we are all fully aware that when you you erase that chalkboard, you get this fine particulate of dust just about everywhere. But in the chalk tray beneath the chalk board, you can end up with growing globs of whoa, chalky grossness. 

Fraser Cain [00:07:29] Really? 

Pamela Gay [00:07:30] And and this is usually because humidity in the air, someone spills something and you get chemical reactions. And these chemical reactions can allow the chalk. Chalk and the chalk dust tray. It’s not gravitationally holding itself together. It’s chemically holding itself together. And and so once you’ve seen that, it’s easy to go from. Well, if dust in my classroom is capable of growing over time through the interplay of room conditions and dust, it’s easy to imagine that in the universe itself, you have things chemically coming together and growing as well. 

Fraser Cain [00:08:16] But what you said is that it’s electrostatic coming together. 

Pamela Gay [00:08:21] And if you think about it, if you have two charged particles they come together or if you have two particles where one I guess electrostatic is the wrong word. Electro mechanics electrochemical. If you have two atoms that have energy shells for their electrons that allow them to share electrons back and forth, you can get molecules forming. You can get these molecules then glomming on to other molecules. This is where I prove I’m truly an astronomer who understands hydrogen, helium and everything else is a metal. But you can get these molecules binding together through either charges or chemical reactions to form larger and larger things. And eventually this is this is how you start getting blobs. Does that get big enough to gravitationally Lamont one another? And I just like the word glum. 

Fraser Cain [00:09:23] Right? Yeah, that is the scientific term is glomming. Yes. So you get these little planet nuggets? Yes. 

Pamela Gay [00:09:31] How many cosmos. 

Fraser Cain [00:09:32] Planetesimals like, how big are these things before other forces start to take over a gravity? Like, are they asteroid sized or are they pebble sized? Are they sort of mash together, breaking each other up? 

Pamela Gay [00:09:47] Well, and the reason I’m staring off into the distance is we now know that you have objects like Bennu and Rio Goo that are gravitationally held together, but are made of shattered minerals. And so you have this situation where you have things coming together loosely, and they have to get big enough that they gravitationally crush themselves in the minerals before you can really start having anything more than fluffy dust balls in space, which is rather unsatisfying. Exactly what size that’s going to happen is going to depend on. Are they truly fluffy or are we looking at ice crystals growing? What is the composition? Things further away from that baby star where you’re going to get a lot more ice production, are going to start forming solid objects that are interesting at smaller sizes than I, where you’re dealing with more of that fluffy dust. 

Fraser Cain [00:10:56] So we’ve got these fluffy dust particles. Yes. Ice crystals forming. Things are becoming larger and larger. What happens next? 

Pamela Gay [00:11:10] As as they get larger and larger, they start to go from sticking together because they literally stuck together like two snowballs colliding to they start to be able to gravitationally pull material toward them out of the surrounding cloud. 

Fraser Cain [00:11:29] And have you seen that picture of some of the Shepherd moons orbiting satellites? And you can see this. These things aren’t big. They’re like a kilometer across, but you can see this little spirally winding trail of material that’s coming from the ring being distorted by this moon, and then returning to the ring on the other side and leaving these little wakes. It’s beautiful. Yeah. And it’s just this tiny little thing, these tiny little shepherd moons. You could see the gravitational influence, even just from them. Give that process millions of years. You see what happens. 

Pamela Gay [00:12:08] And and in this case, instead of looking at these pictures of Saturn’s rings, systems start looking instead at the Atacama Large Millimeter Array images that we have of the planetary disks, protoplanetary disks around young stars. And you can start to see the gaps in the disk that are created in the places where planets are starting to come into existence. And and now, instead of having so much a shepherd moon, you have a hungry planet eating out that material, and it’s kind of awesome. 

Fraser Cain [00:12:51] Yeah. So now I guess the larger dynamics of the solar system start to, to take over because these things are now clearing out their orbits. So what happens next? 

Pamela Gay [00:13:05] Now you start to get into this competition of who can grow fastest and keep the most material. And it’s also a battle against the light from the star at this point. So you have two different processes going on. You have the planets gravitationally trying to grab the material within their sphere of influence, which they carry around with them as they orbit in orbits in orbit. You have the light pressure and the solar wind coming off of the young star. Then that is also blasting material outwards. So you have the planets trying to grab material from their surroundings at the same time that you have the star lighting up and trying to blast the material out of its surroundings. And this leads to, we thought initially a model where you would consistently get rocky worlds forming right next to their parent planet, and you’d end up with gas giants in the outer solar system. And right now, we’re not actually sure where and how things form. We just know that they do. And. Once they form, we start having these weird gravitational puzzles work out as the worlds don’t start out necessarily in stable orbits. They’re just forming and grabbing whatever they can gravitationally. And this includes grabbing onto each other gravitationally, and you can end up with the worlds flinging each other into new orbits. And in our own solar system, we think at one point we ended up with Jupiter and Saturn in a resonance, so that they were going around in an integer number so that they could both end up on one side of the sun at the same time. And by repeatedly adding their gravities together, they were able to rearrange the other objects in our solar system, including flinging Uranus and Neptune out to much greater distances from where they probably formed. 

Fraser Cain [00:15:17] And we’ve done a whole episode on planetary migration, but this idea that that all of the large planets started out a lot closer to the sun, and then through their interactions between their gravity, they all shifted outward and hurled Uranus and Neptune even farther, and Uranus and Neptune switched places. 

Pamela Gay [00:15:41] Yeah, yeah. It’s wild. And and now, as we look out at other solar systems, we’re seeing that there are gas giants snuggled up next to their stars, losing their atmospheres. And given the fullness of time, these could become little, tiny, rocky worlds that are, in reality, nothing more than the core of a former ice giant or gas giant. And so we can’t necessarily know because we don’t know where planets initially started in our solar system, just how big an atmosphere different worlds had, what was their original state compared to what they are now. And and that is one of those things that I find fabulously intriguing. Now we know that things changed quickly. So what whatever was was over within probably half a billion years. But that that period of complete chaos continued for, for a period of time as everything rearranged. We just don’t fully understand how it started. 

Fraser Cain [00:16:56] Like when we’re talking about this period of mayhem, like we’re even beyond earlier than the Late Heavy Bombardment, like even rocky surfaces. Yet you just got planetoids crashing into planet like, can you just imagine? Like, yeah, there would be dozens of planetoids all attempting to clear out their orbits, attempting to create material there. They’re interacting with one another gravitationally. Three orbits later, they crash into each other. They form a bigger object. That object pulls in other objects. Like there must have just been mayhem in it, and only when most of it had cleared out did you then get the time when asteroid strikes could happen all over the rocky planets. 

Pamela Gay [00:17:45] And we don’t fully know how many worlds there were. What we do know is Earth was struck by something the size of Mars and ended up forming the Earth-Moon system. We know that Jupiter was struck by something big enough to make its core fluffy. We know that Uranus somehow got knocked over onto its side. Venus somehow got flipped over. So that. 

Fraser Cain [00:18:09] Right. 

Pamela Gay [00:18:10] It it is just wrong in every kinematic way of looking at it. There were other worlds than these, and we ate some of them. Some of them left our solar system. Some probably dove into the sun. Right? It’s just wild. 

Fraser Cain [00:18:30] So, like, thanks to the revolution of these large radio telescopes like Alma, right. The large Atacama Millimeter. 

Pamela Gay [00:18:40] A large millimeter submillimeter array. 

Fraser Cain [00:18:42] Yeah, that we’re seeing other star systems that are forming. What clues are we getting? I mean, are we seeing them in those times when these things are crashing into each other and and making a mess? 

Pamela Gay [00:18:57] So, no, because what we’re seeing is the dusty disk where planetesimals are just starting to coalesce out of the desk. The period of of great chaos would have come after the disk had mostly been used up and blasted away by. That central star. So we’re seeing the earlier period of time. But. It’s still cool because it gives us the sense of just how big solar systems are when they’re first forming. 

Fraser Cain [00:19:34] I mean, I know that we have seen a couple of examples of planets that recently collided. 

Pamela Gay [00:19:40] Yes, but but that’s different from seeing the full on complete chaos space. 

Fraser Cain [00:19:46] But I mean, from our perspective as human beings, when we live, say, 100 years at the most this year, this period is hundreds of thousands of years long, millions of years long. 

Pamela Gay [00:20:00] It’s thought that the initial stage is probably measured in the hundreds of thousands, and then the settling down into orbits is in the tens of millions. 

Fraser Cain [00:20:11] And so we wouldn’t see this bonkers, planetary collision happening nonstop. We would see the end, I guess, as you said. Right. Like, like when you’ve got this giant star system that’s forming with tons of dust, it’s the perfect wavelength for all my to observe this incredible dust structure. But after the star has ignited, blown away all that dust, and now you’re just left with a zillion planetoids. Yeah, they’re not easy to spot. And so we do see them when they collide with each other. And you get this, this puff of dust created from the shrapnel from these collisions. And I know, like like I said, we’ve reported on several of these on Universe Today. Yeah. That are happening in those first tens of millions of years. Now, you mentioned though like some go into the star, some get ejected out. So let’s talk about some of the like, what are the forces that are going to push those planets out of their star system? 

Pamela Gay [00:21:18] It’s the three body interactions are the worst man. It’s just that simple. So you you end up with, for instance, that situation of Jupiter and Saturn, and then they can just keep pumping energy into the things around them and either fling them inwards or fling them outwards. And this dichotomy of options means sun or leave the solar system. And. And then you also have. There. There are we. We haven’t seen them yet. And that’s pauses every time I say the sentence. We haven’t seen them yet. You have read some papers that I haven’t yet. So you’re just. 

Fraser Cain [00:22:07] Waiting for me to tell you where we’ve seen this. 

Pamela Gay [00:22:10] This thing. 

Fraser Cain [00:22:11] To describe. Okay, okay. I’m ready, I’m ready. 

Pamela Gay [00:22:13] So, so there is that potential for there to be a double planet system that manages to grab on to a third planet and fling it outwards. So you can imagine we have the Pluto Sharan system, and then Himalaya gets a little too close and gravitationally gets flung away. 

Fraser Cain [00:22:35] I do not know of any examples of rogue planets on escape trajectories where we can trace where they came from. No. 

Pamela Gay [00:22:45] So. So we have SUVs, we have seen rogue planets, but they have been of the gas giant variety, where it is probably this energy pumping of of that things getting into synchronization and the resonance just flings things away. 

Fraser Cain [00:23:06] And we’re detecting it through methods like, gravitational microlensing is the method for finding these rogue planets. And I don’t think you get a lot of really good information about the velocity of the planet and maybe what its origins were. It sounds like a big ask for us to be able to see that kind of thing. 

Pamela Gay [00:23:26] And one to say there was a a planet on its way away from its star that was spotted by the Hubble Space Telescope in the early 2000 of the late 90s, where it was one of those. Is it a gas giant or a brown dwarf? 

Fraser Cain [00:23:43] Right? Yeah. I mean, there have definitely been some brown dwarfs seen at high velocities moving through the interstellar medium. 

Pamela Gay [00:23:50] Yeah. 

Fraser Cain [00:23:51] So then, I mean, one of the theories right now is that there are as many rogue planets in the Milky Way as there are stars. Yeah. So is it then just generally believed that each solar system kicks out at least one big planet? 

Pamela Gay [00:24:15] So how it would have to be more than that. Because if you think about it, not all of the stars have planets because you have the low metallicity ones. Stars in general are quite often found in multiples. So you’re looking at systems that have planets are likely to evict more than one planet on average. 

Fraser Cain [00:24:40] Yeah. 

Pamela Gay [00:24:40] And that’s just kind of amazing to think about. 

Fraser Cain [00:24:43] Yeah I mean, the most extreme estimate that I had heard for that was that there could be like ten planets for every star, rogue planets, ten rogue planets for every star out there, that there are so many of those three body interactions that kick out material, that there’s just mountains of it out there drifting through the cosmos. 

Pamela Gay [00:25:04] And where we really struggle is we have the capacity through gravitational lensing to see foreground high mass objects, gravitationally land background stars. And these are often, dramatic enough to draw attention such that we also see when a planets, that is orbiting that star also microlensing but star. But we aren’t seeing just random blips of light from planets. Microlensing things left and right and right. Those are going to be lost and a lot of the noise, unfortunately. 

Fraser Cain [00:25:52] So mean we’re going to it’s going to give us eventually like the machine is really going to do. This is the Nancy Grace Roman Telescope. It’s going to be looking for microlensing at the largest scale. It’s going to be watching huge chunks of the sky, watching the brightness of the stars, checking to see if any of them dim. And we should get an upper boundary on the number of planets that are out there, even a sense of how many rogue black holes there are zipping around the cosmos. So Niantic is where we can’t come soon. Soon enough. 

Pamela Gay [00:26:27] It’s true. And honestly, Lsst as well, because it’s it’s going to be able to spot more of the these background stars getting lensed as well with it’s constant night after night cadence and all of the software that’s being developed to specifically look for anything that changes in brightness. So here’s to hoping. 

Fraser Cain [00:26:50] It’s at a different time scale, like variety is going to be taking a picture and then coming back three days later and noting the supernova that went off. Well, that’s Chris Roman will be watching a field of view, slicing it up in time and noting for any variations in brightness of any of the objects in that field of view. So they’re just they’re kind of two different ways to two approaches, hopefully overlapping. But yes. But yeah. So it’s it’s kind of amazing the how you just go from gas dust and the corpses of other stars into the desiccated corpses of other stars into new planets, life and chaos. Thank you Palmer. 

Pamela Gay [00:27:31] Thank you Fraser, and thank you everyone out there who supports the show. And as a reminder, our patrons get ad free versions of every episode through Patreon. 

Fraser Cain [00:27:43] No ads. No just sounds so civilized. 

Pamela Gay [00:27:47] It really, really does. So if you don’t want to hear me reading ads, join our Patreon! This week I would like to, thank a subset of our donors, and you can find out how much you have to give to hear me mispronounce your name over on Patreon.com slash Astronomy Cast. I really don’t mean to mispronounce your names, but here goes. I’m gonna try one more time. This week I would like to thank Kenneth Ryan, Benjamin Mueller, Paul de Disney, Omar dahl, Riviera, Janelle Rucker, Aaron segev, Michelle Cullen, Peter Scott briggs, Mark h. Whittock, Mark Steven Rusnak, Philip Grand, Bruce. Amazing, Don Mundus, Abraham. Cattell, Jim McGeehan. Anna. Tour. And I saw Michael Regan, Dean McDaniel, Ninja neck, J. Alex Anderson, father Proxmire, some Frodo Tenenbaum, James Ruger, Roger Dwight Elk, and Paul L Hayden. Thank you. 

Fraser Cain [00:29:00] Thank you everyone, and we’ll see you next week. 

Pamela Gay [00:29:02] Bye bye. Astronomy cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy cast is released under a Creative Commons Attribution license. So love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Doctor Pamela Gay. You can get more information on today’s show topic on our website. Astronomy. Cars.com. This episode was brought to you. Thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at Patreon.com Slash Astronomy Cast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.