Ep. 540: Weird Issues: How Do (or Don’t) Planets Form?

As astronomers started to discover planets orbiting other stars, they immediately realized that their expectations would need to be tossed out. Hot jupiters? Pulsars with planets? We’re now decades into this task, and the Universe is continuing to surprise us.

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Fraser:                         Welcome to Astronomy Cast for a weekly facts-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. Hi Pamela, how you doing?

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

Fraser:                         Good. There is a typo in my introduction, and I look over it every single time. It says and director of CosmoQuest, but actually you’re the director of CosmoQuest, and I make that mental change every single time. But it’s too difficult for me to go and edit the thing, so I’m just going to keep going. It keeps the mind sharp I think is what I’m saying.

Pamela:                        It does. It does.

Fraser:                         Before we get into this week’s episode, are there any interesting things to talk about?

Pamela:                        Yes. Yes. We have two, two, count them, two things that need to come up. The first one is on October 5th, everyone should go out, look up at the moon, and celebrate International Observe the Moon Night.

                                    The second thing is we have a new podcast over at CosmoQuest. This is the Daily Space. And we are putting out short, roughly 10-minute episodes most Mondays through Fridays that will help you understand the latest news. These are episodes that basically give you a quick rundown of everything you might run across on Universe Today, but get the 30-second version and then go read the three-minute article later.

Fraser:                         And we’re recording these moments before Elon Musk gives his Saturday presentation on the state of starship, the Starhopper, the newly constructed prototype, which I’m assuming he will stand in front of and tell us what happens next.

                                    So, for those of you who listening from the future, wasn’t that something? All right. Moving on.

                                    As astronomers started to discover planets orbiting other stars, they immediately realized that their expectations would need to be tossed out. Hot Jupiters, pulsars with planets, we’re now decades into this task, thousands of planets and the universe is continuing to surprise us.

                                    Pamela, if he went back 30 years and talked to a person searching for solar systems, for other planets, what do you think they would’ve expected the universe would probably look like?

Pamela:                        They would’ve said, with probably a great deal of insurance, that it is only medium-sized stars that have planets, that the big ones don’t because they have too much light, and the little ones don’t, because they don’t have enough mass. They would say that solar systems had rocky worlds snuggled up against their stars and had gassy worlds further out. And they would’ve said that planets might even be exceedingly rare. We don’t, but that rareness was always brought up.

Fraser:                         Right, that planets would be rare. And now here we are, like I said – man, I don’t remember. Was it ’89 when they found –

Pamela:                        So, the first –

Fraser:                         – 51 Peg?

Pamela:                        No. So, the first planet –

Fraser:                         ‘95.

Pamela:                        – to 51 Peg was ‘95, and then the first planet in air quotes that you can’t see on a podcast was actually found around a pulsar, and that was during the ’91-’92 school year. So, the first time we found one it was around a dead, compact object, and then it would be nearly half a decade before we got to finding one orbiting a legit star that was a legit planet, and it was a legit planet that looked like nothing anyone had ever predicted.

Fraser:                         Yeah. Yeah. And totally, totally surprising as you said like when no one would have predicted.

Pamela:                        And so, the first problem that we had to try and figure out was well, what is the actual distribution of planets. And at this point, we were still thinking only Sun-like stars probably have planets. And so, people started doing all sorts of mental gymnastics figuring out okay, so we can migrate the gas giants in if we do these things and we do this other thing, and we’re not sure how to stop them, but we were good. We were good.

                                    And then, we started looking at more and more kinds of stars. Initially we were limited on what we could discover, because the way we were looking for planets was by looking for the gravitational tugs on the stars that they’re orbiting, which means we could only look at one star at a time using high-resolution spectra.

                                    Well, as we started to look for them using transit searches that looks for the dips in light from the stars a planet passes in front, suddenly we were able to look at large swaths of the sky and multiple stars at the same time, and suddenly we were finding stars by the dozens, by the hundreds, by the thousands, and we were looking because they were in our field of view at a whole new range of kinds of stars.

                                    And this was when we started discovering that pretty much every kind of star that we looked at happened to have a planet if it happened to be high enough metallicity. And suddenly, oh. Oh dear. We had to start changing what we imagined, and the place that we didn’t change fast enough, in my opinion, was massive stars.

                                    Hi. This is Dr. Pamela Gay. I want to invite you to listen to my new podcast, the Daily Space. Most Mondays through Fridays, our team at CosmoQuest brings you a quick rundown of the daily news in space and astronomy. Check it out at DailySpace.org or subscribe using your favorite podcast catcher.

Fraser:                         Okay. So, let’s – I mean we’ll sort of reevaluate all of those things. So now, as a modern exoplanetary researcher considers what’s out there and in what kinds of configurations. So, let’s start with the massive stars then. So before, stars that big probably didn’t have planets. Now what do they think they have?

Pamela:                        Well, the original thought was the massive amount of energy given off by a massive star would push back all the material trying to form planets, and planets would not form. But now thanks to worlds like KELT-9b, we know that systems form planets, and those planets could be snuggled up right next to the stars such at poor KELT-9b on its surface has temperatures that look like the surface temperatures of a star.

                                    Now this is not it generating that much heat, it’s getting heated that much from the outside, and it’s cooler on the other side of the planet. We did not think this would be possible, and it was sort of a I’m going to go look for an undiscovered country work by the likes of Scott Galdi that led to people looking for planets around these stars that otherwise weren’t being searched.

Fraser:                         So, people were like even just not looking at these stars?

Pamela:                        Yeah. They were just pooh-poohing the idea. Oh. That’s a waste of telescope time.

Fraser:                         Yeah. There’s no point.

Pamela:                        Just don’t do that.

Fraser:                         Yeah. Don’t even look. And now I mean I think that’s nicely reflected in a spacecraft like TESS, which is just like forget it. We’re just going to look at them all. If it’s a star, we’re gonna look and figure it out.

                                    So then, seeing this planet around a much more massive star, then what are the implications for that?

Pamela:                        Well, it starts to tell us first of all that well, somehow the stuff of the disc is still able to coalesce enough to form planets, and may be that the planet formed out beyond the area that was cleared by the star’s light. And we have this problem of planetary migration that we really haven’t figured out where we keep finding these Jupiters. Where we can’t explain them forming there.

                                    So, somehow they’re gradually migrating towards there star and stopping before they get into their star, and I’m not even going to try to explain that, because I don’t think anyone can explain it.

Fraser:                         Yet.

Pamela:                        Yet. It just like – how did you get there? No idea.

Fraser:                         Yeah. But I mean the timeframe for these really massive stars is so much lower. Like for the supermassive stars, the only live for a few billion years at the most, and then they explode as supernova. You know, some of the in-between stages between us and some of those more massive stars, I mean does it make sense to go looking for planets at Betelgeuse or other super red giants? Blue giants? Things like that?

Pamela:                        Well, so it’s looking more and more like some planets can potentially form in hundreds of thousands of years based on some of the models they’re kicking around. So you have planets working to form the same time that star systems are working to figure out to star, and so you have the stars, the planets, all of them forming in a mess, and the model that we had even up until a few months ago was you have this disc that is flowing material in torrents that forming star. The star lights up, pushes back, the flow stops, but in that mix, you’ve had planets beginning to form, and the way they’re forming is small dust grains collect together, form bigger dust grains, those dust grains collect together, form a bigger and bigger and bigger stuff until eventually start to get protoplanets in planet-planets.

                                    And when it was thought this is the key, it was thought even a year ago that when we use the Atacama Large Millimeter Array to look at these young star systems, when we see gaps in the discs around the stars, those gaps must be places where planets form, because we didn’t have another way to explain the gaps, and the gaps perfectly fit what we had in our models.

Fraser:                         So, you’ve got this situation like not – I mean the great thing about say ALMA is it can directly observe a protoplanetary disc from any angle, and there’re some really wonderful images that have been taken with ALMA showing these different discs face on, almost edge on at different angles.

                                    And all of the methods of detecting planets right now, they really require things to be lined up perfectly. A planet passes right in front of the star, yanks it back and forth or blocks the light, but ALMA, we see these records spinning in space or pinwheels for all kinds of crazy shapes and go okay, there’re planets there, and yet there’s also going to be a really hot massive star there like a Wolf-Rayet star.

                                    Something really powerful, and yet you see the planets coming together, and that was not expected.

Pamela:                        And it’s in these systems with things like T Tauri stars that when you look at them, we can now start also looking at the system where we’ve identified the gaps with ALMA, and we can start using other telescopes that look in the sky in infrared in the colors that planets are giving off the bulk of their light.

                                    And what we’re finding is those planets aren’t always in the places that we thought they would be. We’re finding gaps that don’t have planets. And they don’t even have eddies that say there’s a planet here you can’t see.

                                    And this is one of the amazing things about the resolution of ALMA is ALMA can see not just the gaps but in many cases it can also see the eddies that are left behind by planets. And having found those in some places and not found them everywhere tells us that different things are happening in different places.

Fraser:                         All right. So, we’ve talked about one sort of whole class of stars and the planets that are around them. Did you want to consider another kind of place that maybe planets were either thought impossible or unlikely?

Pamela:                        Well, from one extreme to the other, the next place to go looking is those tiny planets or rather tiny stars that we thought had tiny planets. This is where we have systems like the TRAPPIST-1 system that is a red dwarf star that with TRAPPIST-1 is orbited by seven tiny terrestrial worlds.

                                    And so here we have our entire solar system with planets capable of having water on their surface that are probably not habitable with life as we know it, because little red dwarf stars tend to go through violent youth and give off high radiation flares, badness, sterilized worlds, but despite this horrible childhood, these planets are there, and the idea here initially was there’s just not enough mass to have a disc capable of performing planets and okay fine. So that was wrong.

                                    So, now we have this idea of you have a tiny star with a tiny disc, and the tiny disc forms a multitude of tiny planets, and we understood that. And we thought that’s what we would always see. And then today, after we had planned this episode – so this is a well-timed press release, we got news of red dwarf star GJ 3512.

                                    This is an object 12 percent the mass of our sun, and it has orbiting it a plant that is intermediate in size between Saturn and Jupiter. This is a giant planet compared to a tiny, tiny star.

Fraser:                         Well, I mean just to be fair, the smallest possible red dwarf star is going to have say 70 to 80 times the mass of Jupiter, and you’ve got it something with say twice the mass of Jupiter?

Pamela:                        It’s half –

Fraser:                         Oh. Half the mass Jupiter. Yeah. So, it’s definitely a scaled down version of the solar system, but it’s not like you just – go ahead.

Pamela:                        And our models don’t allow for a system with this mass ratio to have formed the plant through this bottom-up, dust hit’s dust hit’s bigger dust, forms planetesimals, forms planet. That model does not work.

                                    So, now what we’re starting to think is it may be possible to form solar systems in multiple different ways, and that is not nice, universe. Not nice.

                                    So, here we’re looking at perhaps we can get these gas giants forming with these baby stars in what’s called a top-down model where that fragmenting cloud of material that formed the star didn’t fragment into one big fragment that spun up and formed planets around the star, but rather it formed into two fragments side by side, rotating around each other where one of those fragments form the planet and the other one formed the star.

                                    And this is similar to how binary stars form. So now instead of a binary star, it’s a start and a planet.

Fraser:                         Right. So, when they have a common center of mass is outside of the star? I wonder if –

Pamela:                        Yeah.

Fraser:                         – they’re going to be orbiting – yeah, a common point, which would be –

Pamela:                        So, this is similar – it’s more exaggerated than Pluto-Charon, but it’s a cool system. Yeah. Will call it that.

Fraser:                         But I mean just the idea. I mean as you said, you’ve got these red dwarf stars. They have this tiny amount of material compared to what a star like our sun does, and yet when you look at say the TRAPPIST system, there are six, seven planets – seven planets known so far. Who knows what else could be orbiting a little above or below the plane of the ecliptic farther out into the solar system.

                                    I mean it is a bustling star system even compared to the solar system. And when you think about the fact that the vast majority of the stars in the Milky Way are these red dwarf stars, you know, mind blown.

Pamela:                        Well, and beyond that, we’re only starting to be able to sample what’s out there. We still aren’t finding the mercurys. We are just starting to find super earths, which are really some Neptune objects that we optimistically call super earths.

                                    We don’t know the full diversity of what else that is out there, because we haven’t been looking long enough. As you started out by pointing out, this is still very much the early days.

Fraser:                         Yeah. Yeah. So, I mean I would be interested at this point now, I mean do we have a sense of what a standard solar system looks like?

Pamela:                        No.

Fraser:                         Star, planets. That’s it?

Pamela:                        So, what we have is –

Fraser:                         Mostly?

Pamela:                        – we can poke the system from a variety of different ways, and as far as we know, low-metallicity stars. So, these are stars that don’t have a lot of iron. They don’t have a lot of carbon. They don’t have a lot of heavy elements in them.

                                    These kinds of devoid-of-heavy-element stars appear to still be devoid of planets. So, that part of our original understanding was true. If you don’t have the stuff to make planets, you do not make planets.

Fraser:                         Right.

Pamela:                        The parts of our theory that constrained little things and big things, totally bogus. We need to figure that out again. The parts of our theory that constrained how big the planets can be relative to the star, totally bogus, need to start over again.

                                    Our ideas of how a disc is able to form and move and migrate planets is really the next big thing that we need try to get a handle on. One of the biggest questions in my opinion in planet-forming models is how do you migrate a planet and then stop its migrations?

                                    The simplistic model that we’ve been using is you have a disc. The very center of the disc is empty, because the early stars’ light cleared out the center of the disc, and when the migrating-inward star runs – not star, the migrating-inward planet runs out of material to interact with frictionally, it stops migrating.

Fraser:                         So, I mean this idea of these migrations, I mean they actually can happen really quickly. I’ve heard that you can actually move your planets on the order of tens of thousands of years once they’re drawing and material from one side of the disc of the stream that they’re in, and depending on sort of where the material is coming from, they will absorb it onto their body, and at the same time, this induced a torque that moves them quickly. But here in the solar system, our planets moved outward, right?

Pamela:                        And this is another one of those confusing points. So, when we see the youngest planetary discs out there, we’re looking at discs of material that are massive in radius compared to the size of our own solar system.

                                    So, there seems to be this two-step process where you migrate the material inward while consuming it into planets or something, you end up with a smaller solar system based on this much larger distribution of material, and then once you have all your planets in the middle, you fling them back outwards.

                                    And this isn’t something that you see talked about in general. What you see is the observations from ALMA of these 10s, over 100 AU planetary discs. Then you see systems like the one we live in that are 55, 60 AU before you start running out of planets and Kuiper belt, and you also see of course all the solar systems with the hot Jupiters in the center.

                                    And dynamically, we have models for our solar system. And that the bottom of everything I say today, your take-home message should be model could be totally wrong.

Fraser:                         Right. Well, yeah. And so, just I mean part of this conversation is like don’t we have a bit of an observational bias?

Pamela:                        Yes.

Fraser:                         I mean the fact that the first thing is found were hot Jupiters is because they’re easy to find. If you ask me to find trees, I will find you Douglas firs until you’re sick of Douglas firs until you’re like fine, you know, enough Christmas trees. Thanks. I got it. And maple trees. I can find you Douglas firs and maple trees.

Pamela:                        I have gum trees and oaks.

Fraser:                         Yeah. There you go, right? And so, they are close. I can walk outside and I can find a bunch of them for you right now. And so, we have this observational bias of big planets orbiting closely to small stars.

                                    As the capability of the tools change over time, are we starting to see some kind of shift to get a better sense of what’s normal? What the new model might be?

Pamela:                        I’m not sure we’re ready to say what is normal. I think we’re ready to say that we were wrong. We were just wrong.

Fraser:                         Good. That’s a start.

Pamela:                        You know, it’s an important start. One of the reasons that scientists do science, and I’ve said this before, and I’m going to keep saying it, is because we don’t know all the answers, and we really want to. These new results with ALMA are going to form the foundation of here is the beginning of what solar system models should look like.

                                    Right now, we’re looking at a few high-resolution images. We need to get to a few hundred before we have statistical distributions. We’re only starting to find planets around tiny stars, and tiny stars make up the bulk of our galaxy. These are the stars. And they’re awesome. And we’re still learning new things about our own solar system.

                                    We are starting to understand that you can explain the size of Jupiter’s core as the result of Jupiter getting whacked hard four billion years ago.

Fraser:                         Right.

Pamela:                        This is new information in the past few months, and by realizing that our galaxy – we knew our solar system had been a supervisor in place. We knew that. We can look at the moon and see that, but to see it in the size of Jupiter’s core, to see it in how we recognize other worlds now, planetary solar system formation is far more complex and violent of a process of things moving in and moving out than we ever imagined, and I’m starting to see it like it’s a really bad square dance done right children who don’t hear the caller over the music very well.

Fraser:                         But do you foresee –

Pamela:                        That is my best modern analogy for planetary formation.

Fraser:                         I’m going to need a different one. I need you to go back and keep work shopping it.

Pamela:                        Okay.

Fraser:                         Do you foresee this time when astronomers will have a standard model of planetary formation where you take the mass of the star, the metallicity of the star, you take the – punch that in and out comes a reasonable expectation of what kind of a planetary system you will probably find around it?

Pamela:                        We need to come up with some cool constraints. These include things like you need to know the metallicity of the star. You need to know the size of the star, mass, temperature, all of that. It’s sort of like one thing. And then going into it, you need to understand what is the molecular cloud that’s fragmenting up? Are we looking at something that’s rapidly collapsing because it took a hard knock, or is it more gradually collapsing?

                                    We don’t yet know how things like that can affect the formation rates in terms of massive open clusters end up with one distribution of – we know the end up with one distribution of stars. They have one initial mass function, but do they also have a different planetary initial mass function? We don’t know how the distribution of planet masses compares from two different stars of the same type that formed into different environments. These environmental effects are going to be a whole lot harder to get at.

                                    But with ALMA today and the Square Kilometer Array coming in the future, we’re gonna have more and more potential to get there. And when if JWST ever launches, that’s –

Fraser:                         It’ll launch.

Pamela:                        – and function –

Fraser:                         Don’t say if. It’ll launch and function. Don’t you say those words. It’s gonna happen.

Pamela:                        I’m going to be cautionary, because if I’m pessimistic, it has to work to prove me wrong.

Fraser:                         I can’t wait until we do an episode on James Webb. It’ll happen.

Pamela:                        But there’s the ability in the next decade for us to get the data that is needed to constrain our models and to encourage our creativity, because both things need to happen.

Fraser:                         And we will bring every part of the story to you as the evidence is discovered and the previous theories are overturned and nature reveals its mysteries one after the other. So, thanks, Pamela. That was awesome.

                                    Before we go, do you have some names that you may want to say to thank our generous patrons for their ongoing support?

Pamela:                        Yes. We have some wonderful patrons that allow this show to happen and allow us to pay Susie to take care of us and take care of all the well, day-to-day activities around CosmoQuest.

                                    And those who are making all of this possible are Mathias Hayden, Ron Thorson, Brandon Volverton, Gregory Joyner, Rachel Fry, Darcy Daniels, Eric Ferenger, Kelsely Penflinko. You guys are welcome to give me pronunciations. I’m so sorry. Ryan and James, Kristin Brooks, Duane, Isaac, Shannon Humbart, Dean, Glenn McDavid, Dan Littman, Paul Veller, Martin Dawson, Russell Petto, Kenneth Ryan, Bart Flaherty, Jason Graham, and Brett Peterman. Thank you.

Fraser:                         Thank you everyone. All right. We’ll see you next week.

Pamela:                        Bye-bye.

Susie:                           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 info@AstronomyCast.com, Tweet us at Astronomy Cast, 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 Suro, and the show was edited by Susan Murph.

[End of Audio]

Duration: 32 minutes

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