This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Over 6,000 exoplanets have now been found, and the number is constantly rising. This has galvanized research into whether one of them might host life. We can break this quest into three parts: first, understanding what makes a planet hospitable to life; second, learning how such planets could be formed; and third, how would we detect life if it was there?

Anat Shahar's recent work has concentrated on the second of these questions, namely how a habitable planet might form, and particularly on a key aspect of this, which is how a rocky planet might acquire water. She is a staff scientist and deputy for research advancement at the Earth and Planets Laboratory at the Carnegie Institution for Science. Anat Shahar, welcome to Geology Bites.

Anat Shahar: Thank you so much for having me. I'm looking forward to the conversation.

Oliver Strimpel: Let's start with habitability. What do we think characterizes a rocky planet that could host life?

Anat Shahar: Because Earth is the only planet that we know of that has life, we kind of take clues from Earth as to what a rocky planet might need to host life. The first characteristic would be liquid water, the second would be some way to shield life from harmful radiation, and the third key ingredient is some way for the nutrients that life needs to be recycled on that planet. The other really important thing is that the planet itself be able to sustain life in the long term, which is where all these ingredients come together.

Oliver Strimpel: OK, so let's summarize each of these and why we think they are necessary prerequisites for life. And let's start with water.

Anat Shahar: So water is fascinating. One of the reasons that water is essential is its properties as a solvent, as a molecule. What it can do is bring nutrients into a cell and also excrete waste out of a cell. On Earth, I think it's fair to say that no life has been found in a place where there's no liquid water. Life has been found in places where there might be no sunlight, no oxygen, but there's not been a detection of life where there's no liquid water.

Oliver Strimpel: So is it a bit far-fetched to think there might be other solvents that would do as good a job? Maybe some organic solvent?

Anat Shahar: I mean, Earth has had 4.5 billion years to come up with that solvent, and water's the only one we know of. So it's slightly Earth-centric, I guess, to say that it's the only one, but it's the only one we have ever found.

Oliver Strimpel: I guess protection from damaging radiation is sort of obvious, but are you talking about magnetic fields protecting a planet from its parent star's radiation, like the Earth being protected from the solar wind?

Anat Shahar: Yes, that's exactly right. We're talking about how our magnetic field on Earth, for example, protects the life on the surface from the solar wind, cosmic rays, just ionizing particles in general. The magnetic field on Earth not only has the ability to do that, but it also plays a role in protecting our atmosphere from being stripped by that solar wind, and so in that sense it keeps the surface habitable as well.

Oliver Strimpel: And can we imagine other ways of doing the shielding, both of potential life and of the atmosphere itself?

Anat Shahar: Another way to protect the surface would be to have a very thick atmosphere. For example, a Venus-like atmosphere is so thick that the surface is protected from the radiation from our star. Another way to do this would be if you can imagine a very thick water layer, where the water would protect the life at the bottom of that layer from radiation. And then another way to do it would be if you had subsurface life, so life that maybe lived below a crust, for example. That would also shield it from radiation.

Oliver Strimpel: Is that one of the ideas we're pursuing on Mars by digging down with these rovers?

Anat Shahar: Absolutely. They have found evidence that there was liquid water on Mars at some point, and there was a magnetic field at some point. And so maybe if we dig down, we can look for some of the life that could have been there.

Oliver Strimpel: So you mentioned the need to recycle nutrients and to find a way to keep things stable. Why do we think that nutrient recycling is necessary?

Anat Shahar: This kind of breaks down into several areas. The first would be that in order to have the nutrients that you need for life, you need to bring them to the surface. They need to be around for life to evolve. If you don't have something replenishing that surface, then you kind of run out: you use the nutrients that you need, and then you run out of them. Things would die. There would be no balance in that ecosystem. So that's one reason why we need the recycling. Another reason is that it creates these disequilibrium environments on the surface, areas where you have catalysts that can spark new chemical reactions on the surface. So for these reasons, you need that recycling. Plate tectonics also modulates our atmosphere and our climate on really long geologic time scales, so it kind of keeps the planet at a nice comfortable temperature for us.

Oliver Strimpel: You brought me to my next question, which is that obviously on Earth that nutrient recycling is plate tectonics. And not only does it recycle materials by subduction, mountain building, volcanism and so on, but, as you just said, it serves through various feedback mechanisms to promote the incredible temperature stability of the Earth over billions of years. And again, I guess I'll ask the same question: can we imagine any other mechanism that might be able to do this on another planet? Or is plate tectonics pretty much the only game in town we can think of?

Anat Shahar: The other way that you could recycle nutrients would be with some sort of volcanism. You don't necessarily need plate tectonics on an entire planet the way we have it on Earth. If you had volcanic activity that could bring material that was buried in the planet's interior to the surface, you could also recycle nutrients in that way. But what plate tectonics does for our climate stability, I think that would be a little harder to do with just volcanic activity. I mean, on some level, if you have volcanoes and you're bringing water out of your planet, let's say, or CO2 out of your planet through volcanism, you need to bring something into your planet as well. So maybe a smaller version of plate tectonics would work, but you do need some sort of way to bring things from the inside out and then back in.

Oliver Strimpel: I said in the introduction that you focus particularly on how a rocky planet might acquire water. Much of the discussion we hear about liquid water on exoplanets is focused on finding planets in the so-called Goldilocks zone: not too hot, not too cold, which means not too close or not too far from its parent star. But there is a question of where the water would come from, whatever its temperature. In an earlier podcast, Lindy Elkins-Tanton talked about how Earth acquired its water, and she was of the view that it was born with plenty of it and that we did not need to invoke the transport of water by comets from the outer solar system. Tell us about your research on this topic.

Anat Shahar: So this is a fascinating conversation, right? Where did the Earth get its water? And then where do exoplanets get their water, if they have any? On Earth, just like you said, there are these two, maybe more than two, but at least two schools of thought. One is that the Earth accreted with the water that it has, and the other is that the Earth got its water from elsewhere, from comets, from asteroids. And a lot of that work has had to do with the amount of water we have on Earth, the isotopic composition of the water on Earth, et cetera. But what we wanted to do is figure out if there was a way to get at this question based on what we know from exoplanets.

What we know from exoplanets is that, like you said in the introduction, suddenly so many of them have been discovered that we're starting to build statistics of planets. And what astronomers have found is that a lot of those planets are rocky, but they have these hydrogen-rich atmospheres around them. So then we started to think, well, when planets form, they have these hydrogen-rich atmospheres, but they're also very hot very early on. And if they're very hot, they're molten. They're in what we call their magma ocean stage. So if you can imagine, the whole mantle of the planet is molten, like a bubbling magma. And so if you have that magma ocean surrounded by a hydrogen-rich atmosphere, one of the questions we wanted to ask was: what chemistry occurs there, and what does that do both to the atmosphere and to the planet?

In 2023, we started looking at this as more of a theoretical question. We did calculations to try to say, OK, what would happen if you just took a hydrogen-rich gas and equilibrated it with a molten planet, a molten magma ocean? And we found that theoretically you should be able to form water through the different reactions. But recently, through very hard work by one of the postdocs in my group, Francesca Miozzi, we've tried to do the experiments, and the experiments are really, really hard.

So how do you do these experiments? Well, you take a sample like a silicate magma ocean that you can make in the lab, and you squeeze it between two diamonds in order to create high pressure on that sample. The reason you want to create high pressure is that these exoplanets are really big, and so even the pressure at the base of the atmosphere, at the surface, is higher than it would be on a planet like Earth. And then you surround it with a hydrogen-rich gas. So you have your silicate, your mantle-like material, you surround it with a hydrogen-rich gas, you squeeze it to high pressure, and then you take lasers and heat it to high temperature in order to simulate the actual conditions of these exoplanets.

When you do that, what we have found is that you actually find water that has formed during that reaction, and the water comes in two forms. It comes as liquid water, which is due to the reduction of the iron oxide that's inside that silicate. It reduces to iron metal, and it forms water. So that's one way you form water. The other way you form water is by actually taking the hydrogen and putting it into that silicate magma ocean. It actually dissolves inside that magma ocean. And so we were able to show, through these two different mechanisms, that forming water is a totally natural consequence of planet formation. You by no means need external sources of water in order to bring water to these planets. Just by going through this natural process of having a magma ocean surrounded by a hydrogen-rich gas, you're able to produce tons and tons of water.

Oliver Strimpel: Do your experiments place any constraints on how big a planet might have to be in order to achieve the kind of pressures and temperatures that you had in your lab, for which you showed that water would be produced?

Anat Shahar: The planets need to be at least the size of, let's say, a third of an Earth mass, or 0.4 Earth masses. But the bigger the planet, the higher the pressure, meaning the atmosphere, because of the gravity of the planet. With the larger gravity of the planet, you have a thicker atmosphere, and the thicker the atmosphere, the longer the planet is molten, and the longer you have to create that water and to go through all those chemical reactions. So the bigger the planet, while it's still a rocky planet, the more it can produce tons and tons and tons of water, because it has maybe billions of years to continually go through these reactions.

Oliver Strimpel: But is there another problem? Hydrogen, being a very light element, would tend to escape from a light planet. So how do you make sure that it sticks around for long enough to combine with your magma ocean?

Anat Shahar: There are two main ways that hydrogen can escape and leave the planet, which will happen on most planets anyway. One is that it's getting radiated from the top, and it's light, so it's just escaping. The other is that the planet is so hot that it's radiating that atmosphere from the bottom, and again it's losing some of the hydrogen. So on a planet like Earth, this process doesn't have a ton of time to occur before that hydrogen-rich atmosphere escapes. We think long enough, but maybe not a ton of time. Whereas on a bigger planet, where that atmosphere is really thick, you have more time to do this before that hydrogen-rich atmosphere escapes.

Oliver Strimpel: And because you said this could happen on a planet as small as about 0.4 Earth masses, do you think that Earth could have made water this way, then?

Anat Shahar: Yes. In our 2023 theoretical study, that is what we say: that Earth could have formed enough water. We say about two oceans' worth of water, which is plenty for what we have now.

Oliver Strimpel: So from this theoretical and experimental work, would you conclude, therefore, that water might actually be a rather common ingredient of planets of a certain size?

Anat Shahar: Yes, absolutely. On some level, I don't know how the process actually stops until that atmosphere dissipates. And so we really think that water production is a natural consequence of planetary formation. Now, I will say that doesn't mean that all the water ends up as an ocean, let's say, on top of your planet. When we say water, what we're saying is that there is water that forms, but some of that could go into the interior of the planet, some of that could become water vapor, and some of that could go into the metallic core of the planet as hydrogen. How the hydrogen and the oxygen separate into the different layers of the planet is still a huge area of research.

But what we can say is that there's a large amount of water production. Now the question is, where does it all go? For example, if it goes into the mantle, the magma ocean, does it stay in that magma ocean? Or, when the magma ocean crystallizes, like our mantle has here on Earth, does that water get pushed out and become an ocean? Or does it stay inside the mantle and then have to come out through some sort of volcanic activity, for example? So there are still questions as to what happens with all that water. I'm not suggesting every planet in the galaxy has oceans and oceans and oceans on its surface, but I am suggesting that it has the potential to have gone through these chemical reactions and to have created a large amount of water.

Oliver Strimpel: And that actually reminds me that I think in the podcast with Geoff Abers, and possibly with Lindy as well, they said that there could be at least as much, if not more, water in the mantle than there is in the oceans on Earth.

Anat Shahar: Exactly. That's why, when I mentioned how much water we think we could have produced on Earth, we say about two ocean masses, because there could be an ocean mass inside the planet. What's interesting about these exoplanets is that we actually think the majority of the hydrogen will make its way into the metallic core of the planet, because iron and hydrogen really love each other. So a lot of that hydrogen will go into the metallic core. Some of that hydrogen and oxygen will stay within the mantle, just like you mentioned. Some of it will go to the surface, and some of it will go into the atmosphere.

Oliver Strimpel: This conclusion of yours, that water might actually be fairly common, brings to mind the Drake equation, which expressed the number of active, communicating extraterrestrial civilizations in terms of a number of factors, most of which were completely unknown in 1961 when Drake formulated his equation. One of these factors was the fraction of stars that have planets, which we now know to be quite high. Another one was the fraction that could potentially support life. And since water is a key aspect of this, as you say, your work now seems to suggest that, at least as far as water is concerned, this fraction could also be fairly high, or at least not vanishingly small. So I guess this might at least say that we haven't found any showstoppers when it comes to extraterrestrial life.

Anat Shahar: I would absolutely agree with that. Based on our experiments, we would argue that, like you mentioned, just the water portion would increase that value. We don't know about the other two, the magnetic field or plate tectonics. But yes, in terms of water, I would absolutely agree with that.

Oliver Strimpel: OK, let's talk a bit about how we might detect extraterrestrial life if it was out there. Can we use the prerequisites for life that you laid out earlier, namely the presence of liquid water, protection from damaging radiation and recycling of nutrients, to constrain the search in some way?

Anat Shahar: That's the ultimate goal, of course: to actually find life on another planet. The problem, of course, is that exoplanets are really, really far away, and the information we have on them is quite limited. If we focus, for example, on exoplanetary magnetic fields, there are both direct and indirect ways in which we can look for them. That is an active area of research that has a lot to do with the interaction between the star's magnetic field and the planet's magnetic field. So you can measure that indirectly, through star-planet interactions. Or, I read a paper recently that actually said you can measure it directly, because the magnetosphere of the planet would interact with the star in a way that would actually form sunspots, which I thought was fascinating. That is something that is currently hard to detect, but as far as I understand it, the Habitable Worlds Observatory, which is a big future flagship mission of NASA, might hopefully look for it. The other option, recycling, and how to look for that, is harder. I think the only way to look for that would be some sort of atmospheric signature from the exoplanet.

Oliver Strimpel: Would we be looking for something that could only be there if there was this kind of disequilibrium, as you mentioned?

Anat Shahar: Yes, there are what we would call biosignatures, signatures in the atmosphere that would point towards life. The problem with biosignatures is that we don't know all the ways in which we could get false positives for those biosignatures, and so figuring out exactly what that would look like is important before we go out saying that we found life. But that is, of course, an active area of research.

Oliver Strimpel: What about water? Could we detect water if it was out there?

Anat Shahar: Yes, there have been detections of water vapor in exoplanetary atmospheres already. And that is a good sign in the sense that, like I mentioned earlier, if we're producing water, some of it will go into the atmosphere. I don't know how good water would be as a biosignature, but it is definitely something that has already been detected in exoplanetary atmospheres.

Oliver Strimpel: How is it detected?

Anat Shahar: This is a fascinating thing. The composition of exoplanetary atmospheres is detected through a planetary transit. What happens is that the planet comes in front of the star, and some of the star's light filters through that exoplanetary atmosphere. Each molecule in that atmosphere has a different spectral signature, or fingerprint. And so we see the spectra of that atmosphere through telescopes like the James Webb Space Telescope, for example.

Oliver Strimpel: When the planet is transiting in front of the star?

Anat Shahar: Exactly.

Oliver Strimpel: Recently there's been a lot of excitement around the discovery of phosphine in Venus's upper atmosphere, because on Earth it's produced by living organisms. But as you say, with all such detections, we first have to rule out abiotic production mechanisms before we conclude that life is present. Do you know where things stand with respect to phosphine at the moment?

Anat Shahar: Phosphine was a very exciting detection. It was a big announcement, and it's been quite controversial. There have been studies since the detection first came out that have suggested that actually we don't see it anymore and it's not detectable. There are other studies that say no, actually it is detectable. But the important point, I think, from my point of view, with regard to phosphine is that when it was first announced, the idea was that it cannot be formed by abiotic processes. But since then, people have noted geologic or other chemical processes that could form phosphine in an atmosphere. So this is really important, because it's the first big announcement that has really made us understand how important it is to determine all the ways in which abiotic signatures can make their way into the atmosphere, so that if we find something different, we can really say we have found life.

Oliver Strimpel: This reminds me of a very analogous debate with respect to finding the earliest life on Earth. I did an episode with Martin Van Kranendonk about stromatolites, and even earlier, the rocks in southwest Greenland. And people sort of debate in a very similar fashion, as they do on Mars as well, as to whether certain collections of chemicals are a smoking gun for life, or whether there could be abiotic ways of assembling those particular mixes of materials.

Anat Shahar: Absolutely. When I was in graduate school, the discussion on this was all about a Martian meteorite and a physical signature within that Martian meteorite. Could that be a fossil, for example, or could it have been made abiotically? This was a huge area of research. And it's very important for us to remember that there are abiotic ways to do a lot of things.

Oliver Strimpel: Now that you've shown how water can be created on an exoplanet, what about some of the other elements that are essential, at least for life on Earth as we know it, such as carbon, nitrogen, oxygen, phosphorus and sulfur?

Anat Shahar: These are all super interesting topics of research, because, like I mentioned, we have a hydrogen-rich atmosphere and then we have this magma ocean. What happens before that is that all the elements are formed and then they move into the protoplanetary disk before the planets come together. All the elements you just mentioned are ones that will condense into these little grains and then make their way into the planets. So when you have your hydrogen-rich gas and your magma ocean, those elements are mostly in the magma ocean, but they're also coming out of the magma ocean and interacting with that hydrogen-rich gas. And we actually have very limited data and a very limited understanding of how that works. Would those elements, like the hydrogen we just showed in our experiments, dissolve into the magma ocean? Or would they stay in the atmosphere, or would they form some other condensable phase? We're not really sure.

In our lab, we wanted to start with hydrogen, because there was so much of it around, and everybody loves to talk about water. But the next phase is really to go through all those other elements as well. You know, what is the solubility of carbon at these high pressures? This work has been done for the conditions of Earth, but it has not really been done for exoplanet-like systems. We would like to see how carbon and nitrogen in particular move between the different layers, the atmosphere, the metallic core and the mantle, at these high pressures and temperatures.

Oliver Strimpel: And are you doing that theoretically, or are you crafting experiments to look at that?

Anat Shahar: Crafting experiments. The hardest part of the hydrogen experiments was really figuring out how to do them. Now that we know how to do that, we're going to try to copy those experiments for these other elements, specifically carbon and nitrogen.

Oliver Strimpel: Anat Shahar, thank you very much.

Anat Shahar: Thank you. That was really fun.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find 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.