Science Talk

Super-Earths: Bigger, and Maybe Better

Dimitar Sasselov, professor of astronomy at Harvard University and the founder and director of the Harvard Origins of Life Initiative, talks about his new book The Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet

Podcast Transcription

Steve:          Welcome to the Scientific American podcast, Science Talk, posted on June 21st, 2012. I'm Steve Mirsky. On this episode:

Sasselov:          We always wonder how tiny we are compared to the vastness of the universe, but that's because we compare ourselves in the three dimensions of space. But now think about the four dimension of time—there we're not tiny at all. Our entire heritage goes back to those microbes, which existed here 4 billion years ago.

Steve:          That's Dimitar Sasselov. He is Professor of Astronomy at Harvard and the founder and director of Harvard Origins of Life initiative. He is also the author of the new book The Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet. He was in New York City recently and dropped by the Scientific American offices.

Steve:          You've been quoted as saying that biology is the future of astronomy, so let's start there. What does that mean?

Sasselov:          Biology is the future of astronomy for two reasons. One is that as a science, biology studies life on Earth, but we're already looking for life on other planets, even Mars, if you will; and going to other planets is the domain of astronomy. But there's a deeper sense in what I'm saying. The big question about what is our place in the universe and its scientific approach to it, comes down to a major shift, which we're experiencing right now. That shift is from the question being mostly astronomical—are there any other places that are suitable for life, other planets? To becoming now, mostly biological—how does life start to begin with, what is life in the bigger sense, not just here on Earth? And from that point of view, the shift is from the uncertainties in astronomy now to understanding the biology, and so the two are coming together in where, the future of this universe is probably a biological future.

Steve:          Is that because we are just inherently interested in the question of whether there's life out there or is there astronomical information that you as an astronomer find especially interesting because of the biological aspect?

Sasselov:          Of course, we're interested in what is going on there, because we are part of the phenomenon. But that's exactly the point—it's not only that. There is a deeper, a deeper point here and the deeper point is that—let's look from the point of view of the stars, the astronomical view of this. Four percent of this universe is ordinary matter; we're made of ordinary matter, the stars, the galaxies, the planets are made of ordinary matter. And if you look at the history of the universe, it's the transition from a very mechanistic state, in which you only have hydrogen and helium in a very unorganized pattern—in the early universe, which we observe—forming galaxies, stars and planets and then some planets forming complex chemistry to the point that we have here on Earth. So, if you then project that into the future—and we in astronomy can do that; we can see a universe, which goes out for much longer than it has already passed, essentially not changing significantly in any other way. You know, you'll have galaxies, you'll have new stars, new generations of stars, you'll have even more planets around those stars and in those galaxies. But there is one thing which is going to be different, and that thing which is going to be different is thanks to that evolution of matter, of those 4%, which allows for complex chemistry, more and more of those elements in the table of the elements are produced by stars. More and more planets have the rocky structure that we see here on Earth. And more and more of those planets become living. So, if you think about the future of the universe, there is also transition from purely astronomical stars in galaxies to a future which is a biological planets and living planets in particular. So, those 4%, which seemed so insignificant as a whole in the universe, are really driving the exciting changes in this universe; as opposed to the dark matter and dark energy, which really from the perspective of what we know, about them right now do almost nothing.

Steve:          You sound almost, to me, deterministic in the sense that if the universe is evolving in a particular way, so that the chemistry changes to a particular way, statistically, there's going to be life out there. Is that correct?

Sasselov:          In a certain sense, that's exactly what I'm saying. What we can say deterministically is that the future of the universe is that of more chemistry and more complex chemistry. Simply because of the trend of the transformation of hydrogen and helium which do nothing by themselves chemically, through the stars, which are the objects in this universe that transform them into heavy elements—carbon, oxygen, nitrogen, metals and so on. That is the trend. We have a trend from zero of those complex chemicals, to more and more in the future. And as we see it today, we are in the beginning of that evolution, not somewhere in between or towards the end of it. So, the universe has just started producing chemically rich environments. So that much we know. The part which we don't know—and that's why I was saying that the future of astronomy is in biology—is what does it take for that chemistry to become so complex, that it has the features of Earth life? And the subject which I didn't even talk to much about, which is, what does it take then for that to become intelligent? But even if you just try to answer the first question—what does it take for chemistry to become life?—it is a question which we don't know, but it's the next thing which we're trying to do.

Steve:          Sure, I mean, intelligent life is one thing, but even if you found microbes or evidence of microbes on an exoplanet—that would be the biggest story ever.

Sasselov:          That will the biggest story ever, for sure, for many reasons. And that is practically what we can do today. We can actually look for them. And what compelled me to write this book is to say, "Look we know how to do it, we haven't done our homework yet; to be sure, we'll know it when we see it, but we know how to do it."

Steve:          What do we still need to do to be able to identify the signature of life—even microbial life—on an exoplanet?

Sasselov:          What we know how to do today is we know technically how to do remote sensing. And what this is, is essentially looking for gases in the atmospheres above the planets. We've learned that throughout the 20th century, we can do it very well to very distant objects; stars across the universe, literally we're already doing that. So, the question now is could we detect the same signatures which we see on Earth today—and which are due to life here, the biosphere—the answer is yes. But isn't that a foolish assumption to say, "Well there would, life in other planets would be a carbon copy of life here."

Steve:          Literally carbon.

Sasselov:          Literally carbon. So, I think that's where we haven't done our homework. There's a lot to be done and lot to be learnt and probably lot of surprises; but now it is the time to develop that biological aspect scientifically, so that we're intelligent when we look for those signatures. But technically we can already do it.

Steve:          As a very simple example, if you were able to tell spectrographically that a planet had an atmosphere, and the atmosphere had a fixed amount of oxygen, that to you would be a real red flag, I would presume.

Sasselov:          That will be a real red flag: If you see an atmosphere, which has large amount of free oxygen in combination with, say carbon dioxide or water, and you will say, "Well, why is this oxygen there, if there's not enough evaporation of water to produce it before it combines with something else  and oxidizes it essentially, something must be producing it, and it's not volcanoes. So, that's how we can find a twin Earth, somehow a carbon copy, as I said, of Earth and its biosphere. But what if the biosphere is based on a slightly different metabolism? Which we know now is possible to some extent; we know something about biochemistry to say, "Well that's not the only option that we have there." What if they're methanogens mostly and they produce other gases;  are we prepared to actually recognize them? And the answer is, I think, no. Could we do that homework? Sure. People just haven't fessed up to it and done it.

Steve:          One of the things that really changed the way I was thinking about this problem, in your book was your description of the time span of life versus astronomical time. Because we always think of astronomical time in the billion of years and the time scale of life, at the most, in the thousands of years. But you point out in the book that, no, the time span of life is also in the billions of years. So they're not exactly the same time span of the universe and the time span of life, but they're much closer; they're almost a younger brother, older brother relationship.

Sasselov:          Yes, the time scale—and let's call it the time dimension; as we know now in 20th century physics, we live in a four dimensional parameter of space; three dimensions of space and one of time. It is very important to understanding life and its connection to the rest of the universe, in particular the planets. The planetary time scales are those of the stars, and the stars have time scales which are measured in billions of years. Geological processes takes millions of years. Life it seems to us, takes seconds, hours, weeks, years. But this is life in its individual units. Life as a biosphere has exactly the same duration in time, as that of the planet Earth. So, in a sense, it is comparable to everything in the universe. And there are two things which I want to say about that. The first one is, we always wonder how tiny we are compared to the vastness of the universe, but that's because we compare ourselves in the three dimensions of space. Yes, we're tiny in the three dimensional space. But now think about the fourth dimensions of time—there we're not tiny at all. Our entire heritage who we are, goes back to those microbes, which existed here 4 billion years ago; and that's true for every living thing on this planet. The biosphere is a phenomenon which is really large in the time dimension and comparable to planets and stars. So, in that sense, it is a significant phenomenon. The second part is the way we figured out how microbes inhabit the Earth—including below and above in the atmosphere and below the surface—is that they're really self sufficient in the way in which it's very difficult to obliterate them totally. So the part which we can say is at least life is as long living as the star itself; because, you know, once the planet Earth is engulfed by the dying sun in a few billion years from now, that will be the end of the biosphere. But there is one event that happened and changed that as a potential, and that is us going to the moon. So, if you look again from the eyes of an astronomer, just from another star and look at the planet, you essentially saw a piece of the planet go somewhere else and then come back. You know, it was not that something hit the planet and the pieces just went everywhere. It very deliberately went somewhere and came back. If that is part of the phenomena which we call life, the ability to actually transcend the point of origin, then you're actually transcending also the time constraint which the star gives you. You can be transplanted; you can actually move. And hence in principal you can think of life then, as a system with potential for eternity. It could last as long as the universe itself.

Steve:          And on a simpler level, as you point out in the book, let's say the Earth flew out of its orbit, and was just passing through the galaxy untethered to a star; the surface life would all whither away faster than that, but the stuff that's deep down would probably keep going for a good long time.

Sasselov:          That's a new realization too. In fact, all those deep biosphere studies have been done literally in the past 10 years. And even up until last year, we didn't know whether it was just microbes and bacteria that inhabited miles below the surface, but now we know there're even small animals there. There are worms, but hey, these are animals still.

Steve:          They're multicellular. That's a huge jump.

Sasselov:          Right, which tells you that the biosphere there may be equal in mass, in totality, to that what we have on the surface. Now the amazing thing about them is not that they are deep below the surface. The amazing thing about them is they don't care about the surface. They're completely self-sufficient on the heat of the Earth, and what is available down there—and frankly everything is available except sunlight. And so the point there is that whatever happens to the surface, including if the Sun is gone, as long the planet is intact—it doesn't get really destroyed inside the star—then the planet has the thermal capacity to keep going for quite awhile; and especially if it's a super-Earth, because these are bigger planets and they have a bigger reservoir, so to say. It could go on for the same duration the universe has gone on now, literally.

Steve:          The thermal capacity and the chemical capacity.

Sasselov:          And the chemical capacity, yes—both of them.

Steve:          Sulfur organisms…

Sasselov:          They usually are, yes;. both of them.

Steve:          So, you are bringing up super-Earths. Why don't we define super-Earths for people?

Sasselov:          Well, super-Earths just what the name literally stands for: they're planets that are bigger than the Earth, but not much more than a factor of two in size, not much more than a factor of 10 in mass; and some of them are likely to have exactly the same characteristics—bulk composition, surface environments—that our own Earth has; so hence, super-Earths.

Steve:          And, you talk in the book about the importance of tectonic activity, and biology, and super-Earths, and that's a really interesting combination of, you know, we often think of life happens on Earth, but life is also affecting the planet and the evolution of the geology of the planet.

Sasselov:          Exactly, absolutely, that’s the point of.. the planet itself is a system. I'm sorry I'm using this word a lot; but, system, in the more generic sense. In life the biosphere is a system, and those two systems have very many common characteristics, and they co-opt each other and they work together and as a unit, so in fact, they end up being the same thing. So, if you take a planet, which is like the Earth, and it could be a super-Earth, but the planet, which is rocky and has some water or some kind of solvent, liquid solvent near the surface; the way you can imagine it, it's a large, round reservoir with a lot of heat and chemicals, gases included, in its interior, which are moving all the time. But that reservoir has a lid and that lid is the crust, on which we live happily on our own planet. But now the important thing about this lid: It's semipermeable. In other words, it lets gases and chemicals from inside, and heat, to the surface and to the atmosphere, and then brings down some of those that have already gone through the chemical change and have gotten oxidized, for example, brings them down, recycles and brings them back up. This happens without any help from life; that is a simply inorganic, normal geochemical cycle for a planet like this, which can go on for a very long time; and that's essentially we call it the plate tectonic activity of a planet like this. The plates move around, they get subducted, the volcanoes take stuff out from the inside, and it gets recycled all the time. Now why is this important? It's important because it provides surface environment, which is chemically rich. The environment doesn't get stuck to a certain chemical level where you can't make many changes. And life is all about changes. The ability to find energy sources; to transform one chemical to another, and then move on from there and explore different environments. So the planet, having that geochemical cycle, is a system which is just naturally co-opted by a biosphere, which also has its own cycles, and then they work in unison together. So in a certain sense, it's more appropriate to think about the planet being living; essentially planet and life are one and the same thing.

Steve:          There's a fascinating part of the book where you talk about the scenario; let’s say the atmosphere was stripped away, but we still have plate tectonics and therefore volcanic activity. And within a very short amount of time—not by a single human lifespan—but what was it, 400,000 years?

Sasselov:          It's about half a million years.

Steve:          You would have a new atmosphere!

Sasselov:          You're on, yeah.

Steve:          Not an oxygen atmosphere.

Sasselov:          Not an oxygen one, because obviously the oxygen is due to the cyanobacteria and all the living creatures that use sunlight. But the gas will be back; the liquid areas will come back because the water comes from inside the Earth; the microbes, which lived in the crust would not have even noticed that something happened. And some of them eventually will end up on the surface one way or another, will get used to living, and we'll have another cycle of life. And the biosphere again will take over the surface as it has in the past.

Steve:          Because life is really tenacious.

Sasselov:          Yeah.

Steve:          You talk in the book about the effect that synthetic biology, we'll call it—you call it various things and some of these terms have different meanings within particular specialties, but basically the idea of trying to create an artificial cell that is for all intents and purposes truly alive—how that effort is going to inform the whole field of astrobiology. What's the connection there?

Sasselov:          This is  the most tantalizing part about this field. And, in a sense, what made me write this book is the connection between the new developments in biology—in particularly what they call chemical synthetic biology—and astronomy. This unlikely marriage between the two, which doesn't seem to go necessarily both ways, but that's exactly the point. It does go both ways. It goes from biology to astronomy in the obvious way we already discussed. We need to do our homework and know what we're looking for when we astronomers look for signatures of life on other planets. But it goes the other way to inform what we should be doing in the labs, and that's the part which I want to tell you about now. The way it goes from astronomy to biology is that the question about the basic chemistry and what is the nature of life has remained unanswered throughout a hundred years of working with modern molecular biology. And the reason it has remained unanswered is because we've never had the ability to do the kind of experiment or tests we've been able to do in chemistry, physics and astronomy. You have a system, in which you're trying to understand what are the basic rules; and you push it left and right and see how it reacts back, the feedback mechanism, so to say. In biology, you don't have the luxury of that. Even the simplest microbes, the simplest biological systems, the simplest biological, biochemical cycles, are very complex, as we realize. They have a heritage which goes millions and sometimes billions of years back, which we still don't understand. It's real heritage, in fact that's the beauty of life, because it remembers all the geological trials and tribulations of the environment and uses them in different ways. It's. kind of, a very complicated software system. But what it means, we can't really understand what is the basic operating system of that software system because we can't push it left and right and have veritable feedback. So what synthetic biology is trying to do for this fundamental question is to create a chemical system which has the basic functions of what we call life, but has gotten rid of all the other complications. A very, very simple cell which is called, technically, the minimal cell or artificial minimal cell, which is chemistry that does what life does—you could say mimics what life does—but can be subject to those experiments. So, you can see how the different planetary environments will make the system go sulfur as opposed to carbon. You could see how the changes in the temperature on that other planet would make the choice not towards DNA but say GNA, another molecule, which looks like DNA and could be a genetic molecule, but is not in Earth life. So, these are the kind of experiments, which this new field, chemical synthetic biology, is allowing us to do now. And that is the connection, where they're looking for the initial conditions, as we say, for that input from those new planets in order to experiment with their lab set ups. So that's the very unusual synergy between astronomy and biology that has happened in the last couple of years.

Steve:          And one of the people you talked to about this is Jack Szostak who won the Noble Prize last year, and we had him on the podcast a few years ago; and he's really one of the foremost people in that field, trying to tease out what the origins of life on Earth were or what they could have been. Part of the problem is, the place is so teeming with life now that the origins are washed out, the signal to noise is just not big enough anymore. So what is that like when you're together just chatting about this problem?

Sasselov:          This project started really with us chatting about that and saying, "Well there is something which can be done about this." Jack was excited about the exoplanets that we were discovering and the direction in which we were going, where we were going to do chemical analysis of those different planets. And he realized exactly that, that if we only stay with the initial conditions that we have here on Earth, we're not going to be able to do in the lab, the experiments that might be interesting and tell us something about what is important and what is less important; that is: This is essential to life, while this is due to the environmental conditions. So that was what he was looking for. And I was looking for the homework that I mentioned before. I was going in the direction of looking of those bio-signatures, and I knew I was fooling myself that I knew what I was looking for. So, I needed somebody to tell me: What about those other gases? What about these other possible signatures? So that is the synergy that I’m talking about. Now technically speaking, what Jack Szostak and his lab have managed to achieve in the past few years, is that they've managed to create very simple cell-like structures; empty but having the membranes which are necessary for chemical concentration and which are crucial to create a minimal cell, that chemical system, which I call the artificial chemical cell. So, the project is to synthesize the necessary molecules and put them into Jack's little bubbles, those little empty cells, and watch them operate as a chemical system; not very different from a living cell, because they can function like a cell, given the membranes that he managed to create. So, technically this is really the direction in which the project is going, and that's why I’m so excited about it because it's looking that it's going to happen.

Steve:          And then when the biologists come up with different possible scenarios for how life could be, then you can tune your instruments in different ways to look for alternatives to Earth-like life on exoplanets?

Sasselov:          That's exactly what I'm hoping for, that when the biologists have those experiments in the lab, they will say, "Hmmm... you know these super-Earths which are sulfur-cycle planets as opposed to carbon-cycle planets? You shouldn't be looking for oxygen there, you should be looking for hydrogen sulfide in this kind of concentrations." So I'm going to look at those super-Earths, and I'm going to tell them, "There is a whole gallery of chemical signatures from those, and we see that the sulfur dioxide to carbon dioxide is those ratios; what would you do in your lab?" They go to their lab, look at these artificial cells with which you can experiment and they say,  "Well, the metabolism will go in different directions." So the gases that will really be mostly out in the atmosphere, probably those other ones, then I go to the telescope and look for those. So that is the view of the near future for the science that I'm trying to do.

Steve:          You really need those telescopes to be in orbit; you can't do this with earth-bound telescopes.

Sasselov:          That's always true. When we need to see further away and see tiny Earth signals, going in space is always better for a telescope—you avoid all the problems with the atmosphere. There is a second problem with our Earth atmosphere, when you're looking at other Earth-like atmospheres; well you're looking through the same portfolio of molecules, aren't you? So, essentially already their signature is imprinted on the light that comes from that distant one. So you have now to tease out not only the weak signal from its star and the other planets, but your own planet. So, it always helps to go in space. However, it is not always absolutely necessary; there are some tricks one could do in which we could the two spectras displace from each other; and the truth is, as much as we can do great things in space, the biggest telescopes are always going to be on the ground. It's just a matter of scale, and so there're lot of things you will be able to do with the next generation of large telescopes, so we're already planning for that as well. I would say it's a combination of both, doing some of the science from ground based telescopes and some with the next generation of space telescopes.

Steve:          One of the really interesting notions that you put forth is that, you know, the only planet we know of that has life, obviously, we're sitting on. But this might not be the best planet for life. There might be super-Earths out there that are even better for life to have come in to being and diversify on.

Sasselov:          It just happened that in our own solar system, the Earth is the largest rocky planet. Maybe there was a rule which will tell you that this is how it should be, and our solar system is just an example of how other solar systems are. Now we know this is not true. Now we know that the planets, which are rocky but bigger than the Earth are plentiful; they're around other stars in big numbers. So what happened in the solar system is that, well, there were no super-Earths that formed. Or you could say one formed, and that's the Earth. Venus and Mars are smaller. And now we know a lot about Venus and Mars, and we certainly know that if you're smaller than the Earth, a lot of things go wrong. Your climate is less stable, your atmosphere can go away like Mars, plate tectonic activity and chemical enrichment goes away as well. Mars had huge volcanoes, but they stopped. Why? Because its lid got too thick, and it stopped fairly early on; never had plate tectonics, so it's chemically stuck on the surface. So, when you look at all those things, and you say, "Well, the Earth is great, and sure it is great, everything works. But if you're a little bit bigger than the Earth, all those things are the same or even better—more active, more chemistry, more stability—and nothing is worse, I mean, that you have little bit higher gravity, that's not a big deal, especially for the microbes.

Steve:          The planets concentrate the chemicals, and then life concentrates the chemistry. And here we are—the Earth is the ultimate distillation.

Sasselov:          Precisely. I mean, a piece of rock is essentially a crystal in equilibrium. A planet like the Earth is a huge crystal which is not in equilibrium, over time scales which are comparable to the time scale of the universe. And so that's where life comes to be. That much we know from astronomical perspective. What we don't know is the biological aspect of it—what exactly is the nature of it?—and that's where we're going next.

Steve:          So, why do you say that this completes the Copernican revolution?

Sasselov: I say that it completes the Copernican revolution for two reasons. One of them is just an obvious and trivial one. Copernicus really brought back the ancient idea that the Sun is in the center, and that the Earth is just a planet like the other six they knew at the time. So, when he did that, and when his followers figured out  the orbital motion of the planets and all of that, the logical thing was to say there must be similar planets and planets like the Earth around the other stars, which are just suns like our own sun. It was obvious to everybody, but it was never proven; and you could imagine many ways in which maybe this was not the case. Even with our 20th century science, although nobody doubted it, there was no evidence that there are planets like the Earth or even any planets around the other stars. So, in a sense, trivially speaking it does close the chapter. You say, "Okay the evolution started back then." It took 500 years for us technologically to be able to say, "Yes, the other stars have planets, and planets like the Earth also form, end of story." It is also nice in a way that—and I think that's a coincidence—that this technological ability that we have to complete the Copernican chapter is also the same one that made us globally aware that we live on a planet where we understand that we're part of a biosphere and that this planet is really an entity of which we’re part of. And somehow to turn now to the next chapter which is, let's call it the biological revolution to understand the nature of that aspect of what the Earth is about.

Steve:          Would you bet that we'll have incontrovertible evidence of life on an exoplanet in your lifetime?

Sasselov:          I would bet, and you know why? Because this is what I’m actually doing as a scientist. So if the bets were wrong….

Steve:          You bet you're career on it.

Sasselov:          Yes. I wouldn't even have tried. I have many other projects which I can do. I think to be serious about it, I think we have a fair chance to do that, and again, we have the technology to do it. So we have to try, that's one thing. The other thing is it is a journey. It is a journey in the sense that you learn so many things along the way and so many of them give you new insights about the world. So, many of them give you practical things which you can use in your daily life. This is exactly one of those things in science. So, it is fundamental science, it looks like a pie in the sky, but along the way you learn so much.

Steve:          That's it for this episode. Get your science news at our Web site, where you can check out the section on Citizen Science. You'll find research projects that you can take part in. For example, if you're an astronomy enthusiast, the folks at the Lowell Observatory in Flagstaff, Arizona, can use your talents as part of the Lowell Amateur Research Initiative. And follows us on Twitter, where you'll get a tweet every time a new article hits the Web site. Our Twitter name is @sciam S-C-I-A-M. For Scientific American's Science Talk, I'm Steve Mirsky. Thanks for clicking on us.

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