Transcript for Sara Seager on Exoplanets

Sara Seager:  Let's talk about evidence for life on another planet.

Jim Fleming:  That's Sara Seager, one of the pioneers in the hunt for life beyond earth.  She's an astrophysicist and planetary scientists at MIT where she works on exoplants, planets that orbit stars other than our sun.  In the past year along astronomers have found dozens, maybe hundreds, of new exoplanets.  Seager is searching them for bio-signatures, evidence of life.  Steve Paulson asked how she'll find it.

Seager:  We're going to find it by what we call remote sensing.  We will use space telescopes to look at atmospheres of other planets beyond our solar system.  And in those atmospheres are gases so our goal is to find out which gases are in an exoplanet atmosphere.  On our own planet some of the gases in our atmosphere are produced by life and we'll hope to be able to attribute some gases on another planet to life.  When can this happen?  In the most optimistic view if we are really lucky and planets that can support life are everywhere and the ones that do support life generates a signature we can detect then we may be able to find signs of life on an exoplanet before this decade is out.

Steve Paulson:  So when you say a gas you don't expect, explain that.  So what are the gases that you would expect from a lifeless planet and what might be the unexpected gasses?

Seager:  The best example we have is our own atmosphere, earth, where we have oxygen that fills our atmosphere to 20% by volume.  20% - that's a large fraction for a gas that if we didn't have plants and other photosynthetic life should be miniscule, almost undetectable.

Paulson:  So really the only way you can explain so much oxygen in the earth's atmosphere is that there is this life on the surface and it is excreting oxygen I guess you would say?

Seager:  Yes, our life on our planet is excreting oxygen.

Paulson:  Now you mentioned methane as another unusual gas.  Why would methane be found?

Seager:  Well, methane on our own planet is produced by some bacteria called methanogens and these methanogens live in environments without oxygen.  They take hydrogen, that's molecular hydrogen, and carbon dioxide and they actually extract energy from the reaction between hydrogen and carbon dioxide and one of the byproduct gasses, you would call it excreted gas, is methane and it is a sign of life.  But let's actually take a bigger look for a second.  One thing I love to hate is terra-centrism.  We're great at coming up with what life does on earth and thinking about other planets that way.  We're not so good at coming up gases that don't belong that are not already attributed to life on earth.

Paulson:  Now I suppose one of the great mysteries here which gets to the central question of whether there is life elsewhere is that we still don't have a clue how life originally started on earth.  I mean how did we get from non-life to life?  How did those non-life particles turn into the first living cells?  Is that a basic stumbling block?

Seager:  This is one of the biggest questions of all time.  However, as an astrophysicist I get to just forget about that question because it's really great.  What we do is we don't worry about how life came to be, we don't worry about what life is, we just focus on what life does.  And life on our own earth puts out chemical byproducts, some of which will go into the atmosphere in enough quantity to be detectable remotely. So we focus in the search for exoplanets and the search for remote signs of life on what life does. Life metabolize, life puts out gases, some of those gases are bio-signatures.

Paulson:  So it sounds like your educated guess is that there is probably a lot of life elsewhere because it would seem that there would be these kinds of planets that would be conducive to life.

Seager:  Let's put it this way - in our own galaxy alone there are 100 billion stars.  In our own universe we believe there are upwards of 100 billion galaxies and planets are everywhere.  The chance that life has evolved on another planet somewhere else has to be 100%.

Paulson:  Wow, okay.  100%?  So you're sure there is life elsewhere?

Seager:  I'm sure there is life.  There are 100 billion stars.  There are 100 billion galaxies.

Paulson:  How many planets have we found so far?  I mean planets outside our own solar system.

Seager:  I'm actually looking it up because you know what, it changes.  Did you know it changed by almost a factor of two recently?

Paulson:  Wow.

Seager:  Right now we know of over 700 planets around other stars.  The number of planet candidates out there is over 2,000.

Paulson:  And how many of these planets would seem to be earth like planets?

Seager:  As of yet we have no truly earth like planets.  I want you to understand how hard it is to find an earth.  Earth is so small, so dim, and so less massive than our sun that it's extremely difficult to detect earth.  Most of the planets that have been found so far are either much closer to their star than earth is to its sun or much bigger than earth.  But we're on the verge.  You know every year more and more planets are found of smaller and smaller size or smaller and smaller mass.  So we're pushing down, we're working hard.  It's like an archaeological dig.  You go deeper and deeper.  But we're pushing the planet finding techniques to more and higher and higher precision so it's really just a matter of time before we find those earth size or earth mass planets out there.

Paulson:  Suppose you were to find an earth like planet and all the indications are that there are these unexpected gases, let's say oxygen, maybe methane.  Will you ever be able to say definitely there is life there?

Seager:  It's a touch question to answer - will we be definitively able to say if there's life in that planet.  In the short term we will never be 100% sure.  We could be 99% sure in some cases, 99.5% sure even in other cases.  In some cases we may only be 50% sure but there are certainly techniques that in the future could be used to identify life basically to the 99.9% level but we'd need very large, very capable space telescopes.

Paulson:  You know, I could imagine that you might have two very different reactions to the discovery of life on another planet.  And let's just sort of for the sake of argument here say that if you're 95% sure, 99% sure, that there is life there on the hand I'm sure you would be ecstatic to find such evidence.  And yet, I would think you might also be very frustrated because could you ever actually figure out what kind of life that might be?

Seager:  We're getting into a generational question here.  We're just starting out on a very, very long journey for humanity.  The goal of my generation is to first find out whether or not earth like planets exist and whether or not they are common.  We hope to be able to find signs of life and identify them.  We will have to leave it to future generations, perhaps 100s of years from now, who will find out whether or not life is really there.  And the only way to be sure is to send a probe that is a spacecraft out to another star system and to look more closely at that planet.

Paulson:  But that would take years and years to send a probe all the way to another planet in another solar system wouldn't it?

Seager:  It would take years and years but what is so fascinating is that for the first time a group of people got together to discuss this question seriously and a lot of people still think it's fringe but you have to understand that I have a series of questions I get asked most often from people of every kind of walk of life - children, other professors, other scientists, people I meet on the airplane, journalists - and out of those questions the one I get asked most often is can we go there.  And no matter how carefully I explain that we have no way to go to another planet beyond our solar system now I'll still get asked it again and again and again.

Paulson:  Okay, and I'm another one asking that question.

Seager:  Well, I take this as a sign that our deep desire to travel in space will someday be realized.  So DARPA, a government agency, has sponsored concept studies in how would we do this.  Right now the most bold engineers think there is a way to go, would be a way to eventually get to 10% the speed of light or more.  So if we imagine that Alpha Centauri, our nearest sun like star system has a planet like earth, Alpha Centauri is about 4.2 light years away.  If you could travel at 1/10th of speed of light, that's 40 years.  You could imagine for a future generation that's not that long.  I mean someone could launch a probe and then 40 years later we'll get information.  And to answer one of the most important questions that is ever come across humanity it's not that long to wait.

Paulson: And what motivates you now?  I mean why do you want to do this work?  Why do you want to find evidence for life elsewhere?

Seager:  Yeah, you know I can't really answer that question because I just don't have a great answer.  It's like why does a child learn to walk?  Why do children learn to talk?  They don't actually know why they do it they're just hard wired or compelled to do it. I can't really tell you why I get up everyday excited to go to work to figure something out.  But I came from a slightly different background where I'll say in my past before I had a job I was a very adventurous person.  I would spend one or two months of summer in Canada in very remote places with one other person and the sort of adventurous exploration feeling people still try to do that on earth.  But let's face it - most of earth is explored.  It's mapped.  We know about it and you can do an extreme adventure just for the sake of pushing human limits.  But here we're the new generation of explorers.  We're setting it up to find the planets around nearby stars so that if they find a way of how to do this, future generations of explorers can actually go out to the other planetary systems.



Comments for this interview

Response to yours (Prof. Seager, 02/14/2012 - 8:05am)

My belief in life elsewhere, indeed is a personal opinion, and should have been preceded by such a statement. If people on Earth do not understand the origin of life, that doesn't mean it cannot happen elsewhere.

The book "Rare Earth" is decidedly pessimistic. For a more balanced view, and one I agree with, see this review of the book. ""

Prof. Seager

extraterrestrial life. (James Pawley, 02/12/2012 - 3:14pm)

Dear Prof. Seager,

I was appalled by your casual assumption of life elsewhere. 100 billion stars in a 100 billion galaxies may indeed be a large number. But that is no reason at all to assume that "life" will have occurred elsewhere. It is less than the number of molecules in a can of soup. You give the impression of having assumed this is true, therefore it must be true. Have you read Rare Earth? And they assume life emerges, at least at the level of bacteria. No basis for doing this. Like many physicists, you seem to think that life is far simpler than it is. Usually by are happy to contemplate "replicators" of some sort. But that isn't half of the problem. To form bods that won't break at RT, you need to be able to break the bond on one compound and transfer the energy to form an almost equally energetic bond between two totally different molecules. I agree, that somehow molecules shaking at 10*13/s for billions of years, "learned" this trick. But neither you nor I have any basis for assuming that this has happened elsewhere. I feel that your careless assumption that life is common is more likely to be seen as a grab for research funds. This not only brings science into disrepute, but it diverts resources from the real problem of humans getting through the next 50 years without crashing the atmosphere, the climate and civilization.
Please restrain your enthusiasm to those things about which you have some knowledge. Intriguing though it may be, the origin of life is not one of these things.

I look forward to your reply.

Prof. James Pawley