Summary: Cosmos collaborators, Ann Druyan (widow of Carl Sagan) and astrophysicist Steven Soter, talk about the possiblities for finding life elsewhere and the implications for understanding humanity's own place in the universe.
Ann Druyan, the widow of renowned scientist Carl Sagan, and astrophysicist Steven Soter collaborated with Sagan over many years to create the famed television series Cosmos and numerous other projects. In this interview with 'Astrobiology at NASA' executive producer Kathleen Connell, Druyan and Soter discuss a range of subjects, such the possibilities of life beyond Earth and what the discovery of extraterrestrial life could mean for humanity.
Kathleen Connell: Help us take a stab at understanding the extraordinary proliferation of planets beyond our solar system. What is the evidence, and why should we care?
Steven Soter: For a long time, there was a theoretical expectation that other stars have planets, that planets are common. But there was no observational evidence, because it's extremely difficult to see planets of other stars. They're relatively non-luminous, and they're very close to their stars compared to the distance between stars, so they're lost in the glare of their stars. If you were to look back at our solar system from the distance of a nearby star, you could not image any of the planets, even the largest ones, directly.
But starting in 1995, some sensitive new techniques were developed that could detect planets indirectly, because, when a planet moves around its star, it actually tugs on the star by its gravity. The star makes a small orbital motion around the center of mass of the planet-star system. Since the star is luminous, you can detect that motion by the Doppler effect, because in part of its orbital motion the star is moving away from us, and the spectrum is red-shifted. In the other half of its motion it's moving toward the Earth, and the spectrum is blue-shifted. You can detect that subtle motion in the spectrum of the star. From the period of that motion you have the period of the orbital motion of the planet which is causing it. And from the speed of that motion you can calculate a lower limit to the mass of the planet. You can also detect more than one planet around a given star, because each planet makes its own contribution to the complex motion of the star. That's been done in a number of cases.
Starting in 1995, this technique began to yield results. And they're being discovered at a rate of about one a month. It's looking quite likely that the number of planets in the Milky Way could outnumber the stars. That's a major discovery of our time.
Kathleen Connell: We're using inference-we've not imaged these planets, correct?
Steven Soter: Correct. There's one other kind of detection. In one or two percent of all systems of exoplanets the plane of the planetary orbit is such that the planet will move in front of the star as seen from the Earth. It will make a transit, and will diminish the light output of the star by some small fraction, because the planet is a shadow across the disk of that star. And that will be repeated every orbit. That kind of transit detection method has also been achieved in one case. Again, it's indirect. You don't see the planet itself. But there's nothing else that could be responsible for these two kinds of observations. No one has any doubt that these are companion masses that are orbiting the star.
The masses are in the range of the giant planets of our own solar system and larger. We cannot yet detect small planets-Earth-size planets-by this gravitational method, because they don't make a sufficient motion of the star. And we've not yet seen even Jupiter-mass planets that are at the distance of Jupiter from the Sun, because their orbital period is on the order of ten years or more, and these observations have only been going on for the last seven years. But as they continue, we will be able to extend the detection to more and more distant planets, which have longer orbital periods.
Kathleen Connell: So, Ann, we have a consensus in the scientific community that exoplanets are real. We're talking about hundreds of billions?
Ann Druyan: We're talking about more planets than stars. We're talking about a galaxy and a universe of planets that far outnumber the stars. That was the great revelation to me of working with Steve on writing the show: to look up at the Milky Way and not only see a galaxy of four hundred billion stars, but to realize that hidden from our view is a galaxy of worlds that outnumber the stars.
We are just a few years away from launching an ultraviolet all-sky survey which could look directly at thousands of stars at once to find not these giant gas planets, but small, Earth-like planets. There's no doubt in my mind that we will.
Why should we care? Well, imagine that we do inhabit a galaxy of worlds. And imagine that all those beings-if there are beings on any of those worlds who are conscious-have evolved in the same spatial quarantine that we have. So there's a period of infancy just as we've experienced, in which we imagine ourselves the only beings, the only world in this vast galaxy and even vaster universe. And imagine that there comes a time when we are sufficiently mature, and they are, to develop the methods of science, and to be emotionally mature enough as a civilization not to need to be the infantile center of the universe. That rite of passage, that recognition of these other worlds, of these ways of living, of being, of seeing, of thinking--that's a great moment in the history of our species, as great as leaving the oceans to come up on the land.
There's a tendency we have to think that we are the end of history, that we are the end product of science. And lots of great discoveries have been made. But I keep thinking about all those possible worlds, and the fact that we're just around the corner from really finding them, and how, as Seneca wrote, we're just standing in the anteroom of the temple of nature. I think it will have a huge effect on us.
Of course, most urgently I hope that it will effect the way we see each other. Obviously, the violent spasms of superstition and fundamentalism that are torturing our species and our civilization right now are a kind of last ditch battle against the completely inescapable insights of modern science and the scientific revolution. I think what should happen is that we won t be able to help but recognize our genetic commonality with each other, our shared history, and the fact that the things that divide us would be unrecognizable, undetectable by species of other worlds with their own separate history and evolutionary pathways.
Kathleen Connell: So what you're suggesting, perhaps, Ann, is a new perspective that on the one hand, recognizes the unity of our species, but at the same time allows for our incredible diversity.
Ann Druyan: What I'm doing is merely echoing Carl Sagan's brilliant impulse to make us look at this tiny planet, at the pale blue dot, and to see it in its real context, in its actual circumstances, in its true tininess. I don't know anyone who's able to really see that one-pixel Earth and not feel like they want to protect the Earth; that we have much more in common with each other than we're likely to have with anyone anywhere else. And, I'm echoing Carl's dream of exploring the universe, of putting our house in order so that we're in a position to actually explore the universe, and to really find out how it works and how it's put together. So while science has been a traumatic experience for a lot of people-and maybe it precipitated some of this upheaval-it's also in the end the only hope that we have to get through this period of adolescence that's been so violent and so disturbing.
Kathleen Connell: I've seen this in various ways, including actually showing the Mars rock-the Allen Hills meteorite-to children. We had an open house at NASA Ames Research Center, and 220,000 people showed up. The advertising had mentioned that the Mars rock was going to be there. I attribute a lot of the attendance to that talisman from the universe. And yet to the kids, it looked just like any other rock.
Steven Soter: The thing is, it does look like any other rock. Yet we can prove that it's from Mars. But the profound implication for geology is that there are going to be similar things throughout the universe to what we find here. It's not necessarily going to look weird and exotic. The same might go for biochemistry, we don't know. But just as there's a range of geological types that we understand, we might quite well recognize a lot of other biologies in the universe. The first great discovery of astrophysics in the 19th century was that the visible universe was made of the same chemical elements as we have on the Earth. And that it obeys the same physical laws as we know on the Earth-the same laws of gravity and atomic physics and electromagnetism and so on. There's a unifying principle that came from astrophysics: the entire visible universe has this commonality. And that may extend to biology as well. But we don't know. We don't have the observations for the biology.
Kathleen Connell: Ann, I know you're busy on the Internet, and the Internet is a people's medium, for now. How do you view that in terms of both communicating and really engaging in a meaningful way in the search for life?
Ann Druyan: Well, actually, SETI@home, I think, is the absolute best example, because here are three-million-plus people, participants in a world Internet community who are accepting from the University of California at Berkeley the data from the Arecibo telescope. As their computer is going about its business it's analyzing those parcels of data, returning them to Berkeley. Conceivably, if any signal is ever isolated in that noise, you or me or any other participant in SETI@home will be credited with some part of that discovery. That is about as democratic an approach to science, this notion of massive distributed computing, as any. It's exactly on the theme that is so dear to us, which is the idea of making it possible for all of us to participate in the experience of science. We are in the process of becoming an intercommunicating organism.
Kathleen Connell: What are your personal feelings about the possibility of the existence of life outside of our Earth?
Steven Soter: The problem is, of course, we have no direct evidence. And [in terms of theory] we're not much better off, because we do not know how life began on Earth. We're almost clueless there. If we knew that, we would have some grounds for knowing how common the process is. But I'm basically a Copernican; I believe that there's nothing special about the Earth's position in the universe. I'm impressed by the ubiquity of the chemistry that makes life. We see complex organic molecules in interstellar clouds. It's everywhere. And I'm impressed by the fact that life began on Earth almost as soon as it was possible, almost as soon as the intense early bombardment by asteroids and comets tapered off and a stable environment emerged. The oldest evidence for life follows very soon after that, which suggests that where it's possible, it will take hold. And then on top of that you've got, it now looks like, something on the order of a trillion planets in our Milky Way galaxy alone, and a hundred billion other galaxies. Those numbers are staggering. My own opinion--and it's, I stress, still only an opinion--is that the universe is full of life, that we're not alone. And, that we may be close to finding out in our own solar system if there's other life; and, on a somewhat longer time scale, whether there's life on the planets of other solar systems.
Kahtleen Connell: In other words, are you saying you believe that life is a cosmic imperative, in a way?
Steven Soter: Oh, no. I don't think it's an imperative. That's going too strong. But I would be surprised, very surprised, if we found that life is very rare in the universe.
Kathleen Connell: And Ann, what are your feelings about it?
Ann Druyan: Well, not surprisingly, I agree with what Steve is saying. It would be a giant surprise. You look at any image of a star-choked field in the sky, and the notion that life and intelligence only came to be on our one particular planet, when we're talking about hundreds of billions of stars, and then perhaps five to ten times as many planets, is just untenable. The odds just don't sound likely that this is the only place where life has come to be. And then of course if you factor in the ubiquity of organic molecules, the building blocks of life, it makes it even more of a stretch to imagine that life only happened here. It just doesn't make any sense. I think it's very likely that there's life.
During the next 15 years, American and European scientists hope to launch more than half a dozen missions to search our corner of the Milky Way galaxy for terrestrial planets. To search for Earth-like planets around stars beyond our solar system, the space-borne telescope Kepler Mission is scheduled for launch in 2006. Kepler will simultaneously observe 100,000 stars in our galactic neighborhood, looking for Earth-sized or larger planets within the "habitable zone" around each star - the not-too-hot, not-too-cold zone where liquid water could exist on a planet.
One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth's size, 185 planets if most are 30 percent larger than Earth, and 640 if most are 2.2 times Earth's size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars.
After Kepler, NASA is considering a 2009 launch for the Space Interferometry Mission (SIM). SIM's primary mission will be to measure distances to stars with 100 times greater precision than currently is possible. This will improve estimates of the size of the universe, and help determine the true brightness of stars, allowing us to learn more about their chemical compositions and evolutions. SIM also will look for Earth-sized planets in the habitable zones around some 200 stars.
SIM will be an interferometer, which means it will combine interacting light waves from three telescopes. This interaction, called interference, makes the individual telescopes act as though they were a single, larger telescope with greater light-gathering ability.