Alien hunters (from left): Tori Hoehler, Sara Seager, Gibor Basri, and John A. Johnson

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 (click here for video of the event).


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.