Inside the Stars: a chat with Dr. Gael Buldgen

Welcome to Growing Worlds, the podcast where we explore the science of exoplanets through conversation with early career researchers on the front line of discovery. I'm Lorenzo.

And I'm Jenny. In each episode, we meet a guest who takes us behind the science of planets, stars, and the search for life in the universe.

Today we focus on the stars that host planets. Because if we want to know a planet's size and composition and its potential for life, we first need to understand its star.

And to help us navigate this stellar side of exoplanet science, we're joined by Dr. Gail Buljan from the University of Liège. Gail, welcome to Growing Worlds.

Thanks a lot for having me.

Girl, he's up.

we're very happy to have you. Thank you for coming on the show.

You

Gale is a postdoctoral researcher at the University of Leish, where he works on detailed characterization of stars, which is the very foundation of our understanding of exoplanet. He specializes in astrocysmology, which is the study of stellar oscillations, which allows scientists to probe the internal structure of star with exquisite precision. It's a bit like doing an ultrasound on stellar interior. Through this, Gale's refined key stellar parameters such as mass, radius and age, which impact directly the shape and how we interpret the properties of orbiting planets. He also contributed to missions like Kepler, TESS and is involved in preparing the ESA's Ariel and PLATO missions to make sure that when we analyze exoplanet atmosphere we truly understand the lights of the star that is behind it.

So Gail, before we get into the science, right, we should talk about how you got into the science. So it's like your story, you what drew you to study stars and to astronomy? Was it a particular moment in your life or was it you were just always obsessed? You know, what drove you into stellar physics?

So, Stella Physics itself, started third bachelor year at the university. So I did a bachelor in physics. I was hesitating between physics and engineering. It was a very pragmatic choice. My brother is an engineer and he's older than me. I didn't want to do the same as my brother. So I went into physics. So, exactly. So I did my bachelor in physics and I did a...

Mm.

You don't want to put all the X in the same place, right?

Yeah.

was a small internship in the group of stellar physics in Liège. And I really liked it. So I liked the idea that you could, you know, that you could model what's happening inside a star. And the fact that there is a wide variety of physical processes happening there. have the largest scales to the smallest scales. have convective cells in the sun, for example, the size of Europe, and you're probing also atomic physics like to 10 to the minus 15. meters. So that's something that I was very interested in into and got me into continuing in Stellar Physics. when I chose the Masters, I just picked every course that had the word Stellar in it. And we were lucky we had a lot of options and I could continue and specialize and then it became quite naturally. But the primary choice to do physics? Yeah, I just didn't want to do the same as my brother. That was the primary choice. Yeah.

sibling rivalry! it's not that I actually liked physics, I just wanted to get back at my brother, there we are!

Exactly. No, I mean, I think I would have liked physics anyway, but really, think the way I approach things, I guess I was much more happy doing physics the way I do now in research than being in a company or something like that. I'm better off, I think. So it paid off in the end, but it was a very pragmatic choice.

Yeah. Yeah. Yeah. Were you interested in astronomy, as a kid, or was it just kind of like when you got older and you were looking at university choices, it just seemed like a good option.

I always liked astronomy. I always liked astronomy. I mean, already as a kid, you know, and I was just... wandering around the internet looking at stuff. Also, I got interested in space missions really early, but I didn't know whether I would go on the engineering side, designing missions, trying to get into ESA, things like that, or going really more on the scientific side. I was very hesitant on these aspects. But at the end, yeah, I chose to go for the physics.

I think what's quite telling is that when you said you're always looking at space, it wasn't you out looking at the sky, you were looking on the internet. And I think that that is so telling that now that you've gone into theoretical, using computers, building models, it was there right from the beginning, that little seed.

No, no. Yeah, yeah So I really think that the way I, that's also why I enjoy stellar physics itself is because I see stars as laboratories. So that's what I liked about that. know, it's like, okay, like with seismology, we're going to probe the internal structure of a star, its dynamics. And at the end of the day, you're looking at dynamical processes and you're talking, I ended up talking to people doing, for example, opacity measurements or plasma physics or things like that. And so I'm really, for me, it's really, yeah, I'm kind of an experimentalist in the end. It's just that I use oscillations to do my experiments in a way.

I mean, to be fair, you can't put a star in a lab. A lot of people wish we could, but we can't.

No, you can do a few tests in stellar conditions but not many and yeah that's what makes Delta Stars very interesting at least from a physical point of view. You have a lot of detailed physics you can test.

When we talk about characterizing exoplanet, we often hear about the stars and light curves and retrievals. But behind all these lies the star that you love and that you use as a lab. So can you put on your lab coat and walk us through how knowing this star properties like temperature, composition or age is so fundamental in understanding the planets?

The key thing with stars really is that they are opaque. So the first thing when you look at them, only see, it's like looking at a bowling ball, you only see the surface. So when we think about temperature as the surface temperature, we see. the luminosity is the total energy radiated by the star that's going to be very interesting for the planet because it's going to determine the amount of energy it's receiving. Then you have the chemical composition where it can actually be, see the surface chemical composition, it can help you constrain also the population in which the planetary system is formed because you have to put your stellar models in perspective with the galactic evolution also. But really for the age, you need to go inside. And that's where seismology really comes in. So for there, it's really, you mentioned doing an ultrasound. I like the analogy. What we do often as an analogy also is to say, you know, because it's most of the oscillations we'll see at the surface of solar-like stars will be acoustic modes, it's pressure waves. So when I explain this to students, I say it's like you listen to a musical instrument and you try to reconstruct the musical instrument from the sound it makes. That's what we do with stars in the end. You try to probe the internal structure of it and determine some key characteristics like how evolved is it, how much hydrogen remains in its core if you're looking at the main sequence star, for example, how massive is it, because essentially the mean density of the star is going to determine the period of the oscillations, the dynamical time of the oscillations. From there, get

I love that.

you always kind of get the fundamental parameters mass radius age that you will need to characterize your exoplanetary system. Because if you do a transit, I mean, you get the size of the planet relative to that of the star. Radial velocities, well, it's the mass of the planet relative to that of the star. And when you see these, you know, in exoplanetology, you plot often, you know, the mass and radius diagram and things like that. And sure, but where is the time in this? How did these planets land in these positions? You know, and they have evolved around the star and they've

Mm.

with the star. How much atmosphere a planet will remain depends on how long it has been irradiated by the host star. So that's why when you look at Plato for example you have detailed objectives. We have requirements on how precisely we need to characterize the stars. And also for Ariel, you have a working group dedicated to that, actually, to seeing how far we can push the stellar models and how well we can characterize the stars. So...

I want to follow up on... Sorry. Nope. Okay. Okay. I'll throw this. So, you said two... a lot of fantastic things. There are two things I want you to expand a little. First thing, in order, as you said, you mentioned that we need to put the stars and the planets into the population.

So with these key, no, go on, cut this bit out, carry on, follow up.

You

were deformed. And you are not referring here, I guess, the planetary population in sense of the planets and all the other planets on the same orbit. You are talking about the stellar population. Am I right? How can this affect how the planets build? And before you...

Yeah.

Diving into that, I also throw you the second question, which is, you mentioned, and this is fundamental for today's topic, that you cannot have a measurement of the planets without a measurement on the star. So, can you walk us through a little how this determination of planetary mass and planetary radius that you mentioned with the two methods that you mentioned before? But first, the population.

Mm-hmm. So the population aspect, well, there is, as you know, in astrophysics, we kind of separate the elements, you have hydrogen, helium, and then all the other heavy elements, right? And there is this tendency.

We're not too picky, just the first two and the other ones are just the other ones.

Yeah. It's hydrogen, helium and metals. I mean, you don't need anything else. Like, periodic table is just too complicated, frankly.

Exactly, the other ones are metals. In this respect, you have for example the question of how much metals do you need to actually be able to form a system with terrestrial planets, for example. This is not going to be possible when you're looking at the first stars.

Go

because these will not have synthesized metals. All the heavier elements in the universe, mean heavier than helium, are synthesized in the core of stars. So you need the proper environment to be able to form your planets. So this is why some surveys that are looking at different populations, for example, of exoplanets are very interesting in that to understand where they can actually form. Or if you think about environment like stellar clusters, how many planets do we know in stellar clusters? I wouldn't be able to answer that question, but we haven't looked at many of them. But if you imagine if you're in a stellar cluster, you have all these stars forming together, a lot of interactions, how stable actually can planetary systems be? I don't know if we have an answer to that question yet. So that's why, for example, also analyzing the planetary populations, you have to think about it. If we want to understand the formation process, we need to look at a wide variety of stellar environments to understand how planets form, how they migrate actually in these conditions, depending how the protoplanetary disk will last and how long it will last and things like that. there is a lot of uncertainty in this and that's why I think a lot of the synergies between, at least between stellar physics and exoplanetology have not yet been fully exploited perhaps, because there lots of things still to be understood about stars and about stellar populations. Another question was about transits and radial velocities, right? So to understand, yes.

Yes, the measurement of the planetary properties compared to the stars.

So essentially what you're looking at when you're looking at the transit method is you have your star and you have your planet passing in front of it and you'll see a small dip in luminosity because the star the planet is essentially just you know eclipsing a small part of the small portion of the the stellar surface. So that dip in luminosity of course will be proportional to the size of the planet with respect to that of the star. And of course the star is much bigger, much brighter, so that's why you have a lot of programs dedicated also looking at very small stars, like what we would call Amdorfs, because the transit depth then will be actually deeper, because the star is less bright than, let's say, a giant, for example. So that small dip will give you the relative size. If you know the size of your star, you have the radius of your planet. You're good.

How do you find the size of a star?

Ha, you have different methods. So for that you need models or if you're lucky you have interferometry. So you can actually get the angular diameter from a star directly from interferometric measurement if it's close enough, if you can resolve the surface with interferometers.

Isn't that amazing? You can actually resolve nearby stars. How nearby is nearby?

Exactly. That's a good question. So I... That's a good question. I would have to look actually at the value itself because it depends also on the instrument. It's gonna depend on the instrument I think. The closest one to give you an idea...

Close enough if I'm out of sugar.

would have to look at systems. The most well-known system of solar-like stars would be 16 Cygni, which is also a binary system, but I wouldn't be able to tell you the distance by heart.

But are we talking sort of like hundreds of light years instead of like thousands of light years or tens of thousands? How far out can you actually sort of resolve?

No, no, no, you wouldn't be able to do that in thousands of light years. You have to be closer than that. You have to be in the solar vicinity.

So it really is just the very nearest.

Yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, yes, Other option, if it's a binary system where you actually can actually see the motions of the the star and have a detailed characterization, that helps a lot. Otherwise you models, models. You're stuck with the models. You'll need to fit a model to your observations.

But if you can't do that, yeah. So... okay. And what's in these models?

lot of things. you have to, if you have your, so in your stellar evolutionary models, usually you have to also keep in mind we're not modeling just the star at a given precise point in time, right? We're simulating its entire evolution. So you have to, it's like a kitchen recipe if you want, you have to decide what chemical mixture you're going to put in it, how much hydrogen, how much helium, how many metals.

Mmm.

which will be dominated by oxygen, carbon and nitrogen mostly. Then you have the how you're going to treat turbulence in your model. What are the nuclear reactions you will choose? What transport of chemicals you will actually consider apart from turbulence? If you have, you can actually think about it. It's an interesting case, but in sunlight stars effects like sedimentation actually play a key role. Like, you know, if you think about it from the surface of the star, if you think about, say you have an oxygen atom in the outer layers of the Sun, in the outer convective layers of the Sun. It actually is heavier than its surroundings, so it's going to sink. It sinks very slowly, but since it has billions of years to do so, it significantly influences the chemical stratification inside the star. So you have to take that into account in your models.

Yeah. wow. Yeah.

You need an equation of state for the plasma. You need to model the interaction between matter and light. So all what we would call opacity. So all of these have to be modeled, usually taken as standards from physics computations like lab computations or theoretical computations. And then you have to do your entire simulations and you hope to reproduce your observations. that's, yeah, there's a lot of...

And then, so when you say reduce the observations, when you say reproduce the observations, if I can get my words out, so this is things like you can observe how bright a star is, for example. And so it's like, if the model can reproduce that luminosity, well then the radius that it's telling you is probably pretty accurate.

Yeah. Yeah. Yeah. Yep.

you're going to try to have, usually for the radius, if you think about it, you're going to try to get... the thing you're going to get is the surface chemical composition from spectroscopy. So that's what you're going to try to at least have that. If you that correct, and then you're going to have the luminosity, the surface temperature. And those would be, I would say... Now, there are lots of people, we're still doing that a lot, but these are, I would say, the weak constraints.

Okay. Have that correct from the model.

But then the interesting case is when you can actually have oscillations. When you detect the pulsation modes, so observationally, if you look at the solar, you just... I mean, to detect observation from solar-like stars, it's just a photometry. You just point your telescope, you look for a significant amount of time. For Kepler, it was four years, for example. You do the four-year transform of that. You look at the time series and you'll have peaks. It's going to be a com-like structure. You're going to see the peaks of the modes regularly spaced. And then you're going to try to reproduce that. So you have your evolutionary model. And the question is, is it going to reproduce the music you're seeing? So you're going to simulate the oscillations.

That's amazing.

It's here to beat.

Yeah.

reproduce the music of the stars, I love that. So I looked at like Kepler stared at these stars for sort of four years, because it stared, it gives you that baseline to then put it from a time series into frequency space. And then yeah, look for the frequencies which stand out and then that gives you the vibrational modes. And if the model gives you those vibrational modes, you're like, yes.

And then you're gonna just... Yeah? Yeah, continuously. Yeah.

Wow.

No. No. No. Yeah, exactly.

the structure should be good, the structure should be good enough and to put this into perspective, you're gonna think, Kepler is amazing, we've done that for four years, for The Sun we've been doing that for 30 years. So the data set, you have that. Yeah, yes, yes. If you look at, so it's been launched, was it 1996? And it's still going. So the time series there are insane. To give you a...

We know how big this star is and yeah. my goodness, have we actually been staring at the sun continuously for 30 years?

It's just been, yeah. And I guess because the sun is so close, you know, you can maybe sense modes which you can't detect maybe on other stars just because of distance and the signals aren't strong enough and right.

Exactly. Exactly. You have something that we call geometric cancellation. So for the sun you see the surface. To describe the oscillation modes, actually will have, you you can detect them in velocity also in spectroscopy You'll have one region that will go towards you, one region that will go under, you know, or you actually have these pressure variations. So, mathematically they would be described by what we call spherical harmonics that people doing quantum mechanics have seen a lot and essentially if you think about it, if you have a mode that has a lot of these oscillations up, down, up, down, up, down at the surface, when you don't see the the surface of the star, you're going to have cancellation. You see, you're to do some of all these parts, you're not going to see it. So you only see what we will call the low degree modes. And to put that into perspective for other stars than the Sun, we see the order 0, 1, 2, or degree 0, 1, 2, sometimes 3. For the Sun, we go up to 1000.

Mmm, yes. course. Mm-hmm. my goodness, so you can sort of see the sun vibrating on like the tiniest. Yeah. Wow.

wow.

you do whatever you want with the sun the sun is really

And how good are our models of the sun by now then?

That's a good question. The models of the Sun, I can tell you that's why also we are a bit confident when we provide the age of a star. For the Sun, we can really do a full tomography of its structure and dynamics. And we see that the models reproduce the internal structure down to something like... The worst disagreement we have is in one specific region of the structure of the Sun, it's about 1.5%. the worst and you know we're obsessing about that but it's it's very they're very very accurate yeah

The worst. 1.5 % is the worst. So how, yeah. It's very accurate. Yeah. So how do the different structures inside the sun sort of manifest in these vibrations so that you're detecting through doing the astrocysmology?

So if you think about the sound waves, right, they're going to be sensitive to local sound speed. It's like if you think about the sound propagating into a medium.

Hey.

it's going to be sent, the frequency is going to change or the path is going to change depending on the local sound speed. And that's exactly what you have inside the star. So the first thing that's going to manifest itself is the way the sound speed is changing inside the sun or the star. So it's essentially pressure over density or temperature divided by chemical composition. And that's how you can actually directly then probe, for example, the conditions and the thermodynamical conditions in the plasma. So you can check it out. If you go on Google, you look SoundSpeed profile inside the sun and then you'll see actually the profile measured. from a model, measured. Actually measured. is...

Hey. Okay, yeah. No, but actually, that's wild. Isn't that wild? And so then when the sound speed changes, that then tells you, okay, now the plasma is structured like this and it's structured like that. And then you get your different layers. So do you want to take us from the core of the sun out?

major.

Yep. So. Yeah, sure. Sure. So in the core of the sun, what you have is essentially the nuclear reactions occurring. So you have the proton-proton chain. So hydrogen being fused into helium. And essentially, you have quite high sound speed there. The medium is quite dense, actually. And then it decreases. And you have to imagine 97 % of the mass of the sun, in 97 % of the mass of the sun, the energy is transported by high energy radiations. The sun is a big nuclear reactor. You have energy being produced in the core, you need to transport it outwards. So you have a decrease in sound speed as you're moving out from the core.

So how much of the core is the sun? Like a percentage? Is the core like 10 % of the radius of the sun?

in radius it's going to be about between 20 and 25 percent of the core yeah yeah they increase relatively sharply in temperature the efficiency of the nuclear reactions so you can actually see that you see the the the change of hydrogen inside the sun you can actually see it

20 and 25%. So you've got all your nuclear reactions going on in that 20%.

Yeah. How do you see that? I mean, you can't really see that, right? You, you measure it how?

Well, from the sound speed, mean, essentially, because the sound speed is going to be sensitive to changes in chemical composition, and that's what's happening in the core of the sun. So you'll actually see that. So I told you before, we have the disagreements down to 1.5%, right? Worst case scenario, 1.5%. That's the disagreement. The uncertainty we have on that, I think it's 10 to the minus five.

Okay. Okay

Basically, well, in astronomy, there's no uncertainty. We know what is happening in the sun. So is it, because you were saying how it like will drop off in the core. So is there kind of like, because of the temperature change, so is that like, there's right in the middle of that 20 % is there's loads and loads of nuclear reactions and then it's still happening in the core, but as you get further out in the core, it drops off.

It's Yeah, exactly. The efficiency of the nuclear reactions drops as you're moving out. And then what you can actually map, what you can actually have a very good trace of, is the switch. So I told you 97 % the energy is transported outwards by radiation. And then at some point it's turbulence taking over, right? The outer layers of the Sun are convective. We have that look.

Mm-hmm. And that's where we've got physical motion. So convection, it's like boiling water. It's, know, moving around like that.

Exactly. So the position of where that happens, we have it down to one per thousand in radius.

Like that's just what you just know exactly where it happens. It's...

We know because we know that seat full marks.

And because the waves will actually see a change, when the modes will propagate, they will actually see this change in energy transport. And they will give you that location extremely precisely. so we have been able to do this for the sun. We have been able to do that for some stars. So we are reaching that.

Mmm. Yeah. Wow. That's amazing and it's and have we found that with the stars that we've been able to do it for that it's quite similar is In terms of as to what we find in the Sun

Not always. That's the interesting take. So not always. Like we have the models for the sun work very well. And I always tell when I have students, know, the sun is a very controlled environment because if you think about it, the mass of the sun, we know it very well. It's not from the models, the radius also.

Yeah, because we can measure. Yeah. No, no, it's an actual measured. Yeah.

the age also, but it's from the meteorites. We know it from the meteorites. We don't know it from stellar evolution. So when you compute a solar model exactly, you know when you have to stop your model. So in a very controlled environment, but for other stars, we don't know. So the uncertainties then become much larger actually.

Mm-mm.

So that's why we can go from having this insane precision for the Sun to, for example, for a 1.6 solar mass star, for example, you can have a change of 20 % in age in no time.

and that's only slightly bigger than the sun.

The problem is actually the changes in the nuclear reactions that are dominating inside the star. You go from the PP chain to the CNO cycle, which occurs in a turbulent zone, because you can understand this from a physical point of view, the temperature dependence of these nuclear reactions occurring in the more massive stars is much higher. So you have a lot of energy produced in a narrow range of temperature. You need to transport it outwards very efficiently. So radiation is not enough.

Even,

so you have turbulence taking over. And when that happens, you actually have what we call in stellar evolution a convective core. You can look up, if you're interested, the literature on this. It's been a plague for stellar evolution for as long as stellar evolution has existed. And the problem is that you can change then very efficiently the amount of hydrogen you have for your star. So you can have something that looks... I mean, you don't know exactly how much hydrogen the star can burn. because it can be transported.

It's mixing, yeah.

and your mixing is not well reproduced in your models. And that leads to large uncertainties in age measurements. So this is why in this case, again, seismology can help in the sense that we can then map where these transitions occur. We can try to give extensions of these mixed regions to improve the ages. Otherwise, mean, you don't know if it's a young exoplanetary system or an old exoplanetary system. You can have relatively wild effects. And if you think about the luminosity for example or the mass, if you have a massive star the luminosity can change quite a bit and also the way it changes over time changes drastically with mass so that changes a lot for example the irradiation history of your planetary system and that can have dire consequences for the evolution.

Right, okay, yeah. Yeah, yeah, because we like a stable star. Stable planet, stable star.

Exactly.

You mentioned all these uncertainties and the fact that if you don't know the age, this affects the understanding of the planets. But you also mentioned before that all the measurements of the planets, the direct measurements like radius and mass are dependent on the stars, right? So do you want to articulate for us or expand for us the concept of how the uncertainties of these parameters can change our understanding of the planets?

So the direct effect for the radius, it's a direct relation, it's R squared, it's the surface, so that's clear. And for the mass, it's in your radial velocity measurement. So depending on the mass of the planet or the mass of the star, you'll see a different changes in the way they orbit each other. And so what we measure when we measure what we call these radial velocities, it's like what you do in binary stars. You measure the relative motion of the star and the planet or the two stars. And so there directly you have a direct relation, it directly propagates. The uncertainty that you have on the stellar mass will directly also propagate on the planetary mass. I don't remember the factor to be honest. I don't know if it's linear or not. remember I would have to check. Depends on the formula but essentially it's a direct proportion. If you don't know the star, don't know the planet. And it's an issue for Kepler, for example, actually. We have a lot of planets detected with Kepler for which we don't have a good radial velocity follow-up. So the masses remain quite uncertain for some of the other Kepler stars, actually, because you didn't have all the southern hemisphere high-quality instruments that could do the follow-up, because Kepler observed mostly in the north.

And so if you're uncertain on things like how stable the star is and if its luminosity is changing and stuff, can that then impact how we think whether a star could be habitable or not or a planet, sorry, could be habitable or not?

Yeah, definitely, because it's going to change. So the problem is that the luminosity of a star is going to evolve a lot, especially before it lands on the main sequence. On the main sequence, it's relatively stable, but you have a direct relation between mass and luminosity. The higher the mass, the higher the luminosity. And so if a star is more luminous, you can think about it. It's going to give more energy to the planets. The planet is going to lose much more atmosphere. The habitable zone is going to be pushed out. also is with more massive stars, they tend to rotate faster also, and that leads also to more activity, so more high energy radiation. That is also bad for habitability, because when we think about habitability, again you have to define it. It's not just one thing to be in the region where you can have liquid water. If your star is ejecting, is doing coronal mass ejections like every now and then, it's going to be a bit difficult.

Yeah. Yeah, yeah, yeah. It's like we get Fun Aurora, but because our star is, it only ejects comparatively the tiniest amount of stuff. It just kind of does a little puff. It's not screaming at us every few days.

So that's.

Yeah. And it's, the sun is a very quiet object. mean, and already if you think about it, if you can have a massive event, like if you think about the Carrington event, where you had all the electronics, I mean, at that time there were not electronics, but I think you had, I remember with the event.

telegraph system.

exactly that were actually reactivated at some point, although they had been disconnected, but they were actually transmitting again just because of the event. yeah, have to be, I mean, we don't necessarily realize how actually dependent on the star we are. You can have actually planes deviating their routes because of solar activity. So if you think about that and people tell you, the sun is a quiet star.

Yes.

Okay, but how quiet? Well, very quiet compared to other objects and that can change a lot the evolution of the system, the habitability, but also the evolution of the planetary system. Even if you have a planet that is in the habitable zone compared to its luminosity, if it has been strongly irradiated before, it can have lost a lot of its atmosphere.

Yeah, yeah. Yeah, yeah, and then that inhibits the chances for future life.

Exactly.

So do you think, you know, with our current understanding of planets, you know, of course it's somewhat limited in terms of, you know, we're now moving into that era of exoplanet characterization over exoplanet discovery, but is our kind of characterization of exoplanets going to be significantly impacted by our lack of understanding of the stars that they're orbiting? what do you think is going to be the dominating factor actually studying the like planets themselves or the fact that we don't really understand the stars that they're orbiting?

I think that there is merit to both and I think that the best approach is actually to not isolate them in the sense that the star can be influenced by the planet. You can actually have, if you have a star engulfing a planet, it can spin up a bit. It can lose mass. So it can have chemical peculiarities. So that's interesting also from a point of view of stellar evolution.

Okay. Mmm.

And of course, you can have star-planet interactions, can have tidal interactions, the planets migrating, you can have radiation, as I mentioned before, if the planet is close enough, you can have magnetic interactions also, you can actually have the reconnections between the lines and stuff like that, and it'll actually couple the evolution of both objects. So I would say that there is... What I'm very enthusiastic about is starting to have characterization of planetary atmospheres. That's something that is actually funny in a way because, okay, we're gonna be able to relate the composition of the planets to the composition of the star. And that's gonna improve a lot of understanding of the formation of the systems and understand how both actually influence each other. So for me, that's something that looks very interesting, but I would say that in this respect,

Mmm.

I would not see, for example, like some people would say, Plato is a stellar mission or Plato is a planetary mission. For me, it's both. Or aerial, same. You know, it's going to focus on exoplanets, but we're going to learn a ton of stuff about stars in this respect. And I think that we have to see both and we have to understand that it's a coupled system. It's mutually evolving and it's mutually influencing each other. And the sun is a very good example of that. Again, if we go back to what we

Okay. Mm.

kind of no, if I tell you, for example, that the way the solar system is formed is actually not influencing the outer layers of the sun, but the inner core. It's actually influencing the neutrino fluxes we see, because we talked about seismology, but the nuclear reactions produce neutrinos, and the neutrino fluxes are influenced by the way the solar system formed.

Mm. Huh. That's... That's...

And that's an interesting take because it's completely unexpected. It's actually two colleagues of mine that worked on this. And they started asking me for some results and I said, okay. But then they showed me the results and I was like, what? But that's not going to influence the sound speed. That's going to be more important for neutrinos. And then we checked the neutrino fluxes and yeah. So you see, you can have surprises like that when you take into account planetary formation. And that's why it's important also for stellar physicists. to take that into account. So that's where we're going to learn a lot, I think. If we do this synergetically, we're going to get the best of both worlds.

I like that.

Amazing. Before we wrap up, I want to ask you our signature question.

Haha, so we love this. It's always fun to end on.

If you have to choose one celestial body, which could be a star, a planet or something else entirely that represents you, what would it be and why?

Mm. It's difficult to answer.

This is the question that's flummoxing you. We've like thrown stellar models, planetary evolution, the insider stars. This is, this is the one that gets Gale.

What I... Yeah, that's the one that throws me off. So... No, I would have to stick with the one I like the most, I think, which is the sun, because there's a lot of hidden stuff and there is a lot of fun parts in the sun that don't necessarily always look fun, but that are very... I mean, there's a lot of chaos going on. there is a lot of unknowns and there is a lot of diversity in it in the sense that you have a lot of various influences and various physical processes happening in there. And I think that's why I like the Sun and that's why I think the fact that you have tiny, I'm a person that pays attention to a lot of details, I think, and in the Sun that's what you have to do. So that's why I identify a lot with the Sun and why I match with the Heliosysmology and solar modeling.

Mmm.

even I would say not more than stellar modeling but for me the Sun is a star so that's why I I would I would identify with that at least

Brilliant, I love that. Gail, the man who has more than meets the eye, just like the sun. Look at that. Thank you so much for illuminating how stars are far more than just background lights. They are the keys to unlocking the secrets of their planets.

Yeah Thank you very much for having me.

And thank you all, everyone, for listening, understanding worlds behind our own stars, with understanding the stars that give them light. Join us next time as we explore another corner of the universe and the people bringing it closer.

Boom, one take.