Fifteen years ago, two Swiss astronomers discovered a planet orbiting the sunlike star 51 Pegasi. Until then, nobody had known if our solar system was unique; now we have a catalog of more than 500 extrasolar worlds.
We still have no idea whether any planet beyond Earth harbors life, but that could soon change too: Scientists are increasingly optimistic that they will find evidence of biological activity on an alien planet within the next few years. In collaboration with the Thirty Meter Telescope and the California Institute of Technology, DISCOVER invited four top researchers in the field to discuss how that extraordinary discovery might unfold.
Gibor Basri is an astrophysicist at the University of California, Berkeley, who studies stars that have planetary systems. John A. Johnson, an astronomer at Caltech, searches for and characterizes planets around other stars. Sara Seager is a planetary scientist and astrophysicist at MIT whose research focuses on understanding the atmospheres and interiors of exoplanets. Tori Hoehler, an astrobiologist at NASA’s Ames Research Center, studies how living things on Earth create detectable changes in their environments. The conversation took place at Caltech and was moderated by DISCOVER’s “Bad Astronomy” blogger, Phil Plait.
What is most exciting to you about the recent discoveries in astrobiology? How are they changing the way we think about our place in the universe?
Gibor Basri:I’ve been teaching astronomy for 28 years. Early on, I always had to say to the class, “It’s probably true that planets around other stars are out there, but we really don’t know.” It was amazing to be able to stop saying that and start saying, “We know there are planets out there, and we’re finding more and more of them.” That was a real breakthrough.
Tori Hoehler: For me, one part of it has been the discovery of this absolute zoo of weird planets out there. You really have to stretch the way you think about what life is and what it could tolerate and where it could be, and then try to place it in the context of all these strange, different worlds out there. There are also the questions that have been pondered for such a long time: “Are we alone? How commonly does life arise?” It’s not something we can constrain based on our one example of life here, but now we have the potential to get some valid statistics on that—to look at places where we think life could arise and get some evidence as to how often that happens. Those kinds of observations are a long ways off, but the potential is there, and exoplanet research is what got it started.
What is the next breakthrough for studying planets around other stars?
Basri: The big news is the Kepler Mission [NASA’s search for Earth-size exoplanets, launched in March 2009], which is gathering data right now and has been for more than a year. This is the first time humanity has been able to seriously search for terrestrial planets around other stars. And we hope it will tell us about the frequency of terrestrial planets in our galaxy or in the universe. That’s a major piece of this puzzle. Kepler is basically a giant, 98-megapixel digital camera. It looks for planets using the transit method: You just wait for a planet to cross in front of its star as it’s orbiting. When it does that, the planet blocks a little bit of light from the star. The camera will measure a dip in the star’s brightness, and if a planet is really what’s causing that dip, it will come around and cause the same kind of dip again and again. That’s the essence of the mission. Kepler has already found a lot of potentially interesting things, but it’s easy to be fooled. For example, stars crossing in front of other stars can also cause dips in the signal. So right now the project is sifting through 700 of these potential discoveries, trying to figure out which of them are actually planets. But I think it’s safe to say that terrestrial planets will be announced within the next year.
Sara Seager:We have to be careful about this. To astronomers, “terrestrial” only means rocky and roughly Earth-size; it doesn’t necessarily mean habitable. There are planets at all distances from stars. Earth is pretty far from the sun. The first Earth-size planets that Kepler finds will probably be very close to their star, so they will be very hot, probably too hot for complex molecules to exist and too hot for life. These probably won’t be much like Earth.
So when we find a rocky planet, it might be more like Venus—with a surface temperature of 900 degrees Fahrenheit—than like Earth. How do we take the next step and figure out whether any planets we find could support life?
Hoehler:There are two ways to think about habitability. One way helps to give us a sense of possibility for life in the universe. So when we’re surveying with something like Kepler, we should allow ourselves to think of habitability as broadly as possible, to think of life as being as capable as possible, and get some overall sense of how much habitable real estate might be out there. [In our own solar system, for example, it is possible that areas beneath the ice of Jupiter’s moon Europa or in the geysers of Saturn’s moon Enceladus might support some form of life.]
But if we want to begin to narrow down to a place we could actually search for life, then we ought to be fairly restrictive. In order to come up with an answer that’s going to convince a lot of people, we’ll need to find a place where we can widely agree, “Yeah, these are signatures of life as we fairly closely understand it.” So what we should do depends on the technology that is available to us at the time. If you have the technology, the next step is to try to say something about the atmosphere of the planet. That is fairly simple in principle. In practice, I think it’s going to be a difficult endeavor.
Seager: To elaborate on that, we want to look at an atmosphere and search for things that are unusual. Our own atmosphere is 20 percent oxygen by volume. If an alien civilization is looking at us from far away, and it knows something about chemistry, it will know that we have millions to billions of times more oxygen than we should [if there were no life on Earth]. It’s hard to come up with any other process that can produce that amount of oxygen, other than the activity of living things. So we’re looking for an atmosphere with chemicals in it that should not be in it by any stretch of the imagination.
Hoehler: It’s really an issue of degree, too. For a long time, there was the notion that finding oxygen and methane in a planetary atmosphere would be a smoking gun for life. But those things are present in small amounts on Mars, and nobody is proposing that there’s photosynthetic life on Mars. Maybe there are deep subsurface microbes producing methane, but maybe not. There are precious few chemical signatures we could look at as evidence of life, and in most cases other explanations exist for those observations. When we find something, we’ll have to ask, could this be made any way other than biologically? When we look at an atmosphere, what matters is this: If you took the entire atmosphere and reacted everything that can react, how much energy would come out? By that measure, Earth’s atmosphere and Mars’s atmosphere differ by about 60,000-fold or so. That’s the sort of characterization we need to be able to do.
Let’s say I handed you a check for $1 trillion to use in the hunt for life on other planets. What would you do with it?
John A. Johnson:My collaborators at Yale and Penn State and I have put in a request—not for $1 trillion, but for a substantial sum of money—to build the next generation of instruments to make the next big leap: finding truly Earth-like planets. With our current detection technologies, we’re finding these interesting things called super-Earths. These are about 5 to 10 times the mass of Earth, and we find them by looking at the gravitational wobble that the planet induces in the star. What we would like to do is move down to about three, two, and one Earth mass, and to do that you need to make sure the instrument you’re using to measure those wobbles is rock steady. We have the technology at hand to stabilize our instrumentation to get down to about three Earth masses for planets in the habitable zones around stars. With the next jump after that, we can push down to one Earth mass. We’re a check away from making that next step. If you handed me $1 trillion, I would build three of these new instruments and use them to find the closest, most interesting planets. And then I would hand off the rest of the money to NASA, which would need about one-thousandth of that to build satellites that could go out and take images of the planets to see if they really are habitable.
Seager:I’m glad you only want a few billion, because I can definitely use the rest of that! We think that every star has a planet. That’s basically what we’re seeing right now. Our nearest star, Alpha Centauri, is actually two stars, A and B. They’re sunlike stars. No one has found any big Jupiter-size planets around them; we’ve ruled that out. But maybe there is an Earth-size one. I would say for $1 trillion you could develop a way to travel at one-tenth the speed of light. Alpha Centauri is four light-years away, so at that speed, you could get there in 40 years. I would find a 20-year-old volunteer to go there and tell us what she sees. For a lot less than $1 trillion, with just a little more technology development, I think we could figure out how to tell the difference between a Venus-like planet and an Earth-like one. They’re about the same size and mass, but Venus is not habitable. We could build a special kind of space telescope to help us tell the difference. It would block out the light from the star so we could see the planets directly, look at their atmospheres for oxygen or ozone or other things that shouldn’t be there, and move forward that way.
Even without trillion-dollar funding, you are all making huge advances in our understanding of planets around other stars. Where do you see this work leading in the next decade or two?
Seager:About 10 years ago, I was giving a talk here at Caltech about atmospheres on other planets. I don’t think a single person in the audience believed we’d ever have any measurements. Ten years later, we now have measurements of more than three dozen exoplanet atmospheres. [Seager recently edited the first textbook on the subject, titled simply Exoplanet Atmospheres.] These are big, hot planets that almost certainly have no life. But we’re practicing, using the tools we’ve developed over the past decade to understand them. I think in 10 years we’ll have several examples of planets in habitable zones around small stars, and we’ll have data to work with to understand their atmospheres. John will find us a bunch of planets that we can follow up on. The planets won’t be just like Earth—they’ll be bigger, and orbiting smaller stars—but we’ll find them.
Eventually you might be able go out at night with your children or grandchildren and point to a bright star and say, “That star has a planet like Earth.” We need to look at the atmospheres to do that, and that’s what we’re planning to do. We may even find signs of life that soon, 10 years from now.
Johnson:A recent result that highlights the way surprises keep popping up is the discovery of a batch of planets that are orbiting in the wrong direction. All of the planets in our solar system orbit in the same direction, the direction in which our star spins. That reflects the way we think planets form, which is from a flattened disk of gas and dust around a star. We set out to make what are called spin-orbit measurements of other planets. At first it looked as though everything was aligned, like in our solar system. Then all of a sudden we found some tilted orbits, and then we found one planet going backward around its star. So I’m afraid to predict 10 years out; this kind of result shows that it’s almost impossible to predict. But everything leading up to the point where we can detect biosignatures on other planets is going to be exciting.
Basri:We’ve learned that we really don’t know what we’re talking about with respect to exoplanets: how they form, what their distributions are, anything! The very first exoplanet found was a complete surprise. It was a Jupiter-size planet in a really short orbit, which was utterly unexpected. But now we are right on the cusp of learning whether rocky terrestrial planets are a common thing in the universe. That will be a really interesting result, and it’s very exciting to be around when it’s happening.
Let’s say we actually find the smoking gun: definitive proof of life on an alien world. How should that announcement be handled?
Hoehler: The reality is that we’re just going to have gases in the atmosphere of some distant point of light. We won’t even have the gases, just little squiggles in a spectrum, so I don’t know that there will be a definitive “smoking gun.” If you heard an announcement that we’re about 95 percent sure that some planet seems to have a substantial amount of oxygen in the atmosphere, so life is probably there—I’d be blown away by that sort of thing. But for most people, would that be an astounding, satisfying result, or just kind of like, “Okay, that’s nice”
Johnson: The only truly definitive sign of life would be a SETI [Search for Extraterrestrial Intelligence] signal: a message that says, “Here we are!” If that happened, I don’t think you could sit on it. That would be exciting to everybody, and it would be widely influential. Absent that, there will be caveats, and people will react accordingly.
Audience member: It seems that we’re looking only for planets like Earth and life that’s like the life we know. Shouldn’t we broaden our perspective?
Johnson: We could be in the same situation as when we were first looking for planetary systems. We thought we would find nice, well-behaved Jupiters where they were supposed to be [far from their stars], but we found out that planets are all over the place. This is the problem of having a sample size of one. We could be in the same situation with life.
Hoehler:Philosophically, I absolutely agree with you. I want there to be life elsewhere, and I want it to be weirdly different from us. With that said, the more you look into the details, the more our kind of life, broadly categorized, seems to do a lot of things that would be difficult to do in other ways. But what’s significant is that there is virtually nothing that you would look for or detect that is specific to one kind of life–what it’s made of, what kind of biomolecules it has, that sort of thing. Instead, you look for what life is doing. Life must channel and use energy much more quickly than abiotic processes in order to be alive, and that’s what we’re looking for: something that markedly distinguishes life from nonlife by how much energy it seems to be using. Not just whether something is there, but how much of it is there. That’s the kind of signal we’re going to look for.
Basri:The thing I like about this view is that even robotic life would fit the bill. If robots are producing a lot of energy and using it, that would show up. So looking for energy is a more general approach.
Audience member: We’ve made our planet noisy with radio signals that could be detected light-years away. Other advanced civilizations might be doing the same thing. Are we looking for these signals?
Basri: Yes, SETI, which we mentioned earlier, is broader now, but it began with radio waves. People are looking for exactly what you’re talking about. The Allen Telescope Array at Berkeley is engaged in that; Paul Allen gave $25 million for that purpose. There are about 60 radio telescopes scanning the skies for these signals right now. That would be the definitive answer to the search for life: If you get an intelligent signal, then you know for sure.
