Dimitar Sasselov—LIFE: WHAT A CONCEPT!

Dimitar Sasselov—LIFE: WHAT A CONCEPT!

An Edge Special Event at Eastover Farm
Dimitar D. Sasselov [8.27.07]

"Life/ Consists of propositions about life." — Wallace Stevens("Men Made out of Words")

Is Earth the ideal planet for life? What is the future of life in our universe? We often imagine our place in the universe in the same way we experience our lives and the places we inhabit. We imagine a practically static eternal universe where we, and life in general, are born, grow up, and mature; we are merely one of numerous generations.

This is so untrue! We now know that the universe is 14 and Earth life is 4 billion years old: life and the universe are almost peers. If the universe were a 55-year old, life would be a 16-year old teenager. The universe is nowhere close to being static and unchanging either.

Together with this realization of our changing universe, we are now facing a second, seemingly unrelated realization: there is a new kind of planet out there which have been named super-Earths, that can provide to life all that our little Earth does. And more.

DIMITAR SASSELOV is Professor of Astronomy at Harvard University and Director, Harvard Origins of Life Initiative. Most recently his research has led him to explore the nature of planets orbiting other stars. Using novel techniques, he has discovered a few such planets, and his hope is to use these techniques to find planets like Earth. He is the founder and director of the new Harvard Origins of Life Initiative, a multidisciplinary center bridging scientists in the physical and in the life sciences, intent to study the transition from chemistry to life and its place in the context of the Universe.

DIMITAR SASSELOV: I will start the same way, by introducing my background. I am a physicist, just like Freeman and Seth, in background, but my expertise is astrophysics, and more particularly planetary astrophysics. So that means I'm here to try to tell you a little bit of what's new in the big picture, and also to warn you that my background basically means that I'm looking for general relationships — for generalities rather than specific answers to the questions that we are discussing here today.

So, for example, I am personally more interested in the question of the origins of life, rather than the origin of life. What I mean by that is I'm trying to understand what we could learn about pathways to life, or pathways to the complex chemistry that we recognize as life. As opposed to narrowly answering the question of what is the origin of life on this planet. And that's not to say there is more value in one or the other; it's just the approach that somebody with my background would naturally try to take. And also the approach, which — I would agree to some extent with what was said already — is in need of more research and has some promise.

One of the reasons why I think there are a lot of interesting new things coming from that perspective, that is from the cosmic perspective, or planetary perspective, is because we have a lot more evidence for what is out there in the universe than we did even a few years ago. So to some extent, what I want to tell you here is some of this new evidence and why is it so exciting, in being able to actually inform what we are discussing here.

Basically, in order to explain to you why this is interesting, I want to first of all convince you about three things, which are important to my approach. The first one is that what we are looking for is baryonic in nature. What I mean by that is something of which I don't need to convince you, I believe, but you should bear it in mind because this is a feature of our universe, the one we observe. Baryons are all the particles that make up atoms and all that is around us, including ourselves. But that's not necessarily the most common entity in the universe, as you — I'm sure — know about dark matter and dark energy. I think we have to agree that what we are looking for and would call life is baryonic in nature, and there is good reason to believe that dark matter and dark energy are not capable of that level of complexity in this universe yet — or at all.

The second point which I want to convince you of — or use as my background for what I'll tell you here — is that we should agree that what we are looking for, what we call life, is a complex chemical process. Basically, the ability of those atoms to combine in non-trivial ways.  This is actually my point of departure, where I would be looking at life more from the purely thermodynamic aspect, that is from the point of view which Robert here described and H. Morowitz has been very eloquent in defining and actually done some research on. That is, what is the parameter space in which you can have chemistry which is complex enough to lead to a qualitatively new phenomenon, a phenomenon which we don't see in the rest of the universe. That's actually an important point here.

Do we know enough about the universe that we can have sufficiently good feeling about that parameter space? Obviously we don't have detailed knowledge of most of the observable universe, but the last 50 years have been actually a revolution in that field, in the sense of the ability to get diagnostics of very distant objects and a very large number of objects.

The databases in astronomy up until just a few years ago were larger than what biology had. It's only now that biology — and, I guess, telecommunications companies — have exceeded that. But one aspect of these databases is that you very rarely see unusual, unexplained phenomena. Despite what you all would like to write on the front page of your newspapers and magazines, actually there is a lot of very boring amount of data there, which is hundreds of thousands — already millions — of stars, which have exactly the same isotopic and chemical patterns that are predicted by the theory which is well developed and is called 'stellar evolution' (although it has very little to do with evolution as used in biology).

But it is one of those steps that we now understand as the development of our world that is of our universe, of starting with very simple baryonic structure for that matter, which then becomes more and more complex. Stellar evolution is one of those phenomena that did not exist in the first half billion years of the universe. And this is not a hypothesis; we know it. We actually can observe a lot of it, and we know that there were no stars during the epoch of recombination, which is the cosmic microwave background, with all the structure that we see in it. And then there were stars, and then stars started a new process, which did not exist in the universe before, which is the synthesis of the heavy elements. That is — baryons working together as elementary particles and building a structure — the Mendeleev table, which then would lead to chemistry.

VENTER: How many years ago was this?

SASSELOV: 13.7 billion years ago is where we see the precursor of the microwave background radiation, so that's our first very well studied piece of evidence. Then about half a billion later is the time when the first stars can form, from the gas, and they're mostly made of hydrogen and helium. Then they go through a period where over a time of five billion years they produce enough carbon, nitrogen and oxygen and all the heavy elements, where you start effectively producing planets. And then we come to 4.5 billion years, which is the origin of our own solar system and the Earth. And almost within a half billion years, some complex chemistry which we now see covering entirely and co-opting the geophysical cycles of this planet. So that's to give you a quick idea about the time scales.

In that sense life is an integral part of that global development that we see. And although we know only one example of it, it doesn't seem unusual when you think of it that way — as a progression of complexity that the baryonic aspect of this — baryonic matter — in this universe has actually the propensity to lead to. So the question then is what is this good for understanding the origins of life, or possible pathways? And even more generically, could we design experiments in which we can find out whether all these possible baryonic pathways really merge into one — the one that produces life here on Earth — or are there multiple pathways? Even if you could answer that question, that would be very exciting, because it will tell us something about the general rules of complexity that baryonic chemistry can really lead to.

The question then is, the third aspect which I want to convince you of, is we know quite a bit about the universe, but there are only a few places in the universe where you can think of that complex chemistry being capable to survive over a sufficiently long period of time. And vacuum is not one of them, in the sense of surviving in which you were talking about the origin of life; starting with smaller molecules, which then have enough time to lead to more complex ones. And when I think of vacuum, I don't mean the surface of a comet, but really the inter-stellar medium, with its very low density.

I can imagine life that started on some surface then migrating to live in the inter-stellar medium. But I cannot imagine, as an astrophysicist, from what I know, that there is an environment, which is stable enough over the time scales necessary for that chemistry to take place. So I am a little bit biased in that sense to planets and planetary systems as the only environment that we know of today, as far as we know in the universe, which has all of those factors put together — that is, stability over long periods of time, but sufficiently low or moderate temperatures. (Stars are very stable over billions of years, but they all have very high temperatures, all throughout.) And basically the overall thermodynamic window that Morowitz is talking about, which allows complex chemistry. That's actually much broader than simply having water.

When people talk about habitable environments, sometimes they would equate that to the existence of water, or the ability of water to be in a liquid form. That's a much broader view of what is available there. But whatever your idea of what could be habitable is, the bottom line is that there are not that many objects, or places, in the observable universe that allow that. In fact, planetary systems are certainly not only the best, but are probably the only ones on which we are certain that complex chemistry can occur.

Then the question is, how much do we know about planetary systems? Up until 12 years ago, essentially we knew only of one: the solar system.  That situation is very similar to what we have with life. We only have one example. And that's bad from many points of view, and we — 'we' meaning astronomers — learned it the hard way, because it turned out that what we had theorized about planets was very solar system-centric, and we missed a lot of things that we should not have missed, but that always happens when you have only one example of something.

What planets allow you to do now that we know how many different types of them there are, is you can have a pretty good estimate of what to look for. And one of the things that we learned - I guess the hard way - is that we do not necessarily have to look for planets just like the Earth. What I mean by this is that although in our solar system we have a very large variety of planets — you have Jupiter, which is very much bigger than the Earth, ten times in size, 300 times in mass; you have Saturn; you have Neptune and Uranus — all giant planets, all made of gas — then you have very small planets: that's the Earth, Venus, and Mars, and Mercury, going smaller — and then comets and asteroids.

There is a very significant gap in masses between 1 Earth mass and 14, where Uranus and Neptune are. That's actually, as we would say in physics, more than an order of magnitude. And it allows for a whole set of phenomena that could happen in that range that we've been missing. And from what we understand now, both from theory and more recently — meaning in the last two years — from observations of such systems, is that the fact that the solar system has no planet like this is just a fluke. It just happened the way the planets were formed that what ended up being the solar system has no planet which is in that mass range. The majority of planets in that mass range will be like the Earth, and for lack of a better term, we ended up calling them Super Earths.

I get a lot of flack for introducing that, but it comes from my bias as an astronomer. We call stars that are bigger than giants, super-giants; we call stellar explosions which are more energetic than novae, super-novae; so it just made sense that if you have a planet which is larger than the Earth but otherwise is in essence similar to the Earth, you would call it super Earth. I guess I didn't grow up with Super Man.

CHURCH: That's not Super-Earth, that's Krypton.

SASSELOV: Just take it as it is — it's just a term — it's just planets which are larger than the Earth.  Now why is that interesting — if you really limit yourself to planets larger than Venus and Earth, but not much larger than Earth, then you're left with very small numbers in the galaxy as a whole and in our part of the galaxy as a whole. If you allow yourself to count super-Earths as part of the inventory that you can tap, then your numbers grow by two orders of magnitude. I'm saying this is because of two lines of evidence.

LLOYD: What is the concentration of the smaller ones? What fraction of solar systems, or stellar systems, has 'sub-Earth' planets?

SASSELOV: Ah, so that's actually a difficult question — what fraction of the planetary systems have planets smaller than the Earth — because they're hard to see. We have some estimates, which go to about the fraction of an Earth mass; well let's just say one Earth mass. We have no technical evidence now for less than that. That's from a technique that is called micro-lensing, by the way.

The evidence for this is in part statistical, but that's quite often the case. You observe many objects and you build statistical cases for all of that. On the one hand we already have detected a number of super-Earths — the current number is actually five.  That's a small number for statistics, but it is not a small number statistics when you view it as an effort where a lot of other planets have been detected, and despite the difficulty of detecting smaller and smaller planets, you are detecting an increasing number of those in the planetary systems that you are observing.  In other words, as you go to smaller and smaller masses, below about 12 to15 Earth masses to a planet, the numbers actually rise despite the statistical biases of actually having less of those. This anticipates that as our technology improves, which by the way it is, on a monthly basis, we will be discovering more of those.

There is another line of evidence which is a technique which is called micro-lensing for detection of planets, that is sensitive to the entire mass range of planets, all the way down to one Earth mass, and actually in fact a bit smaller than one Earth mass. This technique is scanning without any prejudice a large number of stars and to this point they have actually detected more super Earths — smaller planets — than larger planets. Which then tells you that if you take the current statistical numbers, which we have already figured out pretty well because we have larger planets in large numbers from the last 12 years of study, you can actually estimate what is the expected number of smaller planets just because of this comparison that you do.

There is a third line of evidence, which being a theorist myself I would not really push too hard, but theoretically if you form large planets you also form small planets, and there is no particular theoretical prejudice that anybody has come up with at this point, that you will somehow create gaps like the one we have in the solar system, where you will have only very small planets and only very big planets.

So the final question here is, are these super Earths actually any good for what we're interested in?

VENTER: Can you actually put a number — what's the number in the universe of super Earths?

SASSELOV: Well, that's a good question. Let's take our galaxy as an example, not the whole universe. We now have a pretty good idea that there are about 10^11 — a few times, 2 or 3 times 10^11 stars in the galaxy. So then we know that of those stars, only about 90 percent live long enough for the kind of complex chemistry that we have in mind, which is half a billion years or longer. However, only about 1/10 of these stars have enough heavy elements so the planets that will form around a star like that will either not form at all, or will have a significant deficiency. In fact we have evidence for that. Then the question is, how much do we know about the number of super Earths? Basically of those left over, where we have ten billion or so, you would say that it's only a fraction which is less than 50 percent but larger than 10 percent from those arguments that I gave to you so far. And then you look where in the planetary system you are — you don't want to be right next to the star and you don't want to be too far from the star, and this is following Morowitz's thermodynamic estimates for the temperature range. The bottom line that you end up with is about a hundred million planets that I would call habitable in the sense that they allow this kind of complex chemistry somewhere near their surface. A hundred million in our galaxy.

VENTER: And how many galaxies are there now?

SASSELOV: Oh, that's a large number, but it's a similar number to the number of stars — 10^11.

The question is — I actually insist on doing it for the galaxy, because I'm interested in the experiment; I'm a theorist, but I really trust the experiment — how many of those environments can we study soon enough (while I am still alive) and with enough detail that we actually can help you guys, the chemists and the molecular biologists, to constrain your experiments into those pathways to life. Basically the estimate is many. Because if you have that many, a hundred million in our galaxy, then only in our vicinity, with the experiments which are already underway, we'll have at least about fifty to a hundred in the next five years. And fifty to a hundred for which we can get some data that will be interesting to inform those questions.

VENTER: So your data set would exclude things like Europa?

SASSELOV: No, not at all — Europa is a great place to look for life. I'm just saying this is the minimum.

VENTER: But I mean size-wise.

SASSELOV: Well, the reason that Europa is viable is because of Jupiter. If Europa was just by itself we may not consider it that viable. In a sense I'm trying to be conservative here, and I can tell you that I can promise you only that many. But there is another reason why I actually would like to make this estimate, and why I talk about the hundred or so that we are going to be able to study. And this is because I do want to be able to study them outside of our solar system. And the question is, how do you study Europa in a planetary system that is 50 light years away? Very difficult.

But can you study a planet which is five times more massive than the Earth and two times larger than the Earth? Yes. Even much more easily than an Earth-size planet. So the point that I'm making is that the fact that super Earths are viable as planets in the comparison to the Earth is actually great for our ability to do these experiments, because it's much easier to detect and study a planet which is two times bigger than the Earth and is still viable. You can learn a lot from it.

One of the reasons I call these planets viable, and in fact even more viable than the Earth, is because they have the basic characteristics of the Earth, except in a much more robust way. You probably know that there is a big problem in planetary science, which is the comparison between the Earth and Venus. Why does the Earth have an atmosphere which is not very hot, that's sort of understood — not yet, but sort of. Why does the Earth have plate tectonics, while Venus doesn't have plate tectonics, that's not understood — or we are at the verge of starting to understand that. These are questions that are much easier to answer for super Earths.

It turns out that plate tectonics, as understood from Earth, is a process which has been going on theoretically much more easily on a slightly bigger planet. In fact if you do the theory, as best as you can today, the Earth is at the margin of what is viable in terms of plate tectonics. Probably some of you may know that plate tectonics is a very important aspect of the viability of a planet in terms of surface conditions, because it's a good thermostat, it keeps the climate more or less stable over long periods of time, and also allows you to have easy access to the large reservoir of chemicals and gasses in the mantle of the planet.

In that sense super Earths are as good as the Earth, and I would argue — better.  They have more stable and robust surface conditions. So there are more of them, they're as good as the Earth, if not better, and they are easier to study. So we have a very bright future of being able to at least find out what's going on.

VENTER: What role does gravity play in the larger — in the super Earths?

SASSELOV: It's actually a positive role. In the sense that if you take the general amount of out-gassing, fluxes, which interchange between the mantle and the atmosphere of the Earth, the Earth's gravity is very close to marginal — we know Mars is an example where it's definitely sub-marginal, in retaining a sufficient atmosphere, and hence making this thermostat being viable, and really providing you with stable conditions over at least a billion years. So having more gravity is actually better.

VENTER: It increases the odds of having an atmosphere?

SASSELOV: In keeping it. You always have an atmosphere — even Mercury has an atmosphere: there is some helium that is being punched out of the surface of the planet, but it simply cannot retain any of it. It just goes away.

So I’d prefer to answer questions rather than to continue.

SHAPIRO: Which is the closest known super Earth?

SASSELOV: The closest known is called — in fact there are two of them: Gliese 581c and d, and both of them are super Earths, and are just 20 light years away. Wilhelm Gliese was a German astronomer (1915-1993).

CHURCH: When will they arrive here?

SASSELOV: Next week.

CHURCH: Since they're better than us.

SASSELOV: The names are Gliese 581c and d — that's the number of the stars. c and d stands for 'planet c' and 'planet d'. There is also ‘planet b’, which is a bigger, Neptune-like planet. 30 years ago, Gliese made a catalog of all the nearby stars. A lot of them are very faint, they hence were only identified in this catalog, so it's a common practice to call the stars by the name of the author of the catalog with a consecutive number.

PRESS: Can you clarify the ratio that you're seeing from the microlensing studies of Earths to super-Earths? I didn't quite catch that number.

SASSELOV: Ah, it's not Earths to super-Earths, it's Earth-like planets, which is super Earths including Earth mass, to giant planets. And we have a good number for the statistics of giant planets in planetary systems right now from all the ones which were discovered with the Doppler shift technique over the past 12 years. They have 250 of them, so that already gives you a good statistic.

DYSON: How many micro-lensing planets do you have?

SASSELOV: At this point, seven.

Someone had asked if it is possible to launch a spore (panspermia) from here to there and hit something 20 light years away? Is it possible to hit it? The answer is yes, from the physics point of view, it's possible. And I have a colleague who says that if anything is possible from the laws of physics, it happens in the universe. But he's a physicist. Let me qualify that. I think panspermia was very popular, and in a sense originated in the modern sense in the 20th century, when there was a possibility, or even some knowledge, that the universe may be very old. Older than 20 billion years, or maybe eternal.

Fred Hoyle, one of the people who really supported and developed that — sort of a steady-state universe, which is not 13 billion years old. Now this is very significant, because then it IS much more likely that complex chemistry originated somewhere in the universe and then spread — it's a very robust system as we know it on this planet; it can really spread. And there is plenty of time for it to spread — even over the large distances and all the vagaries of high-energy astrophysics. However, the universe is only 13.7 billion years old, and actually you have to subtract 0.7 just in order to have the first generation of stars.

The first generation of stars, they were made of hydrogen and helium — there was no carbon, no oxygen, no metals — were actually very large, and they could not have even sustained protoplanetary disks let alone have planets form. It took a long time before you could actually start forming small stars, in which the protoplanetary disks would have enough solid particles to coagulate to form planets. And that actually was thought of theoretically but with very large uncertainties, and now we seem to get, somewhat unexpectedly, evidence for this. Very strong evidence, in fact.

There are a lot of searches for planets and planetary systems that are now even targeted towards stars which are slightly older and have less heavy elements than our own sun. You see a very precipitous drop in the detection of such planetary systems. In fact after certain factor, about ten down, in such heavy elements, nobody has been able to detect a single planet, which is kind of strange, especially because there are people who are trying very hard — one of my colleagues has been trying now for eight years and has come up with zero. And for the normal metallicity (that is stars like the sun), the numbers are very high, already 250.

That means that you have to come to that part in the history of the universe when you have stars like that form. Like the sun — or a little bit less rich in metals. Simply because it takes that much time for the previous generations of stars to synthesize through nuclear fusion those elements. And we know actually that number: another of the big successes in the last five years is that you can now go back all the way to that time and see — literally see, by measurements — the increase in heavy elements as you go from one generation of stars to the next and so on.

Basically you are left, I would argue, with about seven billion years. So the first complex chemistry which could have occurred on the surface of a planet would have started about 7 billion years ago. Now in 7 billion years it's very difficult to bring something from there to here, especially if you want to bring it 4 billion years ago. You only have three.

CHURCH: How hard would it be to hit? At that arc angle, and the radiation damage, and all the rest.

SASSELOV: We don't know that. It's very easy to calculate the cross-section for it hitting. But understanding how it's going to make it there is very difficult.

DYSON: There is some evidence. A radio-astronomer called Jack Baggaley in New Zealand — I don't know if you know him or not — is observing dust grains coming into the upper atmosphere and he claims that first of all a lot of them are extra-solar, they actually come from beyond the solar system, and furthermore they are preponderantly in a certain direction in the sky which comes from a star called Beta Pictoris, which is not very far away, 60 light years or something like that — and it is actually quite a young star but it has a very large dust cloud around it. And so it's plausible that dust is coming from this star and actually hitting the Earth. And if dust grains are coming, there's no reason at all why bigger objects shouldn't also be coming, and they would probably be following similar trajectories. So in principle we know that stuff is arriving from other solar systems. Whether anything is alive on Beta Pic is another question.

SASSELOV: By the way, the stuff which is coming from Beta Pic only started coming to the Earth recently. In the last hundred million years or so. Because Beta Pic didn't exist before that, and it took that long for it to come.

DYSON: Right, but it's easy to get here within the time available.

VENTER: Especially: we had these large asteroid hits hitting the Earth after microbial life existed here — we splashed a hell of a lot of stuff into space from those hits.

SASSELOV: So from here on, Beta Pic's hail will go to some other place.

VENTER: That's why I was trying to push you for a number in the universe, you know, ten to the eighth probability in our own system is pretty high probability. There could have been a million origins, all contributing to panspermic events, you know?

SASSELOV: Right, and then the question is, can you cross from one galaxy to another? And that takes a long time. It takes billions of years. And that's the problem. We can't go from one galaxy to another.

DYSON: Isn't our galaxy big enough?

SASSELOV: That what I was saying, that's why I was making the estimate just for our galaxy.

DYSON: For now.


VENTER: So at comet rates how long would it take to go the 20 light years? Comet speeds.

SASSELOV: It's about a million years. And I'm talking about the really fast comets. The fastest that we've observed.

CHURCH: That would be a lot of radiation damage, in a million years, I would guess.

SASSELOV: Sure. The problem is that we don't yet have evidence? I'm talking about these fast comets — we don't have the evidence for a comet coming from outside the solar system. All of them have close to parabolic orbits — a hyperbolic orbit means that the comet came from outside the solar system and left — so if you take those speeds it's still millions of years.

VENTER: How long would it take a .1 micron object to collect three million rads of radiation, traveling through space.

SASSELOV: You said a ten micron object?

VENTER: No, point one.

SASSELOV: Oh, point one, yes.  I don't know.

VENTER: We can find organisms right here that could take 3 million rads of radiation.

LLOYD: And I bet it wouldn't be that hard to calculate what the velocity distribution of such organic objects would be after an asteroid hit, and roughly how many there would be going out?

SASSELOV: I would prefer a larger object in which your prize collection is embedded. Because then you shield it. So it's shielded from cosmic rays, you shield it from any kind of radiation.

CHURCH: It has to be really large, you know, like meters.

SASSELOV: No, I think if you're just worried about the million years and a certain dose OF rads, it shouldn't be larger than a few centimeters.

VENTER: Well, up to recently, we've been dumping all the feces from the Space Station into space—so that's a kind of shield, right?

SASSELOV: I have a colleague at MIT who calls this the garbage belt of the Earth.

SHAPIRO: I have a different question. Couldn't we get some estimate of the probability of material leaving the Earth, splashing off as they say, by examining the surface of the moon in protected areas? Say those areas that are in permanent SHADOW craters.

VENTER: A couple of hundred kilograms a year.

SASSELOV: Actually several people suggested that as one of the interesting experiments to do when going back to the moon — to look for those. And we know which part of the moon would have most of it — it's not an even distribution over the surface, because of orbital dynamics. So that will be a very interesting thing to do.

SHAPIRO: Look in craters which are protected from the radiation, and so on.

SASSELOV: Yes. I think that's an excellent experiment to do. And if somebody else pays for it...

PRESS: And how would we be able to study the properties of these planets? Is there any way to do it other than looking at transiting planets?

SASSELOV: We've thought a lot about that, because this is where we are spending our money right now. And a lot of people are. There is in fact an Exoplanet task force, which is tasked to think this question over, and this is part of the direction which they are writing into their report right now. The idea is that we want to have two parallel paths.

One is the now old-fashioned one, just a few years old, called Terrestrial Planet Finder (TPF)  — direct imaging, which is still viable, but probably will take longer technologically. And by direct imaging, what is meant is you're not imaging the surface of the planet directly, but you are imaging the planet separately from the star and you are able to get spectroscopic information that way- as well as some surface information (if the planet spins then you see variations which can be interpreted as surface information).

In the meantime, though, technologically it's much more viable to look for transiting planets, and to study transiting planets. Because what transiting planets allow you to do is not only to discover the planets, which is not that important, but once you've discovered them, you have actually the ability to measure their mass and the radius very precisely. And by very precisely I mean to an accuracy of one, two, three percent, which is very precise. So that gives you a mean density of the planet.

It turns out that this mean density can tell you whether the planet is really a small Neptune-like planet, that is hiding as a super Earth but is really a very gas-rich planet without a solid surface anywhere. Or it is an Earth-like Super Earth, which is simply a version of Earth, just bigger. Then once you've passed that measurement, the next thing you can do is you can use measurements both during the time when the planet is in front of the star, which is called a transit, as well as when the planet is behind the star, which is called an eclipse.

In the first case, you measure gasses in the atmosphere through transmission, which is like passing through the atmosphere of the planet. In the second case you actually measure surface features. And the surface features give you a map of the surface: a color map if you do it in the infrared, and an albedo map if you do it in the optical. And this is actually the first such map, that I'm sure many of you have seen, which colleagues in our group at Harvard actually accomplished just a few months ago. It was published three months ago. Now, this is a thermal map, of a giant planet, just like Jupiter. We're not talking about super-Earth here. But right now technically it is possible to do this for a super-Earth.

VENTER: What distance is this?

SASSELOV: This one is at a distance of about 45 light years. So if that was a super Earth you could do the same thing with the existing Spitzer telescope. So that's actually where we are putting our money right now and we hope that NASA will put money into the other one, which is TPF (Terrestrial Planet Finder).

PRESS: What is the prospect for actually being able to measure the atmosphere of the super Earth and say, hey, this things looks like it has an atmosphere out of chemical equilibrium, there's oxygen, — there's something there that makes you sit up and say this thing looks like a place that has life. What is the prospect for doing that?

SASSELOV: Five to ten years, where five is more likely at this point, the way things are developing.  If we are lucky it can happen even in a year or two. But we have to be lucky. The projects which are going to discover large numbers of them are coming up. Corot is one of them, but NASA’s Kepler is much more so, and there are a couple that have just started, or are being built, that will produce enough of those planets that you can then cherry pick and say, ah ha, now I have a few that I can study in detail. But in ten years we will have a whole gallery of them, as opposed to just a milestone.

DYSON: Which molecules will you be looking for?

SASSELOV: Anything that we can see. So basically the idea there is to have enough signal to noise that we can see them all. The resolution is not an issue, because most molecules have broad spectral features, so it's a matter of signal to noise. And we will try to see what we see, and personally that's actually one of the reasons why I'm involved in origins of life research. Origins of life, because I felt that I would be very embarrassed to have this gallery of spectra and maps of super Earths, without being able to answer the question, what do you think is going on on this planet — is it chemistry, or could it be biology?

DYSON: Can you see oxygen and nitrogen?

SASSELOV: Oh, oxygen and nitrogen are actually easier to see. Partly through their proxies which are CO2 and CM. The molecules.

PRESS: If you were looking at Earth from another super Earth somewhere else, is there anything that we've done to the environment that we could actually detect? A large increase in carbon dioxide...

SASSELOV: Yes, people have done this research already — partly in preparation for the Terrestrial Planet Finder. So the strongest indicators there are the existence of ozone — free oxygen and simultaneously amounts of methane — the imbalance is what leads you to believe that something unusual is going on and cannot be reproduced by any of the global planetary cycles that we sort of understand. It is easier to complete the parameter space of global planetary cycles, like the carbon cycle, the sulfur cycle, and to say we are outside of any of that parameter space, that is you cannot explain that combination of atmospheric gases with any of those cycles operating. So by exclusion, you will see that there is something unusual here. But from that point of view, my estimate of habitable planets, a hundred million in our galaxy, excludes the Earth. The Earth as it is now is not very habitable. It's a very hostile environment for complex chemistry.

LLOYD: Were you hoping that people on this other planet would detect that we were screwing this one up, and would come and rescue us. Is that what you were hoping?

VENTER: Why would they want us?

SASSELOV: I actually mean it — I really mean the large numbers of free oxygen.

DYSON: But if you were looking at the Earth this way, would there be enough CN to detect?

SASSELOV: No. The Earth is actually quite a difficult case in that sense.

VENTER: So if it wasn't for the influence of religion, wouldn't we just logically assume that the extrapolation from life here to the statistical base, you know, that we will find it everywhere.

SASSELOV: Yeah, I would say microbial life — that is, the complex chemistry of that sort — is very likely, and the more important thing is that we'll have some evidence to say something intelligent about it, rather than just saying it's very likely.

DYSON: Yeah, it could get stuck in any of these phases — I think the phase where you have to invent ribosomes is probably the one you're most likely to get stuck at.

LLOYD: Though it seemed to have happened relatively rapidly on Earth.

SASSELOV: My big question to all of you here is, can we do it by exclusion? Can we develop again this parameter phase of chemistry to such a completeness where I can look at these 50 planets eight years from now, and say, well, I know why all of these have what they have on their surface and atmosphere, but this one has really none of that - it's out of equilibrium, and it cannot be explained simply by physics and chemistry — it must be something which is more complex and is potentially life.

VENTER: Ken Nelson, who is head of the Mars Sample Return had to think of some of these issues a lot and he said the number one thing to look for is the phosphate bond.  That's the single greatest signal for biological life as we know it.

DYSON: In looking for life as we know it, or perhaps life is as we don't know it.

VENTER: Might as well start with what we know.

CHURCH: How easy would it be to detect the phosphate bond?

SASSELOV: That would be very difficult. I was thinking the other way around — we understand physics quite a bit, chemistry I hope enough, and so if we say we understand chemistry and physics, and this is neither physics nor chemistry that we see there, we've got biology.

SHAPIRO: The trouble is we're looking for a separate origin, this one has the great philosophical impact — if we discovered that life was on Mars but just a spillover from Earth it would be a curiosity, but it would not turn our view of the universe on its head. On the other hand if we discovered a life that's different enough that it couldn't have originated here, the spread would really validate what he's been saying - that I've been saying — that life is inherent in the universe.

VENTER: Well the two aren't incompatible, it could be identical to what we have here everywhere, the same chemistry, and we find it everywhere.

CHURCH: But he's just saying it's hard to prove that.

SHAPIRO: Hard to prove — a muddled case.

CHURCH: Well, it's not a theoretical argument; you either find it or you don't.

LLOYD: Well that's what's so upsetting about this work. Of course saying 'oh look, there's something we don't understand; must be life' is perhaps not the most compelling argument in the world. But if there is something weird going on, and it isn't explained by any of the models of life that we have already. Then that would be very interesting.

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