An extract from a presentation by an astrophysicist to a fascinating interdisiplinary Edge symposium held in 2007 concerning the origin and nature of life, which is so filled with scientific facts, hypotheses, and theories that I recommend reading all of it for a sense of where -- and how and what -- we are.
Dimitar Sasselov—LIFE: WHAT A CONCEPT! | Edge.org
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 [in terms of] 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."
. . . . . (continued at the link above)