We’re all looking forward to the next generation of exoplanetary research, where we get to see pictures of planets directly. But astronomers are already making great strides in directly observing newly forming planets, helping us understand how our Solar System might have formed.
ESO image (ESO)
Hubble news release (Hubblesite)
Very Large Telescope (VLT) (ESO)
ESPRESSO instrument (ESO)
Proxima b press release (UNIGE)
SPHERE instrument (ESO)
Keck PDS 70 exoplanets press release (Keck Observatory)
Giant Magellan Telescope (GMTO)
NASA Webb telescope (NASA)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 572, Twists in Planet Formation. Welcome to Astronomy Cast, 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, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how are you doing?
Pamela: I’m doing well, how are you doing on this fine spring day?
Fraser: I’m doing great, the weather’s beautiful, it’s perfect, but we get these snow drifts of cottonwood, so it’s like —
Pamela: Oh, yeah, we’re getting that too, like a giant, fist sized blobule of multiple —
Pamela: – yeah.
Fraser: Is it allergies if a big wad of cottonwood goes into your lungs and you cough it back out? Or if it just sticks in your eyes?
Pamela: I think that’s choking.
Fraser: Is that an allergy? It feels like really extreme allergy when you actually have seeds stuck in your eye because you’ve been walking around outside. So, yeah, that’s springtime, cottonwood. Regular allergies plus, I don’t know, enormous allergies, but we’ll get through that and then into summer. I can’t wait. So, we’re all looking to the next generation of exoplanetary research, where we get to see planets directly, but astronomers are already making great strides in directly observing newly forming planets. Help us understand how our solar system might have formed.
And I think you picked this episode because you saw the pictures from the European Southern Observatory and just said, yes please, let’s talk about that.
Pamela: Pretty much and I have to admit that the timing of this just seems right because with Atacama Large Millimeter Array, they have been finding successively more detailed information on how planets are forming; where they’re forming, and their techniques for processing the data is getting better and better so that they’ve gone from simply being able to say, this giant disk around this distant star has gaps in it that we assume have planets to being able to say, this really ugly massive forming disk has eddies in it and those eddies are forming worlds.
Fraser: So, let’s talk a bit just about the process of planetary formation and sort of – so how, up until this point, we expect it should look over time.
Pamela: Now, I’m gonna put the caveat at the beginning of this discussion that if anyone tells you they know, with certainty, how planets form in a solar system, they’re lying to you. We have —
Fraser: I believe I said the word, should, but —
Pamela: – yeah, yeah.
Fraser: – so, how – current theories say, but I mean, that’s the amazing – that’s the magic of these images is now suddenly you’re able to compare the current theories to the ground truth, the space truth. So, what is the theory on how this is supposed to work?
Pamela: Okay, so start to finish, we are finding from detailed observations of star forming regions, collapsing molecular clouds, that in the heart – the dense core of these collapsing systems, where stars are forming rapidly, you don’t get planets. So, in order to get planets, you have to first of all start out with a star forming on the outskirts of its molecular cloud, it will be on the outer parts of what will eventually become an open cluster, like the ones that we observe out in the sky on a regular basis.
Now the reason for this is simple, on the inside you have gravity from everything else confusing matter, slurping material from one thing to another; you have radiation pressure from all these stars turning on, all these combined factors, it’s just a chaotic mess and planets can’t find stable material to form out of in these kinds of systems.
Pamela: So, you wanna start out in the burbs, —
Fraser: Right, right.
Pamela: – or in the rural areas.
Fraser: Yeah, you don’t wanna be near a bunch of really hot stars that are just blasting with their radiation clearing out. In fact, some researchers came out this week talking about this, which I think is what you’re –you’re – you’re bringing to the conversation, but yeah so, outskirts you’ve got clouds of gas and dust, they start to collapse down and you get spenny blob.
Pamela: So, as this fragment of a molecular cloud starts to collapse down, forming what is called either a proplyd or a cocoon, the system flattens over time and begins spinning – think pizza dough, you have a big blob of dough initially, you start it spinning and it’s gonna flatten out. Now, within the flattening blob of material, you can end up first of all, with things colliding together. So, you have particles that collide together, glom on together through chemical, static electricity processes.
And as things collisionally merge, get bigger and bigger, they start to be able to gravitationally draw in material from around them. And this generates eddies in this disk so the idea is, everything is going around that central star, but as the gravity from that starting to form protoplanet grabs onto things, that material ends up swirling around the gravity of that protoplanet, creating a little spiral eddy. So, one of the cool bits of image analysis that is being done to see this, is you have an otherwise smooth disk of material, without these eddies of planet formation, you can go around the disk and see even amounts of light all the way around.
So, a planetless disk is a nice, smooth blob kind of like a record. Now, when you get the eddies of the planets, the rest of the disk will have that nice, smooth, nothing going on over here characteristic. So, you can take the areas of the disk that have no planets forming, get the essentially flat fielding to remove the disc, subtract off the disc’s light from where the planet is forming and get at being able to directly —
Fraser: Right, right, right.
Pamela: – image the planet by removing the light of the disk.
Fraser: Right, right. So, let’s talk about some of the technique, you talked a bit about the technique here, but what are the instruments that are being used to observe these planets? And I think the part that’s so amazing to me is up until this point, we know about planets from these indirect detections; we detect how the light from the star dims; we detect how the star is moving back and forth thanks to the gravity of its planet, but in these situations, and we can only see the ones that are on – edge on, really.
But in these situations, what we’re seeing is the holy grail. We are seeing these systems face on at every angle that you can imagine and we are seeing these tiny, little Frisbees, records, we are seeing these planetary disks under formation, which is absolutely incredible. So, what are the instruments that they use to make these images?
Pamela: So, the broad scale, let’s zoom in on something faint with a high resolution array of dishes spread out to give maximum resolution possible, that it is being done by the Atacama Large Millimeter and submillimeter Array, down in Chile. This array of dishes – because they are spread out so much, gives them the necessary resolution to make out the detailed eddies in these systems.
Now, while that’s really the workhorse that’s giving us the highest resolution data, it’s not the only game in town. These systems can also be imaged in the infrared, where they give off the most light because they’re just warm gas and warm gas is easiest to see in, well, warm light infrared. So, here we’re to starting to see people using Keck, Gemini, Subaru, very large telescope. All of these massive mini meter telescopes that are scattered across the planet, all have the ability to start to zoom in on these disks and see different characteristics, depending on the age and the size of the system.
One of the more shocking things to me personally, is I expected all of the action to be going on down within 30 astronomical units of these stars because that’s where all the action in our solar system is located, within —
Fraser: Right, yeah, you gotta get close.
Pamela: – yeah.
Pamela: That’s not the case. So, what we’re finding is there are planets appearing to form out hundreds of AUs away from these central stars and it’s the massive size of some of these disks that is allowing some of these telescopes to be able to see what’s going on.
Fraser: And I think one of the things that is also added to that is this idea that right now the planets that astronomers are finding are the ones that are orbiting really close to their stars. The hot Jupiters; the super Earths; the mini Neptunes, but they have a very tight orbit around a red – an M dwarf star and things that are not similar at all to what we have in this solar system and yet, the fainter – the planets that take years to go around the star, decades to go around the star, like Saturn does. You would need 30 years to confirm its existence seeing – under one of these standard techniques.
But in this case, when you just look at the planetary disk from above, you are seeing and you can go oh, there’s gonna be a planet forming right there, at that orbit, at that speed of probably that kind of mass and then another planet over there. Like suddenly you’re seeing this survey of planetary systems in a way that astronomers could only have hoped for, dreamed of, years ago by looking at babies as opposed to looking at fully matured planetary systems.
Pamela: And one of the fascinating things that we’re finding is somehow planets migrate in ways that we didn’t know. We have the NEISS model for our own solar system that says, at some point Jupiter and Saturn were in a residence with one another; they flung Uranus and Neptune out to much greater distances; they bloated out our solar system. But what we’re finding as we look at these forming solar systems is things start way out, in a lot of cases, and so the question starts to become, do they form way out and migrate in? Do they form at all distances and some stuff just gets lost?
We still don’t have those intermediate pictures and part of the issue is, with the infrared and the submilimeter, what we’re looking at is disturbed disks of material. We can see the discs; we can see the gaps in the discs; we can see the eddies in the discs, which are either pointing to there’s a nice, fully formed planet in there that has already stripped out this gap or there is a forming planet that has material spiraling into it. And then, we can see the close-in planets, using Doppler; using transiting methods. What we can see is any of the steps in between or any of the system’s in between.
Fraser: Right, but I mean the most incredible thing with the ALMA is that you’re just – you’ll see a picture they’ll release, oh, here’s another 30 planetary disks and you can see just the variations of all of these different ones. But the image that really blew me away last week is the one that came from the very large telescopes, ESPRESSO Instrument. Did you see that picture?
Fraser: So, have you done any research into how the ESPRESSO Instrument works? I did a whole video on this if you’re —
Pamela: I’m gonna say, your whole video is much better than my few paragraphs of reading.
Fraser: Oh sure, so the way it works, so ESPRESSO – the very large telescope is these four – the biggest telescope in the world is these four 8 Meter – 8.4 Meter class telescopes in Chile, and they act like one singular telescope with the resolution of their separation. And they – and some one of them, they’re equipped with this instrument called ESPRESSO and I forget the actual acronym, but its job is to see the – essentially the reflected light that is coming off of planets that are orbiting around stars because of very – essentially the light, when it leaves the star and it bounces off the planet, it gets polarized.
And so, they’re able to then remove all of the light from this image that isn’t polarized and zero in on this reflected light of planets. And now, multiple times they have been able to see, like literally resolve the blob that is that planet, the newly forming planet, orbiting around the star within this protoplanetary disk. And it’s still – it’s not an absolute slam dunk at this point, there’s still some people that think it could be explained by some other feature, that it’s – there’s just a knot of gas and dust that’s in the protoplanetary disk.
But we are at the point now where astronomers are starting to see these baby planets orbiting those other stars, hundreds of light years away from us, which is just absolutely incredible.
Pamela: And ESPRESSO stands for the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, which just rolls right off the tongue.
Fraser: Yeah, that really cleans it up, right? Yeah.
Pamela: And this is the system where they’ve really been working to figure out, how do we correct for disks versus not disk light, to make sense of everything it has been seeing and it’s just starting to turn on. It was initially – it started being offered to the community in March of 2018, which means we’re only now beginning to see publications coming from it. So, these couple of systems that they’ve already been able to identify, this is just a beginning of what it’s going to be able to do. And we’re starting to innovate new instruments on multiple different systems. Subaru recently had its own detections that it’s made.
Fraser: Yeah, Keck has its version, as well.
Pamela: Yeah. And what this shows is a desperation to figure out how do we fill in the picture book of solar systems forming? We have those early images of the proplyds; we see what’s going on in the star forming regions; we see the disks and then we see the fully formed systems. And it’s going to be systems like ESPRESSO that, as they push their own limits, are going to allow us to start to fill in the pieces. And what we really want to be able to do, and we’re not there yet, is do a statistical analysis and say, okay, we estimate this star is this age, we see its planets here, let’s look at all the stars we can of the same age, see how common is it for planets to be at different distances.
Is it a function of the mass, the formation place? There’s gonna be a lot of physics going into what causes solar systems to form in different ways? But it’s only by seeing myriad versions of each different stage in the evolution that we’re going to be able to build the same picture for planetary system formation that we already have for star formation. I can look at star and if I know its mass and its age, I can tell you pretty much everything else about it because that’s the dominant feature. If I can only measure its temperature and its luminosity, I can then get at its mass and tell you everything about how it formed and its going to die, unless it has a binary companion and we talked about that last week.
Fraser: Yeah, yeah.
Pamela: With planets, there’s more.
Fraser: Well, I was thinking about just sort of the state of the research right now, which is that when you think about what we can do, we know of 4,000 planets; some of which are in multiple planetary systems, but they are the extreme planets, they are the super – they are the mini Neptunes; they are the hot Jupiters; they are the ones that are orbiting and taking days, at most a year, to go around their star. And then on the other hand, we see these snapshots of protoplanetary disks with all of the different parts, and as you say, going out to dozens of astronomical units away from their star.
And we don’t have a complete survey of what planetary systems look like today, we only have a snapshot of the most extreme planets. It’d be like if you looked at the solar system, you could only find Mercury and then use that as a way to describe the solar system, when it is not an accurate description of the solar system. And so, it’s being able to merge those two and then adding on top of that, this idea that they shift; that the planetary systems shift over time and so you have to be able to predict that, as well.
But ESPRESSO is really a prototype for the kind of instrument that’s going to be installed on these next generations of telescopes, like the extremely large telescope. So, what will that be able to show us?
Pamela: Well, so the question often starts to become, how much detail is there to be seen? Now, when you build bigger and bigger telescopes, it gives you higher and higher resolution and I think the key factor with something like that is going to be, with our current telescopes the resolution that we have limits us to only being able to do detailed observations of things that are close by and let’s face it, there’s only so much in the nearest volume of space. But as you build your telescopes bigger and bigger, you can start to get the same spacial resolution; you can start to get the same number of pixels per astronomical unit on the object that you’re looking at, at greater and greater distances.
So, the massive mini tens of meter of telescopes that are being worked on, are going to allow us to see the nearest systems in greater resolution and it’s unclear, at this point, we have no idea what that would show us.
Fraser: yeah, i mean the hope is, like i know the extremely large telescope was built, in theory, with the capability with a coronagraph to block the light from the star and be able to resolve Earth-sized worlds orbiting sunlike stars in our neighborhood. And you get one pixel, it’s not like we’re gonna see continents and stuff, you just – like, does exists is —
Fraser: – what you get. But still —
Pamela: And we’re looking more at Nyquist samplings, more like two pixels, but —
Fraser: – luxurious, so civilized, yeah.
Pamela: – so being able to see at the resolution that we’re currently seeing, objects like Keck’s observations of PDS 70’s protoplanets, being able to replicate what Keck and Subaru and these single mirror 8 to 10 meter class telescopes, what they’re pulling off, being able to replicate those observations, but for a larger volume of space, will start to give us the statistical understanding of, if you see this, then you see this; if you see this, then you see this, which will tell the story of how solar systems evolved that we can’t get at right now.
Fraser: So, what do you think are the biggest mysteries right now that we would love to have answered? And that should be possible with – when you think about all of the tools at astronomers’ disposal now, you’ve got these incredible infrared telescopes; you’ve got next generation radio telescopes coming, like things with a very large array; you’ve got this next generation of the giant telescopes, the extremely large telescope, the Magellan telescope; you’ve got these space telescopes coming, James Webb, – what answers, what questions, what mysteries will start to get progress?
Pamela: So, the story that I’m hoping to get an answer to, is what is the typical distribution of rocky worlds to gaseous worlds as a function of the star’s size? It’s completely reasonable for us to say, and this has so far held up by observations, that in general, tiny stars have mostly tiny worlds because they formed out of one nebula, that could only be so big, if it had been bigger, it would have formed a bigger star. Bigger stars, we believe, are going to in general, be more likely to have massive planets, but the question becomes, as that star gets bigger and bigger, and as it starts to become capable of adding more and more gas giants, what is the typical distribution as the function of star size of rocky worlds to gassy worlds?
Now, that only tells us what formed there. We know even in our own solar system, our solar system formed with more worlds than what we have today.
Fraser: Yeah, yeah, we ate them.
Pamela: We ate them. Well, it wasn’t just us, Jupiter ate one; Uranus probably ate one.
Fraser: Yeah, Venus clearly had a bad day, yeah.
Pamela: And so once you start to get at, well, how do things form? It becomes a question of how do things move around, can we start to get an understanding of what is that story of, do things form massively far out and migrate in until whatever’s driving that migration turns off? And then, in some cases, get flung back out as we see with the NEISS model for our own solar system. So, I want to first get at, what is, in general, the distribution of kinds of planets you see as a function of star size?
Pamela: And then I want to get at where do they form and where do they go on average?
Fraser: Right. And so I can imagine the situation where you’ve got like – like in one situation, each portion of the disk nicely forms a planet, the star clears away the dust and then the planets are mostly locked in place and it’s a very – and then there’s some leftover material that heavily bombards everything around and you’re done.
Pamela: And that’s what we learned in high school and that is not true.
Fraser: Right, yeah, yeah, yeah. And then the other idea is just this mayhem where you’ve got multiple planets forming in inappropriate areas; they are crashing into each other; they are flipping each other upside down; they are shifting inward and outward and you’ve got that – that, in fact, the formation of a planet like the Earth only came through just constant rain of fire and you got a nice planet, then suddenly it consumes another Mars-sized planet and now it’s all molten lava again and it just happens again and again and again, just imagine sort of the chaos and mayhem and it has serious implications for even the formation of life.
Pamela: And for all we know, our solar system may have originally formed with 20 to 30 objects that we would call planets —
Fraser: Yeah, and then they all, one by one, just battled it out to the death.
Pamela: – some got flung away, Oumuamua-style and we also don’t know if we capture other worlds. I mean for all we know, that ninth planet that shows up in mathematical simulations of the outer minor bodies, that could be something we stole ruthlessly from another solar system.
Fraser: Yeah, yeah. It’s —
Pamela: I don’t know.
Fraser: – and I think that for people who are watching, as you watch the news over the coming years as these new instruments come online; as these new telescopes, this is one of the primary questions in astronomy, that we should see well resolved in our lifetimes, is that we should see enormous numbers of planetary systems discovered; enormous numbers of protoplanetary systems discovered; and we should see the statistical analysis of this get to the point where we do know the answers to these questions.
And then you look at – and back to your original point, imagine looking at a yellow dwarf star and going, I think we know how many planets are there, now let’s just confirm that they’re there.
Pamela: And as near as we can tell, while it’s going to be more complicated than stellar evolution, we can already see there is a clean relationship between if you have too little metalicity in the area that formed the star, so globular cluster-like environments, no planets, more metals, more planets. We’re already seeing that scientific relationship documented. We are already seeing documented that solar systems, in the process of trying to form, are distinctly different in low-density and high-density environments.
So, we’re narrowing in on the complicated physics that describes planetary formation. Once we figure out what it takes to get the planets, now we need to figure out how they evolve through their own interactions and it’s – I’ve decided we’re no longer in a golden age of astronomy, we definitely need to upgrade to that platinum card.
Fraser: That we’re in platinum age? That’s awesome, I love it.
Pamela: Yeah, I think we’re platinum age.
Fraser: Yeah, perfect. All right, on that note, Pamela, do you have some names for us this week?
Pamela: I do, as always, our show is supported through the generous contributions of a myriad of individuals that make what we do possible. This show, it’s not Fraser and I that make this happen, we rely on Richard Drumm to do our audio editing; we have Ali Pelfrey, who’s maintaining our YouTube channel; we have Beth Johnson, who’s putting up our show notes; and we’re able to pay these people a decent wage for the amazing work that they do for us and we only do this because of you.
Today, I would like to thank the Patreon supporters at Patreon.com/astronomycast, Omar Del Rivero; Brent Kreinop; Tim Gerrish; Arthur Latz-Hall; William Andrews; Jack Mudge; Mark Grundy; William Lauer; Jeremy Kerwin; Bruno Leitz; Michelle Cullen; J. Alex Anderson; Dustin Ruoff; Joe Wilkinson; Marco Erasi; Mark Steven Rasnake; Brian Kilby; Jessica Felts; Gabrielle Galfin; Jordan Young; Burry Gowen; Berko Rolin; Ramji Enamuthu; Andrew Poelstro; David Truog; Brian Cagle, TheGiantNothing; Dan Litman; Laura Kittleson; Robert Palsma; Corey Davoll; Jos Cunningham; and Paul Jarman.
Thank you, all of you, for everything you do to allow us to do everything we do.
Fraser: And thank you, Pamela, we’ll see you next week.
Pamela: See you next week. Bye-bye everyone.
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