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.