Searching for Earth 2.0

One in every five sunlike stars in the Milky Way may have an Earth-sized planet circling it in the Goldilocks zone—the sweet spot where liquid water could exist. That's according to a new analysis of data from the Kepler spacecraft. Sara Seager, an exoplanet hunter at MIT, talks about what's next in the hunt for Earth 2.0.

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IRA FLATOW, HOST:

This is SCIENCE FRIDAY. I'm Ira Flatow. It's a question astronomers have been wondering about for centuries. Are we alone in the universe? Is there another planet like ours out there? Go back to the year 1610, for example. Johannes Kepler, who has actually a planet-hunting telescope named after him, wrote: We deduce with the highest degree of probability that Jupiter is inhabited.

Early astronomers focused their telescopes on our solar system, searching for life, but modern astrophysicists are looking beyond our immediate neighborhood and have found thousands of exoplanets, and last week they announced that one in five stars like our sun, one in five, might have an Earth-like planet flying around it. That's billions of planets.

My next guest shines in her own circle of planetary scientists. She's an exoplanet hunter of the first degree. Dr. Sara Seager is a 2013 McArthur Fellow, also professor of planetary science and physics at MIT. She joins us from the jet propulsion laboratory in Pasadena today. Welcome back to SCIENCE FRIDAY, Dr. Seager.

DR. SARA SEAGER: Hi, it's great to be back.

FLATOW: What are you doing out there at JPL?

SEAGER: Well, I'm doing two things, and they're at opposite extremes. The first thing is I was here to talk about my project that JPL is sponsoring together with MIT and Draper Lab, and that's the tiny space telescope. It would fit in something about the size of a loaf of bread. It's called a CubeSat and we hope to change the paradigm of how some space science is done by instead of launching one big telescope, we'll launch dozens of little telescopes.

FLATOW: Like little nano-bots.

SEAGER: Yeah, they're nano - essentially like nano satellites.

FLATOW: Oh.

SEAGER: And the second reason I'm out here, the main reason I'm here, actually, is so related to our topic today, and I'm out here on one of a NASA sponsored study to study how do we actually identify an Earth. How do we actually indentify a planet that could be like Earth instead of just finding planets that are Earth size?

FLATOW: Let me get to that in a minute. I want to back up for a second and have you tell us more about the news announced last week that I sort of hinted at. What did they actually find there?

SEAGER: Well, the news from last week, as you said in your introduction, is that about one in five stars like the sun might have a planet like Earth and more specifically it's that one in five stars like the sun have an Earth-sized planet in the so-called Goldilocks zone or habitable zone. But Ira, there are just so many caveats to that statement, it would take us the entire time just to flesh it out.

FLATOW: But there got to be a lot of planets, possible candidates out there, is what you're saying.

SEAGER: Yes. I mean, all evidence points to the fact that small planets are extremely common. They are literally everywhere.

FLATOW: Uh-huh. And is that something new that, you know, in our vocabulary that these are common?

SEAGER: Well, we've been marching towards this statement for quite a while, actually. Different planet-finding techniques have tried to say something similar, but with larger planets. Kepler itself made a similar statement, but with planets that were just on very short period orbits. They were to hot to be in the so-called habitable zone. So we've been marching towards this, but for scientists, seeing is believing, and so we have to keep going and get a more and more precise number.

FLATOW: Where does this leave us then in our count of exoplanets discovered so far? How many? Are they in the thousands?

SEAGER: Well, it depends how you count them. We have about 1,000 exoplanets that we call confirmed exoplanets. We're basically 100 percent sure that they are planets. Then we have another few thousand planet candidates where we couldn't be 100 percent sure they're planets, but we're confident that almost all of them are. So we could put that number at 4,000. However, I think the better statement for everyone is we're confident that every single star in our Milky Way Galaxy has at least one planet.

FLATOW: And how will we know that it has any kind of life on it?

SEAGER: Well, to do that, we actually need to do a new kind of space telescope, one that can actually look at the planet atmosphere. That's our best way, as astronomers, to tell whether a planet has life on it; that is, to look for gases in the atmosphere that don't belong, that could be ascribed to life.

FLATOW: Any of your nano satellites do that?

SEAGER: No, actually. You know, we sort of have this thing in exoplanets where we're sort of joking but sort of serious. We put a dollar per value, so if you want to find a Jupiter, it's not too expensive. If you want to find an Earth, I mean, Kepler was 600 million. If you want to find life, you know, we're talking about at least a billion.

FLATOW: A billion bucks to find life.

SEAGER: Yeah, if you're lucky.

FLATOW: Well, that may be part of an aircraft carrier.

SEAGER: Yeah.

FLATOW: But the Kepler space telescope, which we've been using to find these, as you mentioned, it's out of commission. Is that going to affect the search?

SEAGER: Well, here's the thing. Kepler's goal was to tell us how common Earth-sized planets and Earth-like orbits about sun-like stars are. Kepler wasn't designed to follow up a planet and look at its atmosphere and tell us whether life is there. Kepler was here to do essentially a census. It's like the census of the United States, when someone comes knocking on your door and wants to know how many people live there.

And you use that information for other purposes. So from my viewpoint, Kepler's numbers are telling us that we can go forward and we can build the sophisticated space telescope needed to find out whether or not life is on an exoplanet.

FLATOW: What about the exoplanetary moons? You know, we talk about here, one of the best places to search might be one of the moons of Saturn, like Europa. Would it be important or desirable to look for moons of these other exoplanets?

SEAGER: Well, funny enough, moons are pretty much one of the last items on our wish list of exoplanet discoveries that we actually haven't found yet. But before we favored moons of exoplanets because we didn't think we could get down to Earths, and now we're confident that, you know, time has passed, technology's maturing, that we think we can get to the planets themselves, and so moons aren't really necessary.

And being right near a giant planet, they may be problematic.

FLATOW: All right. So you're out there looking for Earths. What are you looking for? What do you look for?

SEAGER: Well, you know, people are debating this hotly right now because what we found from exoplanets is that the norm just doesn't exist. Exoplanets exist of all masses and sizes and orbits and there's fights going on in the community of what really we are looking for. We could just look for an Earth twin, but then we may be shortchanging ourselves, 'cause the Earth twins may be rare, yet other types of planets that are habitable may be common.

Everybody does agree on one thing, though, and that is we want to find a planet with liquid water at the surface. All life on Earth needs liquid water and although one might argue that life needs a liquid, not necessarily liquid water, water is our most abundant planetary material. So that's the first thing we want to do, is find a planet that we can see signs of liquid water at the surface.

FLATOW: All right. I'm going to give you the SCIENCE FRIDAY blank check question. Everybody loves this question, and that is, if I gave you a blank check for as much money as you want to do what you'd like with it, to find the goals that you would like to have, how much would you need and what would you do with it?

SEAGER: Well, I actually have a very precise answer for you.

FLATOW: Oh, good.

SEAGER: I could give an answer that's off the chart, and then I could give one that's actually more reasonable in our future.

FLATOW: Let's be practical.

SEAGER: Okay. Well, we will be practical then. And what I'll tell you is - you want to know why I know like the exact answer to this?

FLATOW: Yeah.

SEAGER: Because right now...

FLATOW: You're writing a budget someplace.

SEAGER: Well, I'm actually chairing a committee sponsored by NASA. It's called a Science and Technology Definition Team. There's actually two parallel teams. The one I'm chairing is about, what we call direct imaging with the star shade - a big, big, 30 meter shade that would go up in space and fly tens of thousands of kilometers from a telescope and it would search nearby stars for - directly for an Earth and it would be able to get the spectrum and see if there are signs of life there.

So we've done - gone through costing exercises so far and the team at JPL has done a great job designing this. Now, we're tasked by NASA to come up with a concept under $1 billion, and because of all sorts of overheads, and they want to have extra reserves 'cause things always go over budget, it doesn't really leave us with a very big telescope.

We'd be putting a one-meter telescope - 1.1 meter telescope in space. So to do this right - and you know, while I'm talking to you, my team is actually still hashing out right in real time, right now...

FLATOW: Wow.

SEAGER: ...you know, which stars we're going to look at and what our priority is and do we revisit the stars or do we just hope we get lucky and see a planet, then get a spectrum, 'cause it'll take weeks to get a spectrum. So I know the costs I would ask you. We're asked to do it by under a billion for NASA. We don't think that's going to get what we really want, so I would say that if you were to give me the blank check...

FLATOW: You've got it.

SEAGER: ...for $3 billion we will nail this problem.

FLATOW: Wow. That's not a whole lot.

SEAGER: I can't guarantee that we'll find life, but I know that we'll find a bunch of Earths, just based on the Kepler results. And if life is ubiquitous, we'll find some signs of it.

FLATOW: Of course, (unintelligible) in San Francisco's written in that Europa is a moon of Jupiter and not Saturn-like I said it was before. Thank you for being kind enough not to correct me on the air. 1-800-989-8255 is our number. Does this have support in Congress? I mean, that's where your money's eventually going to come from. Are Congress people excited about a search for life, or do they just think it's something that's not - should not be part of your mission? Like some people don't think we should be going to asteroids and coming back.

SEAGER: Yeah, no. The asteroid thing we can talk about. That's a separate issue. Right now people, they don't want to capture an asteroid and bring it back near Earth.

FLATOW: Right.

SEAGER: But in general, I think that - I can't say for sure yet if congress supports this search, but I think the world is getting excited about this. It's not just people calling into your show or writing to me and my colleagues, but it's just sort of around the world, the people knowing that something's going to happen sometimes; they just don't know when. So I'd say that I would have to guess that, yes, people are interested and supportive.

FLATOW: What kinds of signs of life do you look for? Do you look for things like we're looking for on Mars or do you look for something else?

SEAGER: Well, Mars is a little different because you can go to Mars and you can scoop up material and ultimately look for DNA that's probably too terracentric, too Earth-specific, or you can look for amino acids or other things. All we can do is look remotely for gases that don't belong. And there was or is a kind of controversy on Mars right now about methane on Mars. Methane is a gas that shouldn't be on Mars.

If it's there truly, it means either life is producing it or there is some very unusually geophysics going on, which both would be of great interest. But, you know, now that the Curiosity rover is there, it doesn't actually reproduce the measurements seen from afar. So it's similar in some ways and different in others. Essentially, we're looking for gases that don't belong.

But, you know, you've asked what do we look for? And that is a huge, huge challenge. We start out by thinking, well, let's look for gases we have on Earth, and our best biosignature gas here is oxygen. On Earth, oxygen fills our atmosphere to 20 percent by volume, yet if we didn't have the photosynthetic life - that's plants and photosynthetic bacteria producing oxygen - we would have virtually zero.

So oxygen is a no-brainer for us; however, it's pretty specific, you know. We didn't get oxygen until a couple billion years ago and it took a while for it to accumulate, so we do a lot of research on what to look for. I could get into details, but essentially we're looking for gas...

FLATOW: Yeah, give us a little detail. We like detail.

SEAGER: OK. Well, there's no solid answer right now, so the sort of simplest thing is to start with what are earth's dominant biosignature gases? And we have oxygen, we have other gases like methane, but some gases like methane are ambiguous. They could be produced by life, they could be produced just by, you know, coming out of earth's crust.

But we can do down the list and there's other gases. They may sound obscure to you, but we just sort of take the inventory on earth and one of the ones that's been recently favored is DMS - dimethyl sulfide. This a gas produced by phytoplankton in the ocean. No one really knows why, like when they're under stress of they have some reason for producing it.

And it actually accumulates in our atmosphere and for a while it played a role in - people thought it did. It's now disregarded in global warming in terms of seeding clouds. Sulfur would go into the atmosphere and seed clouds. So dimethyl sulfide, people have talked about methyl chloride. So there's a variety of things that life on Earth produces, and people run simulations and models to say, hey, what if this gas was an exoplanet of a different kind than Earth with different atmosphere properties. Would it accumulate and survive?

FLATOW: Yeah.

SEAGER: But you know what? I found looking into this a question asked by a friend was what about Earth's atmosphere? It turns out that almost every gas that exists in part per trillion by volume or more is produced by life.

FLATOW: All right.

SEAGER: Most of those gases exist some other way as well.

FLATOW: All right. Dr. Sara Seager will return after our break. Stay with us. We'll be right back. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

(SOUNDBITE OF MUSIC)

FLATOW: I'm Ira Flatow. You're listening to SCIENCE FRIDAY, and we're talking this hour about the hunt for exoplanets; what we've found so far, what we're looking for, how will we know when we find it? Sara Seager is a 2013 MacArthur Fellow. She's also professor of planetary science and physics at MIT in Cambridge. Our number is 1-800-989-8255.

Lots of people on the phone. Laura. Laura in Antioch, Illinois. Hi.

LAURA: Hi. You can call them exoplanets and maybe people will understand it, but universally please refer to them as Class M planets. They're Class M. Everybody knows what that means, although I thought it was kind of odd in the dialogue over and over. It'd be like, you know, economists driving around and every time they pass a bank they say, "A bank where people keep their money."

Every time they're Class M planets, much clearer and thank you for your work.

FLATOW: That's Star Trek.

LAURA: Both of you.

FLATOW: That's Star Trek that you're talking about, right?

LAURA: Of course.

FLATOW: Yeah, Class M planets.

LAURA: (Hums theme for Star Trek) right, Star Trek. Class M planets. Everyone's heard of them.

FLATOW: Okay, Laura. Thanks for filling...

LAURA: I've been inside too long.

(LAUGHTER)

LAURA: OK, all right. Bye.

FLATOW: Thanks. Have a good weekend. Well, but Sara, think about it. If you start using that kind of terminology, might not people relate to it a little more, get a little more excited?

SEAGER: Yeah, sure, some people. The Star Trek fans will definitely relate to it more. We just still prefer exoplanets, planets outside our solar system, planets orbiting stars other than the sun.

FLATOW: Who invented that terminology, exoplanet?

SEAGER: Well, you know, we had two terminology, two words that were common: Extrasolar planet and exoplanet. But we go for the simpler term, exoplanet, just like exoskeleton. I don't know who invented it.

FLATOW: Let's go to - more people on the phone. Joe in Harpers Ferry, Iowa. Hi Joe.

JOE: Hi. When you see a transit, I assume we're looking into the galactic plain, or the solar plain, and I'm curious. Are all systems potentially oriented towards the galactic spin, or are there different orbital mechanics in different systems that we might be looking into the bottom of and not see a transit?

FLATOW: Hum, yeah.

SEAGER: Yeah, Joe's asking...

FLATOW: In other words, if you're looking up from underneath it, and it doesn't go across it, you won't see it.

JOE: Exactly.

FLATOW: Yeah. Good question.

SEAGER: I'll make a couple of comments here. There's many ways to find exoplanets. One of the more popular ones right now, the one that the recent studies last week by the Kepler Space Telescope referred to our transiting planets, planets that go in front of their stars seen from our telescope. We don't think that stars have any preferential - that their axis points in any direction. It's just random, 'cause the way that stars form.

And yes, planets and stars have to be aligned just right for us to see the transit. And however, some planets, you know, they're not orbiting in their star's equatorial plane. They're actually going over the pole in a couple cases; some planets are orbiting backwards, so we've actually - those are just a subset of planets that are very close to the star. But we've seen all sorts of things.

FLATOW: Have all the planets we've discovered changed the way we think about our solar system or what's normal in space?

SEAGER: Absolutely. Absolutely. We thought before that all planetary systems would be like our own and it turns out we have yet to find a solar system copy. We think now that out of sun-like stars, our solar system is rare. It couldn't be more than a 10 percent common, maybe twenty percent maximum.

FLATOW: What about the thing we read a lot about in science fiction that we are a carbon-based planet, right. Could there be something that's not carbon-based putting out some other kind of gas that we're not detecting?

SEAGER: OK, yeah. Well, first of all, our capability isn't quite there to detect gases produced by life yet. That's sort of in the future. As to what life is, what life is made of, people love to speculate on other, you know, life forms. But in astronomy, it's sort of easy for us to answer that question 'cause we can never answer that question.

All we can see is what live produces and life produces such a huge array of molecules. We don't really map that onto what life is made of. We just say what life does, like metabolizes and life will produce gases that accumulate in the atmosphere, and that's where we stop.

FLATOW: Paul in Sun Valley, Idaho. Welcome to SCIENCE FRIDAY.

PAUL: Hey, thank you. Great topic. You know, I'm wondering when the technology is there and when we do develop it, and we do find gases, what exactly are we going to prove that that was created by an terrestrial life?

FLATOW: Um-hum.

SEAGER: Hmm. Well, when we find a sign of life, and if we can be confident that it's produced by life, what will it prove? You know, that's a good question. I mean, what we'd like to know is are we alone? And although that won't prove that intelligent life produced that gas, it could have been some kind of simple-celled, simple single-celled bacteria, it'll give us a start on answering the question about whether or not there's other life or other intelligent life in the galaxy.

FLATOW: You know, we've been finding some of these planets that are within, you know, let's say 20 light years of Earth or pretty close. Could we actually carry on a conversation with some of the - if there was intelligent life on some of these planets - within someone's lifetime reasonably?

SEAGER: Well, certainly. The SETI, Search for Extraterrestrial Intelligence, they've now turned to listen towards those star systems that have planets at all in planets inhabitable zone. So one could conceive of carrying on a conversation. It'd be a slow conversation. You know, electromagnetic waves radio signals would travel at the speed of light. So imagine that that system was 10 light years away. That's 10 years to send your message, and if they write back right away 10 years to receive. But certainly it's a possibility.

FLATOW: What's most, I guess, talking to you for so many years and following your career, we can see how intensely involved and fascinated you are about this. What is it in you that is so fascinated about studying these exoplanets?

SEAGER: I know that's - I get asked that question a lot and I've yet to have a great answer to it.

(LAUGHTER)

SEAGER: You know, sometimes I'm like, well, why does a child learn to walk? Why does a child learn to talk? You just feel driven to a certain task and accomplishment. But I'll say one thing, and that is it's a special time for us here on Earth. It's the first time in human history that we actually have the technological capability to find other Earths; to identify them as Earths and to hope for look for signs of life on the first handful or the first dozens or hundreds of exoplanets.

And that's a special time in history, so I'm trying to do what I can to make sure that that happens now.

FLATOW: Is it possible, and I know people have asked this of me all the time, and I'm going to get a real answer from you, is it possible to send a space probe if you find one of these planets, to investigate, like we would send something to Mars?

SEAGER: Yeah, you know what I love about that question? That is the question I get asked most often. Out of all the questions about exoplanets and alien life, that's the favorite question. So I'd like to be positive. I'd like to say, yes, some day. I don't know when that day will be. I know that people now are thinking about that already. They're thinking there are ways to eventually travel at a tenth the speed of light.

And so if a planet system is ten light years away, that's still a hundred years to get there. That wasn't including slowing down and speeding up. So this would be an effort that would take many generations and we may send probes without people. But that is one of the reasons I'm committed to this topic because I do believe that hundreds or a thousand years from now people will figure out how to do this, and that they'll look back at us collectively as those people who first found the Earth-like worlds.

FLATOW: Do you think that you'll find visible evidence from your kind of work before we might find, I don't know, radio evidence or signals, things like that, of other life on other planets somehow?

SEAGER: Well, Ira, now you're getting to things that are speculation rather than any science and, you know, as a scientist I'm really reluctant to speculate. But I'll do it for you.

FLATOW: Thank you, thank you.

SEAGER: So I think that my personal opinion is that intelligent life would be more rare than simple life, and I have no way to back that up, and the biologists don't even like it when astronomers talk about the probability for life elsewhere. So given that in our lifetime we can access, you know, hundreds of planets if we're lucky, I think we have more of a chance to find some kind of simple life rather than get a message from an alien.

FLATOW: Is the Drake Equation still working? Is it still operative?

SEAGER: Well, the Drake Equation is still operative, but you know what? I did a takeoff on the Drake Equation. I call it the Revised Drake Equation and now people call it the Seager Equation because I think our focus has shifted.

FLATOW: Yeah.

SEAGER: So before Drake wanted to know, you know, what is the chance that there are alien civilizations out there, whereas he wanted to know if they're are radio signaling alien civilizations out there, whereas I want to answer a question we have actually able to answer in the near future. And that is, you know, what is our chance of finding a planet with a sign of life or how common are life - what is the likelihood that we can find a planet with signs of life on it?

FLATOW: And how...

SEAGER: That approach is still relevant.

FLATOW: So how did you alter that equation, or did you just rewrite your own equation for that?

SEAGER: I pretty much rewrote my own, but I really did copy his format because he has several terms and some of the terms are quantitative. You can assign a number to it, like how many planets are out there? And other terms are more speculative, like what is the chance that life arose on that planet. So, is similar, it just has the different focus.

FLATOW: Did you come up with a number for the chance or the possibility?

SEAGER: Well, the thing is you never can come up with a number. That's part of the point. The equations are illustrative. They're not really predictive. And the only reason I did it was - I actually did it for another astronomer's birthday party, and it was like a fun thing to present to my friends, But what we did, you know, for here when we have our birthday parties, we have symposia.

(LAUGHTER)

SEAGER: We have like a daylong series of talks by our friends and colleagues. But the goal was to tell the world that the real search for alien life is ongoing right now, and it's not necessarily the search for intelligent life, but it's the search for signs of life around planets or orbiting stars other than the Sun.

FLATOW: And it's not science fiction.

SEAGER: No. But here's what I did come up with though. It was a little contrived for kind of a specific reason. Because at MIT we're building a spacecraft called TESS - Transiting Extrasolar Planet Survey Satellite. And this is kind of a follow-on to Kepler. It's sponsored by NASA. It's a $200 million mission that will launch in 2017.

Now, TESS will find a bunch of big Earths, or Earths or big Earths, transiting small stars. And those systems, although they're not Earth analogues, will actually follow up their atmospheres with a telescope called the James Webb Space Telescope, which is to launch in 2018. So if we're - so my equation was actually designed for that TESS/James Webb system to say what is the chance - or how many planets will be accessible to us with signs of life in the next decade?

And so if everything was in our favor and we would have to - literally it would be like winning the lottery five times, but if every - we know how common these Earths are and habitable zones around small stars. If every one of them had life and if half of that life produced a gas we could detect, our chance of finding it is actually 100 percent. I mean, there's a lot of caveats there. But if every planet has life, every planet in the habitable zone has life, and if that life - half of it produces a sign we could detect, then there's probably about two planets that we're going to be able to find some kind of sign of life on.

FLATOW: Wow. We're standing by. Thank you, Dr. Seager, for taking time to be with us today.

SEAGER: Thanks for having me again.

FLATOW: And good luck to you and congratulations on the Genius award.

SEAGER: Thanks, Ira.

FLATOW: You're welcome. Sara Seager is a 2013 MacArthur fellow. She's also a professor of planetary science and physics at MIT in Cambridge.

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