Hello, and welcome to the Physics World weekly podcast. I'm Hamish Johnston. This episode is sponsored by the cavalry prize. The cavalry prize honors scientists for breakthroughs in astro, nano neuroscience, and neuroscience. Transforming our understanding of the big, the small and the complex. The vision for the cavalry prize comes from Fred C, a Norwegian American entrepreneur and philanthropist who turned his lifelong fascination with science. Into a lasting legacy for recognizing scientific breakthroughs and for supporting basic research. The 20 24 cavalry prize in astro was announced yesterday on the twelfth of June. I'm very pleased to have this year's Lau join me down the line from Cambridge Massachusetts. They are David Char of Harvard University and Sara Sig of the Massachusetts institute of technology. They share this year's cavalry prize in astro for their groundbreaking work on the discovery and characterization of extra solar planets and their atmospheres. Hi, Sarah, and David, welcome to the podcast. And congratulations for winning the Cavalry prize in astro. Thank you. It's great to be here. It's great to be here with you. So, Sarah, my my first question is for you. Astronomers have confirmed the existence of more than 5000 exoplanets. And that number keeps rising. And they have wonderful descriptions such as hot jupiter, mini neptune, and super earth. Can you give us a flavor of the different types of exoplanets that astronomers know of? Well, what's really truly amazing is there's a continuum of planets in terms of their mass, sizes and orbits, and it's very astonishing and what's completely unexpected. But these hot jupiter, they're just jupiter, mass, jupiter sized planets that are extremely close to there's host stars. So their atmospheres are heated to 3000 even 3000 kelvin. The mini neptune are particularly exciting because Right now, we think we're on the verge of figuring out what they actually are. Their planets in between the size of earth's and Neptune. Earth, neptune is 4 times the size of earth. And so far, these mini neptune appear to be incredibly common in our galaxy. Yet, we have no solar system counterpart, and they have a very awkward ambiguous average density, so we don't know, for example, if they're rocky world surrounded by a subs, hydrogen or hydrogen helium envelope, or if they're this mysterious water world, a type of scaled up version of 1 of Jupiter's Icy moons that's largely water by mass. But there's so many. We could literally just have a monologue hours long of all the different planet types out there, they seem strange to us, but it it is that simply because we we only know about the solar system, and you know, the the the idea of a of a a hot Jupiter was almost beyond our imagination before we started detecting them. That's right. It's never good in science when we only have 1 example to build an entire theory on planet information based on the solar system. So the large majority of... Astronomers of scientists were completely shocked about the hot jupiter and everything else. But by now, honestly, we've learned to be surprised. 1 thing we're dying to know, at least I'm dying to know is how common is our solar system. Our solar system is actually very hard to find. That works in our favor, we might not be here talking because we've had such a rich diversity of planets to discover. But in the near future, we hope to get an answer for that with 1 of our next telescopes the Nancy Grace Roman telescope that's going to do a micro lens survey and can take the sense. But right now, it could be that, you know, 10 percent or less of sunlight like Sars have an actual solar system. Do you have a you're you're a pioneer in the the study of the atmospheres of Exoplanets. How do astronomers study the atmospheres of these planets that are so far away and and so faint compared to the stars that they orbit, There are 2 related methods that astronomers use and that Sarah and I have used to explore the atmospheres of these exoplanets. And they both have to do with a very special geometry. When our line of sight is aligned with the orbit of the planet around the star, and that means that once every orbit the planet passes in front of the star. And when it does so, some of the light from the star, passes through the atmosphere of the planet. And so if you were if you were looking at the start at that exact moment, you could imagine seeing this little planet in front of it and then seeing an ann, which is the atmosphere surrounding that planet. And so what we're doing is we're using the light from the star as a probe as a as a back light to probe the atmospheric chemistry, and that means that we don't have to spatial resolve the planet from the star. We don't have to be able to take a picture of the planet isolated from its star, which is very demanding in terms of optical design. We can instead use this, this trick and many, many people now use this trick all the time. The the other method is when the planet passes behind the star, then the planet is entirely out of you, and you might think well, that's no good because we're not seeing any life from the planet. But what that allows us to do is to measure the radiation, the thermal radiation from the star by itself. And we can then subtract that from data gathered at any other time when we have both the thermal radiation from the planet in the star and whatever's is left over, okay, However, small that residual signal might be. That is the radiation from the planet. And that's complimentary because that tells us the actual thermal emission, whereas the first method tells us about the opacity. And so we can learn a a great deal from these 2 methods. And David, when when you study the the light that that travels through the atmosphere, are you looking at bit? At light at certain frequencies that's been removed from the solar spectrum, or can can can you actually see light that's given off by, molecules and atoms in the atmosphere? That's right. The the ideas we're seeing the absorption due to whatever atoms or molecules are present in the atmosphere. And so as the light from the star passes through, certain specific wavelengths blanks are removed, and that's because they're being absorbed. And course, that only happens at the exact moment when the planet goes in front of the star. So we we can be be confident that that absorption really is due to the planetary atmosphere. Of But but we can't see emission at the moment or maybe we'll never see emission from the atmosphere because the star is is just so bright. We didn't don't see emission during transmission spectroscopy because the light from the star goes through the atmosphere and gets absorbed. And yes, the light is re emitted, but in all directions. So there's just such a tiny amount we never see it. Yeah. So, no. We do very much wanna study the emission from the planetary atmosphere. But the the best time to do that is not when the planets in front of the star, but rather when it is at some other point in its orbit. So so you can imagine if you were looking at the system and seeing it at quad we're seeing at what we would call a a quarter moon if you were imagining. The analog of the moon around the earth. And so you see some, radiation, some thermal emission from the planet and some from the star, You have to be able to subtract the light from the star and you do that by getting those measurements when the planet goes behind the star. So you take measurements to different times and difference them. And this allows you to not only see the actual emitted spectrum, but to actually see how that spectrum changes as the planet completes its orbit. And and it would change because the the temperature of the of the planet is changing. Is that, could you actually see that? Yeah. Not just the temperature, but you can you can see that the chemistry then changes because the temperature is changing, so you have different... Atoms or molecules that might be present, and we can actually see wins. We can actually see the... There's evidence that you know, the heated gas from the day side of the planet is flowing around to the night side of the planet. So, you know, when the data are up high quality, we get access to a lot of the dynamics and the chemistry of of the planetary atmosphere. Wow. I mean, that is incredible when you consider how far away these things are. So David, using these techniques, what have we learned so far about exoplanet atmospheres. Well, in in many ways, we've learned a great deal, And in many ways, we've learned almost nothing, and there's and there's a very exciting future in front of us. For certain kinds of planets, and in particular for what we call the hot jupiter, we've had access to a very rich data set, and that's simply because they're the easiest to study. Okay? So the hot jupiter, as you might imagine, are both the physically the largest planets that are out there, but they're also the hottest ones. And so when they pass in front of their star, their atmosphere, that ann that you're viewing in transmission that ring of atmosphere, is very large, both because the planet is large and because the atmosphere is puffy because the atmosphere is hot. And so in in that case, we've been able to detect many molecules. So the first detection was sodium atom, but we now have been able to detect water, a carbon dioxide, carbon dioxide, methane, and now we're seeing evidence of photo chemistry. We're actually seeing products that we would only expect due to this specific interaction of the radiation, changing the actual chemistry of the atmosphere rather than just the atmosphere being hot. Okay? And as I mentioned, we can also make maps of the planets. So for these hot jupiter, we can actually measure the thermal emission as they orbit around their star. And we can see to what extent some of the planets are able to take the the the radiation that's dumped onto to the hot bay side because we think most of these Planets are tit locked. They always present the same face to the star, and we can see that red distributed around to the night side. The night side is not cold. Is my point. The night side is a little bit colder, but it's not it's not perfectly cold, and that indicates that some heat is being transported. And and there's of course, this incredibly rich dataset set now coming out of the James webb space telescope, which is really unprecedented, both in terms of its aperture. Okay. I can collect a lot of light during these very special moments. There's only a few hours when the planet goes in front of the star, we have to get as much data as we can, but it's also very sensitive to infrared wavelengths. And most molecules, of course, are active. We see their features at infrared wavelengths. And so the James Webb space telescope is really, very, very special. Okay. So that's the good news. The bad news is that for the smaller rocky planets, planets like the Earth and venus. We know essentially nothing, and that is because the atmospheres are, tiny, and the atmospheres are colder compared to the hot Jupiter. And so we just don't yet have the sensitivity. Now, for certain systems, we are able to begin those studies. And those are for small stars, which astronomers call red dwarf or m dwarf. And by shrinking the star, Okay? We... We're shrinking some of the noise that we're trying to overcome. The bright light of the star is something that we have to to manage. Okay? It allows us to probe the planetary atmosphere, but it also causes a lot of noise in our measurements. And so when we find these small Rocky planets orbiting these m dwarf. Then we can access them if the M dwarf planets are also very hot. And so there's a few systems, where the data are very good, and we've actually learned they don't have an atmosphere. So they might be similar to to mercury in our own solar system where Mercury because it's hot and relatively low mass was not able to retain its atmosphere. And so that's really exciting. We're now at the point of probing what the astronomers call the cosmic shoreline. We're trying to figure out how cold and how how massive does a rocky planet it have to be when we first see it it it's able to retain that at... Sphere as opposed to losing it entirely, which, of course, puts it on a completely different path both for its evolution and also for the prospects of potentially hosting life. So And in in your research, looking at atmospheres. What is there 1 thing that has surprised you the most? In terms of you know, what you've learned about Exoplanet, atmospheres or is, is it is there a new surprise every day for you? Well, I would say the, you know, the story... When you work in Exoplanets, you sort of have to get used to surprises. Right? Because as Sarah was describing, just in terms of the population of planets, most of the planets that we know about orbiting other stars have no analog in the solar system. Okay? They're intermediate and size between earth and, you know, neptune. Okay? We don't have anything like that. In our solar system, and yet most planets that we've discovered are are in that in that middle zone. And so, then, you know, pretty much by definition, their atmospheres and their properties are are also gonna be complete surprises. So I I would say that learning about the atmospheres of those planets, which are sometimes called water worlds. Or mini neptune or super and that just reflects the fact that we don't know what the heck really are. That will tell us their true identities. That will tell us whether they really are more similar to Neptune or more similar to to the earth with maybe some hydrogen on top. So so that's sort of a surprise I would say in waiting. So Sarah, observing signs of life on an exoplanet would be a profound event for humanity. What are the bios that astronomers like you are are looking for? It turns out this is an incredibly loaded question. And as we unfold the observations and get closer to the day when making such a detection becomes reality. It's just incredibly messy. So we've worked like microbe in particular, over the last couple decades to literally come up with a list of every molecule that's in gas phase at a habitable world temperature impression and there's a lot of them. There's, like 14000. I mean, large majority are compounds, but... And we've kind of worked through classes of molecules or... You know, other people are growing group of people work on this as well. The reason I'm hesitating to give a direct answer, is because we will be faced with 3 incredibly important questions. 1 is the signal real. Any gas we're looking for is likely to be a trace gas not there at huge quantities. Second 1 is the signal attributed to the right molecule. And then the third question assuming the first 2 pass some bar is the molecule produced by life or does it have an antibiotic false positive? So what we've largely found is that for nearly every gas of interest, there are multiple options for... For how to assign the gas, and it's gonna be a tricky situation. My favorite gas personally, is so oxygen. Oxygen is a highly reactive gas shouldn't be in our atmosphere unless it's continually produced. I mean, there are ways to Produce oxygen without life. And our telescope the James webb space telescope isn't... Quite capable of detecting oxygen right now. But we have a huge list of other gases, and those include pho. They include you know, dime cell. There's literally a good long list of molecules we're are interested in? And how do you sort of get away from the the sort of earth and and solar system bias when you're coming up with lists of chemicals to look for. Or you you have to be very imaginative in terms of what sort of life you're imagining on a distant planet. Well, we aren't able to go from what the life is to what it will produce. We're just making the assumption that life elsewhere like life on earth should use chemistry to extract energy from the environment to store energy. And to metabolize and in the process produce a waste gas. So we're sticking with this general idea that there should be a gas that doesn't belong that's far out of equilibrium with its environment. And so there's no magic bullet, basically. That's why I started by saying that we've tried to come with a list of all molecules of interests, literally, like, whether or not they're produced by life on earth. And and when you're doing this research, I mean, do you work with Chemists? Who who addressed this problem. I suppose from a from a chemical point of view. Is that... Is that a part of your sort of day to day research? I do have bio on my team, but it's not really necessary at this point because we're still at that point of trying to detect trying to assign the molecule and trying to believe if the signals real or not. So, Sarah, a growing number of earth like exoplanets are being discovered. Are these the only places where astronomers are looking for signs of life or could life exist on other types of Exoplanets? That's a great question. Let's call them at earth size because we don't know if they're earth like yet. And, you know, it's just so hard as Dave was sane and earth... Sized planet likely has a tiny atmosphere. Think of like the skin of an onion on an onion. So tiny. So it's kind of like the person looking for their lost keys and looking under the street lamp. That's what we're kind of trying to do. We're trying to broaden our perspectives so we have a better chance to find signs of life. So the next, you know, biggest type of planet are these so called mini neptune, and there's 2 thrust going on there. 1 is an idea that like Earth has an aerial biosphere here. We have life in our clouds. Perhaps there's life in the clouds and other planets. And these mini neptune, the ones we can access now. They're typically too hot at their surface for life. But in the clouds, just like on earth as you go up into the clouds the atmosphere gets. Cooler, so to on other planets. So there's that idea, but, you know, people have had that idea before for Jupiter, like Carl Saga tried to propose. There could be life on Jupiter, and in that situation, if the surface is too hot, we you have to just have a kind of con confined scenario where the down drafts wouldn't bring life too far downwards too hot. Others take this idea of the so called mini... The many neptune being so called water worlds. And, you know, there are some scenarios where the water world may literally have a liquid water ocean, a hot water ocean. Maybe not too hot for life, but so hot and that there might be life in those oceans. So we're literally, maybe even pushing the pendulum too far, maybe not. But we're trying to be as open as possible so we don't miss our chance to find signs of life with our current James webb telescope. And and, Sir, what's your your sort of overall view on the existence of life? I mean, do you think that life must? Occur out there somewhere. It would just be a almost a mathematical impossibility that it didn't or you know, is it possible that Earth is the only place in the... Well, at least in the near universe where life exists. Well, since it's a physics podcast, we really couldn't answer that question in any way quantitatively until we better understand the origin of life on earth. You know, we have giant holes in our in our understanding of how life on earth arose, but just speculating for a moment, we see the ingredients for life everywhere. You know, meteorites have amino acids. We see all kinds of complex organic molecules in the intra interstellar medium. That doesn't mean they survive when they make it to a planet. But it seems like the ingredients for life are quite straightforward to form. And just given the number of stars in our galaxy alone, hundreds of billions of stars. And the idea, a reality that there are hundreds of billions of galaxies out there. Surely there is life elsewhere. But the question we're facing right now, Dave and I and all the others working on Exoplanets is, is there life on a planet orbiting a star near enough and right enough, for us to find a sign now. And finding a sign of life now doesn't mean we'll be sure there's life out there, but it means it'll be enough for to fuel the search to keep going. And, Sarah, is there always going to be some sort of ambiguity involved in these measurements? You know, you might have... You might see lots of bios, but maybe at the end of the day, they could all have non biological origins, and, you know, you can never say, u, a hundred percent that there's life on that Exoplanet. There will always be an ambiguity with remote sensing. Some people have tried to put forward an idea that if you do see lots, you know, every planet that could have water has signs of water and signs of life that that, you know, collective set of observations may indicate there's life out there. But let's not get ahead of ourselves. We're living in an amazing time. We have over 5000 exoplanets. We know that rocky planets, Our common, we know that planets of all kinds in their habitable zone are out there, and we're on the verge of being able to find water vapor on habitable planets. Maybe some hints of signs of life that will keep us moving forward. And and what about you know, your your research and the research of others into how life could exist on other planets. Is is that informing in any way our understanding of of life on earth? Are we more open to... I don't know looking in, you know, places on earth where we might have thought life couldn't exist, but, you know, maybe it could. I would have to say not yet to that. You know, and earth people have looked literally every everywhere they can for life, like, in the dries deserts and the most acidic environments. And so that research kind of goes on in parallel. We always like to hope though that studying exoplanets, the search for life or exoplanets themselves will feedback onto our understanding of our own planet, and it might well 1 day. So looking towards the future. What new telescopes are are you both most excited about? And why? David, do you wanna go first with your answer? I think the news here is that there are some incredibly powerful ob that will be coming online and are gonna probe regions of parameter space that have been previously inaccessible to to study. Okay? So it's a tremendously exciting future. The next big mission that will advance our understanding of Exoplanets is called the Nancy Grace Roman telescope. It's a Nasa facility. And so what it will do is it will do a micro lens survey. So that's a different method that allows us to find planets that are very, very far from their stars. The the most successful methods to date Namely the transit method when planets pass in front of their star and the wobble method when there's the gravitational orbit of the of the star and the planet around their common center of mass, those both favor close end planets. But if you were trying to understand the formation of the solar system and all you knew about where the inner planets, you'd probably have a very incomplete theory. So we really need to know about Saturn and uranus and neptune, and how common those are around other stars. And so I'm very, very excited about that that mission which will hopefully launch in just a few years. After that, the next big project will be on the ground, and it will be, a class of telescope called the extremely large telescopes, and there's 3 of them. They're being built by different nations and different cons consortium. But they will have, a tremendous aperture, basically be much larger than any ground based telescope that operates in the optical and and for it has ever been. And they will, allow us to gather more photon. More light than we never been able to do and really study these atmospheres at very high spectral resolution. And so they might allow us to really study. It's the the specific chemistry of these atmospheres and really go after potentially even some of these some of these bios molecules, as well. And so those might be coming online in the next well, I would say 5 to 10 years. And then finally, that the next big Nasa emission is called the habitable worlds observatory. And so that's that's sort of a working name So III anticipate that when it launches, maybe 15 years from now, it will be given a different name. But it will really allow us to study the atmospheres of earth like planets orbiting sun like stars. So real twins we hope of the Earth's sun system. I mentioned before that the only rocky planets who's at spheres, we can hope to study in the near future are the ones that orbit these low mass stars, these these red dwarf stars. But maybe there's something about those kinds of stars that preclude the existence of life. Maybe they put out too much uv light. Maybe they always strip away the atmospheres of their planets we're gonna find that out over the next 5 years. So that's why it's important that we simultaneously pursue this other path where we are building the technology to really look at the atmospheres of Earth sun analog. And that's why the habitable world observatory, which will be able to spatial isolate the planets and actually see the reflected light and do spectroscopy on that reflected light. That's gonna be really, really demanding technologically, but I know that, you know, the community's is up to the task. And, of course, the, the idea of having having access to this really profound question is is what drives us all forward. 1 question we have back to your earlier question and the con is, are we too t centric When we go out there with the Habitable world observatory, will we find earth like worlds? Or will they be all like venus? Or something else. And I've just spent a few years on what started out as this tangent that has grown into my favorite mission, which is this consortium I'm leading, we call ourselves the mornings star missions to venus. We're going to be a series of private public, Partnership funded missions to venus with a singular goal to find complex molecules, signs of life or life itself in the venus atmosphere. And this is building on a crazy idea from half a century ago, Carl Saga that if the surface of venus is too hot for life, which we all think it is. That high up in the atmosphere 50 kilometers above the surface as it gets colder. It's the right temperature for life. The problem is that venus unlike earth doesn't have water clouds. It has clouds made of acid, sulfur acid, which is orders of magnitude more acidic than the most acidic environments on earth where life is found. Yet lately, my team and a couple of others have been experimenting with sulfur acid and have demonstrated that some key biological molecules are stable in sulfur gas. Said amino acids, nucleic acid bases, fatty acids growing list of things, and we're building towards generating a synthetic, said informational polymer, not rd dna which is unstable, but swapping out parts to demonstrate that there could be some kind of primitive life, in the atmosphere to motivate people to get us back to Venus. So I would say my favorite missions or my favorite upcoming missions are small disruptive missions that complement these larger community based ones that Dave has mentioned. So guys, I I can't let you go without asking What is your favorite Exoplanet and why? Before I tell you the name of my favorite Exoplanet, I have to prepare you because, of course, in science fiction, I'm a big science fiction fan. Planets have... Have really wonderful dramatic names. You can think of H. Okay, from Star Wars or Ara from from June. Unfortunately, in in the real world, the planets get named after their stars by pending a lowercase b to the name of the star. And the stars themselves only have catalog numbers. Okay? So so so it could be that the first planet that we find that has signs of life on it. Has has an absolutely dreadful name, and I just wanna prepare the community for that. Alright. So then that being said, my my favorite planet is called L nhs 11:40 b. Why is it my favorite planet? Well, it's because it is Rocky planet, a terrestrial world. We know it's mass and it's it's its size, and therefore it's density very well. And we really think it looks like basically a scaled up version of the earth in terms of its bulk composition, the amount of iron and the amount of of Rocky mantle and it orbits a very nearby star, so it's somewhat accessible and the planet as at just the right temperature, it's a little bit cooler actually than the Earth, and and the combination of the high surface gravity of the planet and the fact that it's relatively cool means that it might have retained its atmosphere, and and it's an old star as well. So it could be that life has had time to to take root, and we could study study that life through the planetary atmosphere. But the other connection is that it was discovered by me and my team, and we did that with a, a special observatory called the Mu. Array, which was an array of robotic ground based telescopes, 8 of them in Arizona, 8 of them in Chile, when we first set out to study this this population of planets around these very low mass stars. And so it's... I think I think it's a really citing scientific opportunity, but it's also just very near and dear to to me and my team personally. So that's my favorite world. That's, David. Although, III should've have thought this out, and I should have I should've have asked, what what's the... What's your favorite exoplanet that you haven't discovered? But we'll we'll leave that to a different podcast. And how about you, Sarah? Well, I was gonna guess 11 and 40 be for you. So... Yay. And usually, I answer this question, my favorite Xl Planet is the next 1. It's 1 in the future because we've been so astonished before, surely there are still a lot more surprises left. Well, that's great. That's a great place to end, and Sarah and David, thanks so much for taking the time to come on the podcast and congratulations on winning the C plot prize in astro. She must be very pleased. To be this year's winners. Thank you. Thanks a lot. This has been a lot of fun. I'm afraid that's all the time we have for this week's podcast, which is sponsored by the cavalry prize. Thanks to David Char and Sarah Sig for joining me today. And a special thanks to our producer, Fred Isles. We'll be back again next week. The cavalry prize is a partnership involving the Norwegian academy of science and letters. The Norwegian ministry of education and research and the C Foundation. Since the first awards in 2008, the c prizes have honored 65 scientists from 13 countries. Find out more at Prize dot org. There will be no weekly podcast on the fourth of July. Instead, you can tune in on the second of July for the first installment of Physics world live, which will focus on quantum sensors. Featuring a panel of experts, this live event will explore the extraordinary capabilities of quantum sensors and look at how they could benefit humanity and shape our understanding of the world. You can find out more on the Physics World website. Just click on the Physics world live tab near the top of the homepage.