This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. During the past few decades, we've discovered over 5,000 planets orbiting stars other than our Sun. Using telescopes on the ground and in space, we can infer the mass and radius of these planets. And by detecting minute changes in the spectra of light we receive from these stars and their planets when the planets pass in front of or behind their star, we're beginning to learn something about exoplanet atmospheres and even exoplanet surfaces. But do these observations, and ones we plan over the next few years, enable us to learn anything about the geology of exoplanets?

Sara Seager is a leading researcher in the field of exoplanets, and especially in the detection of exoplanet atmospheres. This work has just won her the prestigious Kavli Prize. She's had leadership roles in space missions designed to discover new exoplanets and find Earth analogs orbiting a Sun-like star. She's a professor in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology. Sara Seager, welcome to Geology Bites.

Sara Seager: Thanks for having me today.

Oliver Strimpel: The number of known exoplanets has increased rapidly, especially since the Kepler mission, which was NASA's first exoplanet-hunting mission, started operating about 15 years ago. Can you summarize the main ways we have used so far to find exoplanets?

Sara Seager: Well, there are several different ways. The most popular one today is called the transit method. If a planet and star are fortuitously aligned so that the planet goes in front of the star, or transits, on each orbit, we will see a tiny drop in brightness of the star. So even though we can't spatially resolve the star, the star will drop in brightness when the planet goes in front of it and will return to its normal brightness after the transit is over, and this will repeat each planet orbit. Transits are actually quite rare, because planets can have any orbital orientation on the plane of the sky. So in order for us to see a transit, we have to monitor lots and lots of stars. Our current space mission, TESS, can actually monitor tens of thousands of stars, even millions of stars, at a single time.

Before transits became feasible, the method used to find planets was called radial velocity. This is kind of an awkward name, but it's when the planet and star orbit their common center of mass. We can actually measure the line-of-sight motion of the star, which we call radial velocity, because it's the velocity along the radius between us and the star. And we can measure that to incredible, incredible precision, on the order of meters per second or even better. Astronomers can then infer the presence of a planet using that technique.

Oliver Strimpel: Can you also detect the, as it were, left-right motion of a star as a planet moves from side to side?

Sara Seager: Yes. In fact, you can measure the left-to-right movement of the star. That would be the movement of the star on the plane of the sky. We call this astrometry. And for a very long time, people have measured motions of stars on the sky. Astrometry has not come into its heyday yet for finding exoplanets. But let me tell you this: astrometry is about to explode, because there's a European Space Agency telescope called Gaia that has been measuring the two-dimensional positions of the stars, right-left on the plane of the sky. They're mostly doing that for an entirely different purpose than exoplanets, but they're going to be releasing the next series of data, where they will be pointing out planets.

Oliver Strimpel: You said that the transit method captures somewhat rare events, when a planet happens to be moving between our line of sight and its parent star. So do all these methods introduce certain biases in the kinds of planets that we're likely to detect?

Sara Seager: Yes, there are definitely biases in each and every planet-finding technique. It's pretty amazing, actually, because we wouldn't be having this conversation if nature hadn't provided such a wide range of diversity of exoplanets. Transits especially have a bias towards planets close to the star, and big planets close to the star. I liken it to throwing darts. If you're very close to the dartboard, it's really easy to get that center bullseye. If you're really far away, your angle becomes much, much narrower. It's very hard. So the planets very close to the star are more likely to show transits, and bigger planets create a bigger signal. So we have some crazy kinds of planets out there called hot Jupiters. They're big, they're puffy, they're very close to their star. They orbit frequently, and that's what enabled this whole field to get a jump start.

Oliver Strimpel: So presumably we know of a lot of hot Jupiters, but we haven't seen a lot of Plutos so far. Are there any new methods of detecting exoplanets, especially ones that attempt to overcome these biases, and especially ones, I guess, that would enable us to see another Earth out there if there was one?

Sara Seager: Right. Well, each method has a different bias, so no method removes all the biases. But there is one very intriguing method called gravitational microlensing. Gravitational microlensing happens to fill in a piece of the puzzle that the other techniques struggle to reach, and that is planets between about one and four astronomical units from their host star. And there's a telescope going up called the Nancy Grace Roman Space Telescope that is going to specialize in wide-field astronomy and has a microlensing program that will nail the problem of getting that inner region.

The way it works is, I want you to imagine a very distant star that's just doing its own thing, just shining regularly. And then, in between our line of sight and that very distant star, another star passes by. There are a lot of stars in the galaxy, and they're all moving around. We're all kind of rotating the same way, but there's motion with respect to each other. I want you to imagine this dark star passes by, and for a moment, while it's appropriately aligned, it bends space, because mass bends space. Once it's lined up perfectly, it's creating this little defect in space, which enables some of the light rays from the very distant star, instead of going off into outer space, to get bent, to get lensed toward our viewpoint, thus allowing that background star to brighten. The amount of brightening depends on how precisely that dark star passes between us and the background star. We can have magnifications of several tens of times.

Now I want you to imagine that that dark, unseen star passing between us and the very distant background star has a planet. All of a sudden, the shape of this brightening and dimming back to normal is altered from a really smooth up and down, and that could alert us that there's a planet there. This is a real, legit method of finding planets. It's amazing. There are telescopes that survey millions and millions of stars every summer, when the galactic bulge, a field full of very distant stars, is in the night sky from the Southern Hemisphere. Computer software identifies these lensing events, where the light of the star rises. And any time the program sees it deviate a little bit, the workers involved get a ping on their cell phone, and they know it's time to figure out how to follow up that one, to see a special tiny defect in that microlensing light curve that will indicate the presence of the planet.

Oliver Strimpel: How do they follow up?

Sara Seager: They follow up with individual telescopes. I want you to think of it as having telescopes that discover that there is a light curve, that the light curve is changing, looking at lots of stars at once. And then there are follow-up telescopes to characterize it. This telescope would be more sensitive; it might be bigger. It'll just focus on that one star at a time.

Oliver Strimpel: Of those over 5,000 exoplanets that I mentioned that we already know about, how many of those have been found by gravitational microlensing?

Sara Seager: Many dozens of planets have been found with the microlensing technique so far.

Oliver Strimpel: So it's quite a small proportion, but significant and growing, presumably.

Sara Seager: Significant and growing. And once the focused program on the Nancy Grace Roman Space Telescope gets into operation later this decade, we'll have thousands, maybe tens of thousands, of microlensing planet events. You know, every technique has its pros and cons. Gravitational microlensing will complete the census. We'll find lots of planetary systems of interest. But those dark stars and their accompanying planets, they go by and they are gone. We don't have another chance to follow them up. So it's a very statistically oriented method.

Oliver Strimpel: Do these methods work better if we're using a space telescope, or can we do some of this from the ground?

Sara Seager: We can definitely do some of this from the ground, gravitational microlensing in particular. It's almost all ground-based. Other methods, like the transit method, do far better from space, above the blurring effects of Earth's atmosphere, where we also don't have Earth's day-night cycle to interfere.

Oliver Strimpel: Do some of the methods that we're exploring involve artificially blocking the light from a star so we can more easily see or detect its exoplanets?

Sara Seager: Yes. One of the most important methods of our future is called direct imaging. We call it that because we will see photons from the planet directly, rather than from the transit, which is blocking out starlight, or gravitational microlensing. In those, we're not seeing the planet directly. So direct imaging is an umbrella term for methods that block out the starlight directly. The main category in operation is called a coronagraph, and it dates back to the 1920s, when a French optical physicist, Bernard Lyot, wanted to be able to study the Sun's corona any time, not just when there was a total solar eclipse. So he developed a device to block the sunlight inside his telescope, and he called it the coronagraph, so he could see the corona. It's a little more complex than just putting a little blockage inside the telescope. It's quite complicated, but we have that in use now on many ground-based telescopes and space telescopes.

So far, direct imaging works for a special kind of planet. Remember the biases: it works for very bright, big planets very far from their star. We want to do way better. We want to do many orders of magnitude better than the current state, so that we can block out the starlight and search for true solar system-type planets, Jupiters, and especially one like Earth.

Oliver Strimpel: But in terms of this artificial blocking, the coronagraph presumably was attached to, or an integral part of, the telescope itself. But you're involved with projects that involve separating the device blocking the light from the telescope.

Sara Seager: That's right. Right now, the coronagraph is what is being planned. It's what's being used, and it's not necessarily part of the telescope, but it's part of an instrument that is part of the telescope. But no matter how you slice and dice it, finding another Earth is incredibly challenging, because our Sun is 10 billion times brighter than our Earth. So we somehow have to separate 10 billion photons that we don't want from the one photon we do want, over and over again. And so the coronagraph is tricky. We have to have a very stable telescope, a very stable wavefront, to separate that out.

The method I'm involved with is like a coronagraph on the outside. It's a giant, specially shaped screen called a starshade. The idea of Starshade is to have its own spacecraft, and it would formation-fly with a space telescope. The starshade is tens of meters in diameter, and it would formation-fly tens of thousands of kilometers away from the telescope. The sheer beauty and purpose of Starshade is to block out that starlight so only planet light enters the telescope. So you've made your job easier, because only planet light is going through the telescope. You don't have a mixture of your 10 billion photons plus one photon.

Oliver Strimpel: How far away would the shade have to be from the telescope?

Sara Seager: The starshade and telescope have to be separated by tens of thousands of kilometers.

Oliver Strimpel: And presumably you could then line up a different star after you've measured one, by moving the telescope or moving the starshade.

Sara Seager: That's right. You could move either, but we traditionally think about it as moving the starshade, which would take a week or a little longer to reach its next target. But in the meantime, the telescope could be carrying out other observations.

Oliver Strimpel: Wow, that's incredible, just to think that we can potentially directly image an exoplanet. What's the status of Starshade?

Sara Seager: Well, right now Starshade is shelved in favor of the coronagraph. NASA has decided to make the coronagraph work for the Habitable Worlds Observatory, which is a big, ambitious space mission that would launch no earlier than the 2040s. So Starshade is still being worked on by myself and a small group of people who believe in its power to block out the starlight so that only the planet light enters the telescope, which gives Starshade some real advantages. By the way, it was actually first invented in the 1960s by Lyman Spitzer, who was one of the people who invented the Hubble Space Telescope. But every decade, Starshade has been shelved, and then a decade later it gets revived, then shelved again, then revived and shelved again. And so this Habitable Worlds Observatory, so far, although it's in the very, very early stages, is being designed as what we call "starshade ready." So the telescope will be able to work with a starshade, should NASA or someone else decide to move Starshade forward later.

Oliver Strimpel: So let's talk about exoplanet geology. Even just putting those two words together is quite startling, to actually think that we can say something meaningful about what is going on in the interior of an exoplanet, or even on its surface. I said in my introduction that existing methods enable us to infer the mass and radius of an exoplanet. That should give us an average density, which is a good start, isn't it?

Sara Seager: It's an amazing start, because it can tell us approximately what the planet is made of. I do have an analogy I like giving. Let's imagine we have three planetary materials in front of us. We have a piece of rock mixed with iron, we have some water, and we have some hydrogen and helium. And imagine I give you a box, but I don't tell you what's in it. You pick up that box, and it has one of those three materials, or a mixture of them. And the box is so heavy, whoa, it falls to the floor. What would your guess be as to what's in that box?

Oliver Strimpel: Well, probably the piece of rock mixed with iron.

Sara Seager: Right. What if I gave you a box and it starts floating upwards?

Oliver Strimpel: Well, presumably the gaseous one, made of hydrogen and helium.

Sara Seager: Right. Now, what if I give you a box and it's just kind of average?

Oliver Strimpel: Yeah, I guess that's not well constrained, is it?

Sara Seager: No, because you can imagine having this heavy iron-rock mixture with hydrogen and helium, or maybe it's water. Maybe it's a combination of all three. So mass and radius work pretty well at the extremes. If it's a dense planet, you can say, OK, look, this is mostly rock; it's got some kind of iron core. Or if it's very low density for its mass, you can say, yeah, this is a giant exoplanet, mostly hydrogen and helium. But it actually does fail in the middle, when it's a combination of things. We don't know what it is. So on the whole, it's a good place to start. I'm saying it in a cheery voice, but when we started this whole field a couple of decades ago, it was a very depressing thought: that we can't tell you what some of the planets are at all, but instead we'll develop statistical methods to give you the whole range of possibilities, with probability assignments.

Oliver Strimpel: Well, that's a different way of having to proceed. What, if anything, can we hope to learn about the actual surface, and perhaps even the interior, of an exoplanet, given that we have the limitation of the ambiguity of just a mass and radius measurement?

Sara Seager: Well, we're working on that, and I can answer that in three separate ways. One is, we do high-pressure physics experiments. For example, one thing that really matters is melt curves, and we can find these out by doing experiments on rocky material, on iron, and seeing where it melts. So we can know if our planet will be molten on the inside or not. That was an overly simplified example, but I wanted to give something concrete for people to latch onto.

That's one way. A second way, and now we're talking about the top of the planet interior that's in contact with the atmosphere, is to observe the planet's atmosphere. It doesn't work for all cases, but look at the atmosphere, see what's present or absent, and try to infer what is beneath that. We do have a favorite example and a favorite planet. This planet's called K2-18b. It's a mini-Neptune. It's two to three times the size of Earth, so smaller than Neptune, which is four times the size of Earth, but bigger than Earth. These appear to be the most common planets in our galaxy, but we have no solar system counterpart, and they fall in that average-density range, so we can't pin them down. There are those of us who want to believe they're water worlds, made of more than 50% water by mass, like one of Jupiter's icy moons scaled up, and others who want to think it's a true mini-Neptune, made of water in its different phases at different pressures. Or maybe it's just that rocky iron planet with a hydrogen-helium envelope.

So one interesting thing about K2-18b is we see methane but no ammonia in the atmosphere. This is interesting because if the planet has a hydrogen-rich atmosphere, we expect it to have the reduced species of molecules. They should all be in their most hydrogenated form. So you should see methane, and you should see ammonia, as opposed to mostly carbon dioxide or carbon monoxide, and N2, although you can't see N2 because it's a homonuclear molecule. So let's take this at face value and for a moment just assume it's the truth that there is methane but no ammonia. That's amazing, because what it means is that the ammonia dissolved out into a surface that is liquid. We don't know if that would be liquid water. Ammonia is incredibly soluble in liquid water, orders of magnitude more so than methane, but ammonia is also very soluble in magma, in liquid rock, so we haven't resolved what's going on yet. And sure, there's a huge difference between a planet that is a water world with some hot liquid water versus one that is rocky with a molten magma ocean. It's incredible that we're making progress in this way, because both of them are totally bizarre.

But let's get to my third way of studying this. We're now moving outward to the actual surface of the planet. We have evidence, and I mean in the community, that some exoplanets have no atmosphere. As you know, our own Mercury in our solar system has no atmosphere. There are other planets close to their star, and they appear to have no atmosphere. How do we know that there's no atmosphere? These particular planets are very close to their star, and they should be tidally locked. That means over time, due to tides, they end up in a very low-energy but special state, where the planet rotates one time for every time it orbits. It shows the same face to the star at all times, just like the Moon shows the same face to Earth at all times. What the tidally locked status means is that one side of the planet is always heated and one side of the planet is always in darkness. With an atmosphere, the energy gets rearranged, but with an extremely thin atmosphere, or no atmosphere, that planet will be heated just the way your mind's eye says it should. There'll be a hot spot directly across from the star, and you can measure thermal emission phase curves. Just like our Moon goes through phases, we can measure phases of these planets through the heat they're emitting. So we can see that one side is cold, one side is hot, and the hot spot is right where it should be. We have evidence for that.

Oliver Strimpel: When we talked earlier, you mentioned the possibility that we could infer the presence of an atmosphere from a time lag in the thermal emission phase curve.

Sara Seager: What I was telling you about before is that if the planet has an atmosphere, and we've seen this on some of these hot giant planets, that hot spot is not directly across from the star. It's actually blown downwind, due to massive, fast winds that are set up. If there is an atmosphere, this thermal emission phase curve I described is all washed out. The hot spot is blown away. There's always wiggle room. You know, in science you always have a tiny bit of wiggle room away from that picture.

Let's take it, for example, that it's the truth that we have some planets that have no atmosphere. Now people want to do surface mineralogy. So now we want to take spectra of the planet. How we'll do that for these planets very close to the star: we can't spatially resolve them, but we can look at their so-called secondary eclipse. If a planet's orbiting a star and it goes in front of the star, it typically goes behind the star as well. So we can observe the spectrum of the planet and star together, and then we can observe the spectrum of the star alone, and we can tease out what that planet spectrum is. We have to get lucky here, but if the entire planetary surface was predominantly covered by a single rock type, as in made of the same minerals, we could see that in the spectrum.

Oliver Strimpel: Because we'd see the vapor form of that mineral?

Sara Seager: We're not expecting to see the vapor form, although that's a good idea in and of itself. But minerals have spectra. If we shine a light on a rock, and that rock heated up and gave off an emission, it wouldn't be a black body. It would have some bites taken out of it, depending on what's absorbing.

Oliver Strimpel: So it'd be a reflection spectrum?

Sara Seager: Well, not a reflection spectrum. Our techniques aren't good enough to see the reflected light, typically. And we're talking about our new James Webb Space Telescope, which is infrared-sensitive. So we're talking about energy that hits the planet and gets reprocessed and reemitted. It's just like when you touch the pavement on a hot day. That pavement is hot; it's not reflected light, it's the heat coming off. That itself encodes what the material is made of, because it's not coming out perfectly uniform. It changes with wavelength, in its own type of spectrum. But it's not black-body radiation either.

Oliver Strimpel: The effect of the mineral is seen on what's emitted from the surface.

Sara Seager: Right. They're weak absorption features within the emission spectrum.

Oliver Strimpel: So, to sum up, these methods that you've described would tell us if there's an atmosphere, if the surface is a water ocean, perhaps if there's a magma ocean, or if there's a dominant mineral. So how do we actually study the atmospheres of these exoplanets?

Sara Seager: The main way we study atmospheres today is through the transiting planets. As the planet goes in front of the star, the starlight shines through the atmosphere, but some of the starlight at certain wavelengths gets blocked by the atmosphere, where the atmosphere is strongly absorbing. So we can look at the star by itself, then we look at the star during transit, and we can subtract those two, and we're left with the planet's atmospheric spectrum.

But I want to explain it another way. Imagine a planet transit. There's a planet of a given area going in front of a star of a given area. The drop in brightness will be the area ratio of the planet to the star. So the transit depth is a fixed size, based on the size ratio of the planet and star. But I want you to imagine a planet with an atmosphere. Now the planet's a tiny bit bigger, because it has this kind of shell around it, this atmospheric shell, like a blanket. As you know, there are some wavelengths where the atmosphere is very strongly absorbing, and other wavelengths where the atmosphere is completely transparent. So we're effectively looking for a tiny change in the size of the planet as a function of wavelength, and we measure that for dozens and dozens of exoplanets.

Oliver Strimpel: Wow. And the only way it would be wavelength-dependent would be if the atmosphere was playing a role.

Sara Seager: Yes, the atmosphere has to be present, and the atmosphere has to be playing a role, with different gases.

Oliver Strimpel: So let's suppose we are lucky, in fact, and we can say the surface of a rocky planet has water on it, and whether there's a dominant mineral on the surface, whether it has a magma ocean. Might we be able to go further and find out if it had continents and if plate tectonics was operating?

Sara Seager: We've thought about every single one of these things, and I can always give you an answer, but it's not going to be easy. Well, suppose we can do direct imaging and block out the starlight, so we can see the planet, an Earth-like planet, in reflected light. Did you know that our own Earth varies? Because the ocean is very dark, sand is very bright, snow is bright as well. We have contrast; forest is dark. And effectively, if we could monitor another Earth, looking at it directly, we would see variation in its reflected light because of this differing pattern of dark and bright.

It's actually quite amazing. There have been experiments done in the past on Earth as an exoplanet, by studying earthshine on our Moon. When our Moon's a crescent, have you ever noticed that the dark part of the Moon can be very bright sometimes? If there's a giant snowfall at night, you'll see that it's actually particularly bright. One person who was leading these studies claimed that when watching the crescent Moon from California, where the observatory was, he could, quote unquote, "see Asia rising," as our Earth was turning and that bright part was being lit up. So earthshine is sunlight that hits Earth, is reflected off Earth and hits the Moon, and then is reflected back to Earth. So oddly enough, you can get Earth's spectrum by looking at the dark part of the Moon that is lit up by earthshine, instead of the rest of the Moon that's lit up by our Sun.

We also once had a wayward spacecraft that got repurposed, and I was part of a team that proposed to use this spacecraft to look back at Earth and study Earth as an exoplanet. And so we could measure changes in brightness and infer the presence of continents and oceans. Now, clouds complicate that picture, but they don't destroy it.

Oliver Strimpel: Well, so the idea would be that if there was a rotating exoplanet and we could block out the parent star, we could then see its diurnal variations, if some areas are much brighter than others.

Of course, a huge aspect of our motivation to explore exoplanets stems from our search for extraterrestrial life. It's been suggested that the Earth's peculiar geology, and particularly certain consequences of plate tectonics, was at least partly instrumental in the emergence of complex life in the late Neoproterozoic and the Cambrian period that followed. Specifically, plate tectonics results in the formation of shallow marine continental shelves, and the stabilization of surface temperatures via greenhouse gas feedback loops between volcanism, which injects greenhouse gases, and weathering of newly formed mountain belts, which draws them down. Do you agree? And if so, do you think that searching for a particular kind of exoplanet geology in a rocky exoplanet within the habitable zone might help us narrow down likely candidates for complex extraterrestrial life?

Sara Seager: Wow, that is a 10- or 20-year research program. To try to answer your question: sometimes I think we overcomplicate things, and it's extremely hard to break away from terracentrism. People won't like this answer, but I like to imagine a planet where things are operating very slowly. So let's say a Venus-like world. It has outgassing; it's got volcanoes that are active, but it doesn't have plate tectonics. But perhaps its deep reservoir is just slowly leaking out, and it is giving enough for some kind of overly simplistic life form that's just eking out an existence and barely getting by. So we like to think that Earth is the place. It has all the ingredients you just mentioned. But I really don't think we have the answers to your questions yet.

Coupled to that, we would love to have such a large number of planets to study that we can pick and choose which ones we think are best. Right now, maybe thankfully so, nature is limiting how many and which planets we have to study, because it's still hard to find planets, and the number of candidates available to us is not giant, so we'd want to study all the ones we have.

Oliver Strimpel: Is it even conceivable that we might be able to send something to a nearby star to take a look?

Sara Seager: One thing I love about exoplanets is that the line between what is considered mainstream research and what is considered completely crazy, out there, is constantly shifting. When I started working on exoplanets 25, 30 years ago, it was not considered solid at all. Most people just kind of laughed it off. Now it's so serious that probably a third of all astronomers, and some scientists from other branches, work full time on it. So this idea of sending something to another star was considered inconceivable and laughable, but there's work going on now that is shifting that line. I can give you two examples of work that's really happening, but is far from being actually realizable.

One is called Starshot. The idea is to send thousands of tiny spaceships, little tiny spacecraft called StarChips, that have solar sails. You'd launch a lot of them, because just like the baby turtles that all go to the ocean, most of them unfortunately probably don't survive. So you hope some will reach the nearest star system. You would launch these, they would deploy solar sails a couple of meters across, and we'd have to have a giant bank of lasers here on Earth sending out photon pressure to accelerate them rapidly to 20% of the speed of light. And they would take 20 years to reach the nearest star system. They'd snap some photos (they have no way to slow down, by the way) and send those photos back. $100 million from Breakthrough Initiatives went to this project to get it off the ground. Now, it's not launching or anything, but it's again to shift that line, and to list all the challenges and start tearing some of those down.

Another idea is the so-called Solar Gravitational Lens Telescope. Remember how we talked about how mass bends space? Well, the idea is to send a telescope to 500 times the Earth-Sun distance. So the focal length of this telescope would literally be 500 astronomical units. We'd have to block out the light of the Sun. That would be fairly easy, but essentially we'd have to choose a target, so that a known planet would be directly behind our Sun, and our Sun would bend the light rays that would normally be going to outer space towards us. So we could get a lot of light, and we could magnify this planet, and we could also spatially resolve the surface to something like 10 square kilometers. So these are ideas for the next generation, but ultimately we'll have to find a way to get a much closer look.

Oliver Strimpel: So the idea would be that at that distance, we could use the Sun as our gravitational lens, block out the light from the Sun itself using a coronagraph or something like that, and then just look at the star that's focused by the Sun.

Sara Seager: Well, there's a star-planet system that we'll be targeting, but we'll just be targeting that planet, and we will get the light from the planet and spatially resolve the surface.

Oliver Strimpel: What are you working on at the moment?

Sara Seager: Well, my favorite project right now is the search for signs of life, or life itself, in the Venus atmosphere. We all know that Venus has a massive carbon dioxide greenhouse atmosphere, making the surface 700 Kelvin, too hot for any kind of life we could ever conceive of. But just like on Earth, when you go above the surface, hiking or in an airplane, it gets colder and colder, and so too on Venus. Fifty kilometers above the surface of Venus, it is the right temperature for life. And there are clouds: permanent, 20-kilometer-deep clouds fully covering the planet. We have Earth life in our clouds, bacteria that get swept up and spend some time there. So it seems conceivable, the way I've put it anyway, that there could be life up there. Carl Sagan first proposed this idea over half a century ago. But at that time, they didn't know that the clouds were not made of a very friendly water substance; they're made of acid, sulfuric acid, a chemical that destroys all of our life. It's very nasty, very toxic. It burns a hole in your clothes and skin in an instant.

Well, we've been doing laboratory experiments on molecules in sulfuric acid. Our DNA is unstable, but we have put our nucleic acid bases in sulfuric acid. They get protonated, but they are stable indefinitely: adenine, guanine, cytosine, et cetera. We took our amino acids, our 20 biogenic amino acids, and put those in sulfuric acid. Those are, with one exception, stable. Half of them get chemically modified, but they're still stable. And that was just the tip of the iceberg. There's something pretty interesting, actually: there's this growing number of stable biomolecules that can survive in sulfuric acid.

We started out this research to motivate space missions to Venus, missions to search for signs of life and life itself. And remember, we talked about this line. This is still on the crazy side. It's a bit too crazy to be supported by large government agencies like NASA and the European Space Agency, so we're trying to carry out a series of missions to Venus as private-public missions. Our first mission is called the Rocket Lab Mission to Venus. It's being carried out by Rocket Lab, and I am leading the science and instrument team, to go to Venus, the first in a series, to see what's going on inside the sulfuric acid droplets.

Oliver Strimpel: Is there anywhere else in the solar system that you would like to go look for life?

Sara Seager: So many places. Mars, of course, is always a favorite, and there's growing evidence of really interesting chemistry there. I'd say it's fair to say most of us are hoping that right under the surface there are pockets of water where there's life eking out an existence. There's Europa, and Jupiter's icy moons that have water oceans. Not like ours; it's probably some kind of sludge, but there could be life there. Titan: anywhere there's liquid, we're hoping that there's life, and Titan has liquid seas of ethane and methane. So there are many, many, many interesting places.

Oliver Strimpel: Sara Seager, thank you very much.

Sara Seager: Thanks for having me today.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.