Since the beginning of human history, we’ve looked up at the stars and wondered: Are we alone? No other generation has been able to find an answer, but David Charbonneau thinks we may be the first. He’s an astronomer at Harvard University and a recipient of an honorary degree from the University of Chicago this year.
Charbonneau has made it his life’s goal to search the stars for habitable planets and alien life. On this episode, he tells his fascinating story about the history of exoplanetary research, his journey as a planet hunter and the stunning discoveries he’s made along the way.
- David Charbonneau receives honorary degree from the University of Chicago
- The Truth about Exoplanets—Scientific American
- An Exoplanet Like No Other Yet Found—The Atlantic
- Scientists Spot A Planet That Looks Like 'Earth's Cousin'—NPR
- Understanding life, here, there, and everywhere—The Harvard Gazette
Paul Rand: The vast and unknown mysteries of outer space have always captured our imagination and ingenuity
Tape: We choose to go to the moon in this decade and do the other things not because they are easy but because they are hard.
Tape: That’s one small step for man, one giant leap for mankind.
Paul Rand: And when we’ve all looked up at the night sky all of us have asked the same question…are we alone?
Dave Charbonneau: Our generation is the first generation in human history that's going to have the technological ability to actually go and answer that question.
Paul Rand: That’s Dave Charbonneau. He’s one of the world’s leading astronomers, a professor at Harvard University and the recipient of an honorary degree from the University of Chicago this year. When I heard him speak at Convocation, I just knew he had to come on Big Brains. Back when he was just a graduate student, Charbonneau took on a job straight out of science fiction…finding life on other planets.
Dave Charbonneau: And we might we might find out that we're totally alone. Right. We really could be it as far as we can measure. Or it may turn out that we live in the Star Trek universe and essentially every star has a bunch of planets with life on them.
Paul Rand: Charbonneau is a planet hunter. He scours the universe searching for habitable worlds and alien life. Butwhen you’re studying something that’s light years away, how exactly do you do it?
Dave Charbonneau: Yeah, I mean, the reality is that we are not going to other stars anytime soon. So I love science fiction. I wish this were not the case. But the reality is that it is it is the distances between stars, even the very close the stars to us are vast. So if you take the, you know, the fastest spacecraft that we've ever built and launched if we pointed that directly at the closest star it would be hundreds of thousands of years for many stars to reach those systems. So we have to do it with telescopes. We're not going to have the opportunity to send robots or send people to go and do this. And the good news is that using telescopes, we can study the atmospheres of planets, even if they orbit other stars. And the other piece of good news is by studying the atmospheres in this way, you can actually figure out what's in the atmosphere and find molecules that we know are uniquely made by life.
Paul Rand: From the University of Chicago this is Big Brains, a podcast about pioneering research and pivotal breakthroughs reshaping our world. On this episode, Dave Charbonneau and the planet hunters. I’m your host Paul Rand.
Paul Rand: Dave Charbonneau was destined to become an astronomer. Even from an early age, his mind was always wandering amongst the stars.
Dave Charbonneau: So I was definitely the kid that was always interested in astronomy. And even as a little boy, I remember going with the scouts on big canoe trips. I grew up in Canada and I would take my star chart and I would take my little constellation map of the sky and we'd go away from the city where you could really see a lot of the a lot of the fainter your stars and I would try to piece out the constellations. That was there from the very beginning. When I was in high school I saw the ocean for the first time. So then through most of high school, I actually really wanted to be a marine biologist. And it was only when I got to college that my interests swung back to astronomy. But of course, my long term hope is that I do ultimately get to be a biologist because of the kind of astronomy that I work in.
Paul Rand: It wasn’t so long ago that a job like Charbonneau’s didn’t really exist. To understand how he was able to become a planet hunter, we have to go all the way back to a groundbreaking discovery in 1995.
Dave Charbonneau: What had happened was the first Jupiter like planet had just been found around a nearby sunlight star. And that was done by two Swiss astronomers. Michel Mayor and Didier Queloz.
Tape: The Au Provence observatory, it’s here that two swiss scientists made the most exciting discovery in the history of astronomy. They found the first planet outside our solar system.
Tape: All the signals were in favor of a planet, but it was such an unusual planet.
Dave Charbonneau: And actually, they recently received the Nobel Prize for that.
Tape: The way we found the planet and the kind of planet we found was entirely unexpected.
Dave Charbonneau: What they had done, is they had used a method called the wobble method. And so they were looking very carefully at a set of nearby sun like stars. And they found a star named 51 Peg that they could see was wobbling back and forth. Sometimes it was coming towards us and sometimes it looked like it was going away from us. And so they concluded that there must be an unseen partner in that dance, that the that the star couldn't be alone, but actually had to have a massive planet. And the two of them were basically dozy dosing in in circles around each other. And that's what was causing the signal they were seeing on the star.
Paul Rand: This discovery revolutionized the way astronomers studied outer space. The race was on amongst astronomers to find more planets and new ways of studying them. Charbonneau was no exception.
Dave Charbonneau: When I arrived for graduate school, I was looking for a project to work on. And then I was approached by an astronomer who was looking for a student and said to me, hey, there is this this new field that is being born. And there are some very basic questions that we're trying to that we're trying to puzzle out. And and it was clear from the very beginning that the big quest was ultimately to find out if we are alone in the universe. You know, for thousands of years, humans have wondered whether or not we are alone. Right. So if you if you go back to the Greeks, they wrote about this question of whether there were other worlds and if we could somehow come into contact with those other worlds and are those other worlds would be inhabited. And then certainly for 500 years, there's been there's been lots of writing about, gosh, you know, now that we know the points of light in the night sky are stars, there must be planets, but they're just too faint to see. And therefore, there might be life on those planets, but we just don't know how to measure it yet. And, you know, even if there was such life on a planet orbiting the closest star, we wouldn't have had the ability to detect it yet. So would have gone completely unnoticed by us. So we really don't know the answer to this question. We really could find out that life is sort of an inevitable consequence of the processes of star formation and planet formation that just like you always make planets, you're essentially always end up with life. Or we could learn that something very, very rare or possibly even unique happened on the earth. And the formation of life is an incredibly improbable event. And to the best of our ability to measure, we are we are effectively alone. If we do find that life is a commonplace in the galaxy. I think that will cause a lot of people to reevaluate their sense of self in the universe, which is kind of the role that astronomy has always had, is giving us a cosmic sense for, you know, that our lives are very short compared to the age of the universe, that physically we're really tiny compared to the size of the universe. I think learning that we’re just one civilization or one form of life among many would be a big change for people, but we do want to think about the impact of those discoveries here on earth.
Paul Rand: Okay, so you’ve decided to search for habitable worlds and alien life out amongst the stars. Where do you start? If we’re going to find life in the universe, it’s more than likely it’s going to be outside our solar system. We’ve already taken a good look around our neighborhood and haven’t found much. So, you need to start by looking at exo-planets.
Dave Charbonneau: Exo-planets are our systems of planets orbiting other stars. And as it turns out, many stars, if not all stars in the galaxy have planets. That's a that's a relatively recent discovery. We didn't know that five years ago.So exo-planet just means a planet orbiting another star. So it's kind of a shorthand, you know, instead of saying planets that orbit other stars like the Sun, which gets kind of cumbersome. We just came up with Exo Planet. We also thought it sounds kind of cool.
Paul Rand: Just as Michel and Didier had found the very first exo-planet. Charbonneau wanted to make his own big exo-planet discovery. Something that would revolutionize the field of astronomy…
Dave Charbonneau: When I was a graduate student, I went out to Colorado and worked with the scientists there named Tim Brown. And we used a very humble telescope. It was a four-inch telescope located in a parking lot in Boulder, Colorado. I was looking for the shadow of a planet going in front of it’s star, and we had been looking intensively at this one star, and we saw these little blips these little moments where the star got fainter by just a small amount, just 1% of the light of the star was being blocked by the planet. And we made the first detection of what we call a planetary transit, which is basically the shadow of a planet passing in front of its star. If our line of sight is exactly lined up with the with the orbits of the planets so that every time they go round their star, they go in front and they make a little eclipse. Then we'd be able to see the star that we're studying and get fainter. And then and then it would brighten up again when the planet moved out of the way. And then that would repeat every year for the planet.
Paul Rand: Charbonneau had his groundbreaking discovery…
Dave Charbonneau: It's not like I guess I thought it would be I thought that it would be. I thought it would be pretty clear when you'd made a discovery. You were looking for a thing, you found the thing, and then and then you sort of jumped for joy. But in reality, it was a much more uncomfortable process because your mind is very uncertain. Right. The other thing that that I didn't appreciate at the time was, of course, what the impact of that of that measurement would be. We now use those kinds of planets to actually get at the atmospheres of the planets. When the planet goes in front of its star, we're able to see some of the light passing through the atmosphere of the star, and therefore we can deduce what's in the atmosphere chemically when atoms and molecules are present. And of course, all of those ideas began to flow once we actually had the first example of one of these transiting worlds.]
Paul Rand: Charbonneau’s transit method put our ability to identify exo-planets into hyper-drive. Now, he’s working with NASA and other organizations on projects that will use this method to take planet hunting to the next level. And they’ve already made some major discoveries. That’s after the break.
Dave Charbonneau: Following my years as a graduate student, then I then I became more senior and I began thinking about what projects I would like to build, and I decided that I would really want to go specifically after small rocky planets and maybe even try to find planets that that were just the right temperature to have liquid water. And so we we did the math in terms of, you know, how many stars would we have to look at and how long would we have to look at each star? And we decided we would need to build an array of telescopes. And we called that the MEarth project. And the reason is we were looking for Earth like planets orbiting small stars, which astronomers called M stars. So M for star and earth for the kind of planets we wanted to find. Gave us the MEarth project. So what we did is we built an array of telescopes. We built basically eight telescopes in a single enclosure in Arizona. And then we also built a copy of those later, a few years afterwards after the project was up and running in Chile. And so with these 16 telescopes, we could look at all of the stars in the sky that were small and really nearby to us. And so for many years, we searched the sky and we made some interesting discoveries, but we hadn't yet found a planet that was that was both rocky and just the right temperature to have to have liquid water on its surface. And then I was working with a very bright graduate student, a guy named Jason Dittmann, and he went and applied a new data analysis method, basically a machine learning method to some of the data that we had recorded previously and went back over many years and found this one star. And he brought forward this one star to my attention and said, I think I think there's something very interesting going on with the star. And ultimately that that led to the discovery of this first planet that wasn't just rocky, but also had just the right temperature for life.
Paul Rand: This was another major breakthrough. Of all the exo-planets that have been discovered, this is one of the most promising to find life. It’s rocky, which is important, and it’s temperatures are conducive to liquid water. All signs point to this exo-planet being habitable.
Dave Charbonneau: There are probably many, many factors that go into making a planet habitable. But, you know, as astronomers, we can't measure all of those yet. In fact, we can't even measure most of them. But what we can do is we definitely know the planet needs to be rocky as opposed to being gassy. So that means we have to figure out its density. And that's something astronomers can do. And we need to figure out its approximate temperature. And the way we do that is by figuring out how bright the central star is and how close it is to the star, because largely speaking, the temperature of the planet is just determined by how much energy it receives from its from its sun. And so those were the two things we could measure about this planet. We knew we had a rocky planet and we knew it had was getting just about the right amount of energy so that if there was liquid water, the water if there was water on the planet, the water would actually be in a liquid state. Now, probably we want to learn much more, you know, as does the planet have a magnetic field so that it can it can shield its atmosphere from from radiation from the central star. Does the does the planet have take to plate tectonics, for example, which is essential for maintaining the habitability of the earth over time? And those are things we can't measure yet, but we're just starting with what we can measure.
Paul Rand: Charbonneau is an essential member of a number of missions that are pushing the envelope on planet hunting.
Dave Charbonneau: Well, now I'm part of a mission called the Tess Mission.
Paul Rand: Tess stands for…Transiting Exoplanet Survey Satellite.
Tape: We are almost ready to launch TESS into orbit. TESS will spend two years gazing at almost our entire sky by breaking it up into 26 different sections. Four powerful cameras will stare at each zone for about a month.
Dave Charbonneau: And it launched in April of 2018.
Tape: Launch announcement
Dave Charbonneau: It’s surveying all the nearby stars looking for these planets. It hasn't yet found any rocky, habitable planets orbiting nearby small stars wuite the way that we did with MEarth. But I fully expect that that there are a few more out there and that by looking at the entire sky from space, of course, we probably will find them. So it's really exciting moment. Basically, in the next I would say in the next year to a year and a half, we will know of all of the closest rocky planets orbiting the very nearest stars to us. So we're kind of going to meet the neighbors for the first time. We've been doing it, you know, here and there with smaller telescopes. But now we're going to do it in a uniform way. Find out who all the neighbors are and then figure out what the next step is in terms of picking a few of them and really going and drilling down and setting their atmospheres in detail.
Paul Rand: If the TESS satellite wasn’t impressive enough…the next project blows it out of the water…
Dave Charbonneau: Then looking a little bit beyond that, then there's two projects them. I'm very, very excited about. The first is the NASA James Webb Space Telescope. And it will be very, very good at learning what the atmospheres of these planets that have been discovered by, for example, the TESS mission. The James Webb Space Telescope will be launched into orbit, so it'll be it'll be operating from space and hopefully it'll launch in just a couple of years.
Paul Rand: The James Webb telescope will be able to observe planets and events in our universe that no other telescope has been able to capture. And it will be able to use the data that TESS has collected in order to know exactly where to look.
Dave Charbonneau: And then the other the other big project is called the Giant Magellan Telescope. That’s of course and exciting project that both the University of Chicago and Harvard are partners on. And that's a that's a ground-based telescope. It'll be in Chile. But at the time that it becomes operational, it would be by far the most powerful telescope ever built. And because it can gather so much light, it would be able to go and really look for, for example, for oxygen. That's the first project that really could go and do that. And that would be, of course, transformative ifwe made that discovery. And we're also hoping that the giant Magellan telescope will we'll get we'll get on sky in something like five years or so. So it really is this kind of special decade in astronomy.
Paul Rand: These revolutionary technologies will push the search for extraterrestrial life into the future. But all these exciting new possibilities beg another question…how exactly will we know when we’ve found alien life?
Dave Charbonneau: Well, we don't know. You know, we I think we're going to start by looking for life as we know it. And that's for the simple reason that if we're going to design an experiment, we have to know how to interpret the results. And so if we look for life that is broadly similar to life on the earth. So it needs liquid water, maybe it involves photosynthesis. And so we can recognize things like oxygen that are made by that photosynthesis. Those are the sorts of signatures that we could figure out how to study with telescopes when looking at a distant planet. It could be that life on other planets is radically different. You know, it's not carbon based, doesn't need liquid water, makes all sorts of different gases that we don't recognize as being biogenic. But I think in that case, then we're then it would be much more difficult to actually know that we were even looking at life. And my guess is we might record the signals of life, but not actually recognize them and just pass them over. So the current thinking is we are going to look for a chemical disequilibrium similar to what's going on the Earth. So if you were an alien astronomer and you found the solar system. So you're looking at another star, but it happens to be our sun. And you. Found there were four rocky planets in the inner part of that solar system. And you studied the atmospheres of each of them. You would notice that there's something really different about the third planet from the sun. So the third planet from the sun has a lot of oxygen. And and you would know that oxygen is very reactive molecule. It would it would it would react, for example, with all the iron that's around and form rust and be pulled out of the atmosphere. And the fact that there is that large amount of oxygen, particularly in the presence of other things like methane, which are very reactive, would be a strong indicator that that there was life and in particular that there was photosynthesis. So of course, what's really going on is, yes, that all that oxygen is produced by photosynthesis that's been, you know, hard at work for billions of years on the surface of the earth. So oxygen is the key, but it is not enough to just detect oxygen because it's possible that oxygen is made by other processes, basically by light from the from the star that's breaking apart water molecules and making oxygen molecules. And so to distinguish between those scenarios, then we have to find other molecules like methane and water and also find out how much carbon dioxide there is in the atmosphere. But the good news is that all of those molecules are readily accessible. So so, you know, we know what it would take to go and look for oxygen. Look for water. Look for the carbon molecules. We just need more powerful telescopes than we currently have because, of course, the planets themselves are very small compared to their stars.
Dave Charbonneau: I think that, you know, space has always played this really essential role for humanity, but especially for the American public, you know. And that that this is something we're very proud of is our history of space exploration. And there's an enormous wealth of excitement in terms of going out and exploring space, both with telescopes and looking for life on other planets in and actually with rockets and sending people. And we should continue to do all of that. So I think that if exo-planets can help keep people interested in space and we can genuinely express to people the very real opportunities for discovery that are at their doorstep. That's a very positive thing.
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