It is a rare researcher who can fundamentally change our picture of our place in the universe. In the 16th century, Nicolaus Copernicus did it by arguing that Earth is just one planet among many revolving around the sun; in 1924, Edwin Hubble did it by showing that our galaxy is just one among many. This year DISCOVER honors David Charbonneau, a Harvard University astronomer whose research could soon lead to an equally stunning revelation: By studying alien worlds, he may find the first direct evidence of life beyond Earth, a sign that our living planet is—yet again—one among many.
Astronomers currently know of roughly 200 planets circling nearby stars, and more and more of these so-called exoplanets are discovered every year. Most of the newfound bodies are so strange that scientists have had to coin new terms, like “hot Jupiters” and “super-Earths,” to describe them. Playing the celestial detective, Charbonneau has systematically gone about investigating these impossible planets and uncovering their secrets. In 1999, he led the team that made the first observation of a transiting exoplanet—one that passes directly between its parent star and Earth. By examining how the planet blocks out some of the light from its star, Charbonneau can see what gases are present in the planet’s atmosphere. In 2001, Charbonneau and astronomer Tim Brown of the High Altitude Observatory in Boulder, Colorado, used this technique to “sniff” the atmosphere of a huge, broiling planet called HD 209458b, even though it is 150 light-years away—4 billion times as distant as the moon. Just a few months ago, Charbonneau’s team at Harvard made another breakthrough and created the first weather map of an exosolar planet. The forecast: hot and windy, same as yesterday, same as tomorrow.
Charbonneau’s personal journey to becoming a planet hunter began with his desire to be a marine biologist. Born to a physician and a geologist, Charbonneau was no stranger to science. As a teenager growing up in Ontario, he visited the tide pools at the Pacific Rim National Park in British Columbia during a family vacation and witnessed firsthand the wild diversity of life at the border of the sand and the sea. His dedication to biology gave way to a passion for physics when he encountered special relativity, quantum mechanics, and Stephen Hawking’s A Brief History of Time. Theoretical physics then led him to astronomy, a passion that now colors every part of his life (his daughters are named Stella and Aurora).
For his next act, the 33-year-old Charbonneau wants to move beyond the exotic and bizarre planets he has studied so far. Now he is looking for something far more familiar: a smallish rocky planet with an atmosphere that bears the chemical imprint of life, like the abundant (and otherwise inexplicable) oxygen that plants pump into our own air. Charbonneau hopes to refine the transit technique so that even the faint wisps of an Earth-size planet’s atmosphere can soon be detected and analyzed. If he spots the signature of alien biology on such a world, we will know that we are not alone in the universe. If he fails, it will strongly support the idea that we are truly unique. That is why David Charbonneau is DISCOVER’s Scientist of the Year.
You were one of the first people to use the transit method to study exoplanets, and suddenly that technique is really taking off. Why now?
Why it’s suddenly working may have two factors. One, the astronomical community has slowly figured out how to get very good data on tens of thousands of stars, night after night after night. We’ve also gotten very good at understanding most of the little winks and blips in our data. The other answer is the same reason as “why the four-minute mile?” Why didn’t people run a four-minute mile before 1954? There was this perception that it was extremely difficult and perhaps couldn’t be done. Most astronomers thought that most solar systems looked like our own. That meant that the planets that were big enough, the ones that blocked enough of the light, were far from their stars. That meant that they would only transit once every few years instead of once every few days. The probability of a transit was very small with this model. No one was looking because we had entrenched ideas.
What are some of the planets that you’ve studied like? How strange are they? 189733b orbits a K dwarf, a smaller, redder star than the sun. Basically, its star is more of a lightweight compared to the sun, so it’s a bigger planet orbiting a smaller star. With 189733b, the excitement is that it’s the first planet that we really have a feeling for what it looks like. We actually have a weather map. It’s the first planet that I have a mental map of in my head because we’ve actually measured, to some degree, the physical map. We know where the hot and cold areas are, and so on. The nightside of the planet is actually quite hot. It didn’t have to be the case—it could have been that these planets were very, very hot on the dayside and very cold on the nightside, but apparently there are these very strong winds that can move energy around to the cold side, so the nightside on those planets is really quite hot. In a sense, that planet feels the closest because we have this image of it.
TrES-4 is a newcomer on the scene. What we know about it is that it is extremely low density. I think TrES-4 is really going to be difficult to explain—it really pushes the laws of physics to try to understand how it can maintain such a low density when it should want to contract under its own gravity.
HD 209458b is very hot. It’s tucked in very close to its star; it orbits its star every three and a half days. Its temperature is probably about 1400 degrees Kelvin! It’s very puffy, so it’s very low density, which means that given its mass—which is less than that of Jupiter—its diameter is bigger than we expect, and so the puffiness of this planet is actually still somewhat of a puzzle. Its star is rather like the sun, maybe a little bit hotter. Basically, its star is a twin of the sun, so that’s why it’s intriguing, because the star is similar to the sun in terms of its age and its mass, and yet the planets around it are obviously so much different from the planets of our own solar system.
Does that mean that our solar system is exceptional?
We don’t know the answer yet. We don’t have any clue about systems with terrestrial [Earth-like] planets because no one has yet looked with enough precision to find them. What we have learned is that the diversity of exoplanet systems is immense. The basic architecture of our solar system, where things go in circles, and there are small rocky planets close to the sun and big massive gas giants far from the sun, is certainly not the only architecture. It may not even be the most common architecture. There are many ways to make a planetary system, so, for example, the planets could be on eccentric orbits, or you could have the most massive planets right up next to the star, even closer than Mercury, and those might even be more common.
What’s it like to be the first person to see an exoplanet? You know, the discovery moment now in astronomy isn’t at the telescope looking through the eyepiece but at a computer screen when you’re analyzing that data. But there is still that moment where you make that first plot, and you look at it—and right there, no question, there is the signal. The first time that we measured the actual emitted light from these planets, or the first time that we detected that one of them had an atmosphere, those were very unambiguous signals. And the first time you see that, that’s the most rewarding moment in science.
You’re now working on the MEarth project, which is going to look for Earth-size planets orbiting close to “M dwarfs,” which are small, dim stars. How long until it’s up and running, and how long until it gives its first results? The project is being built now in southern Arizona, and we hope to have two telescopes working in October and then six more by January, so we hope to start the survey early in 2008, and the nominal survey will take three years. Our telescopes are pretty humble by astronomy standards. The telescopes are 16-inch telescopes—tiny compared to what we often use for our research, telescopes that are 10 meters [about 400 inches] in size. But I think that it’s not unrealistic that someone will make the first detection of a transiting planet in the habitable zone of its star in the next couple of years.
Suppose you find a planet the size of Earth. How do you then look for life?
We are very biased by having grown up here on Earth, but there’s a huge challenge in asking yourself the question, “What different forms might life take?” It would be so difficult to recognize life if it were very different chemically from life on Earth. The easier question is to look for life that is very similar to life on Earth. That’s probably going to be our first step. When we talk about life on other planets, we’re talking about life as we know it.
The first measurement is to determine that the planet has an atmosphere. You need a thick atmosphere for life as we know it. Then the trick is to examine the atmosphere spectroscopically for the presence of certain molecules. If we look at the spectrum of Earth, we see there’s a lot of oxygen. All of that oxygen is driven by biological activity. The only way Earth’s atmosphere has this large quantity of oxygen, especially in the presence of methane and other things that would like to react with oxygen, is that there’s this driver, which is life, which through photosynthesis supports this equilibrium. We look to see if life has done things to that distant atmosphere that we know it did to the atmosphere here on Earth—that’s a nice remote-sensing approach; it doesn’t require any assumptions about the life, like that it wants to communicate with us or anything fancy like that.
If you stepped back from the solar system and you took a spectrum of Earth and Mars and Venus, you would see that there’s something really special about Earth. The atmospheres of Venus and Mars have mostly carbon dioxide, which is not a good molecule for life. However, if you don’t see those [Earth-like] signals, you can’t conclude that there isn’t life, because the life may be completely different; it may proceed in some chemical pathway that we might mistake for nonbiotic processes, for geologic processes.
How do you think people will be affected if we discover that there is another living world out there?
Philosophically, if it were the case that the galaxy is full of habitable planets, and perhaps even other civilizations, I think that people would think of themselves quite differently. Or to know that Earth was truly unique in that it was the only habitable planet would affect how many people view their place in the universe. When I went to school, there weren’t planets around the stars—they were there, of course, but nobody had ever detected them. My daughters will grow up in a world where there were always planets around the stars. They’ll learn in school that of course there are planets around the stars—hundreds of them. By the time they go to school, even a few years from now, there may be a thousand. And maybe even by the time they’re in a university, and hopefully before then, it’ll just be a fact that there is life on some of those planets. There will be this amazing change, and they’ll have just grown up in this world where that was always the case.
What are we learning about our own lives on Earth as we look at these distant planets?
Well, I think there is certainly a very clear answer to that if we look ahead. If we find life on other planets, what we want to know is whether the basic forces of evolution and biology are universal. You kind of wonder about how life started on Earth. Maybe it’s the case that you just have to cook up a planet with roughly the right properties and life is unavoidable—life will just spontaneously get going on any such planet, and that it’s a very universal process. Or maybe that’s a really rare process. Maybe it’s not enough to have all the right conditions. Even if you have those right conditions, it’s still one in a billion. So I think it really is something that is very close to home.
Has there been any specific new research triggered by these studies of planets around other stars? We want to understand a lot of the molecules that we look at in planets around other stars. Those molecules are the exact same molecules as here on Earth, but we now want to see them under very different conditions, very high temperatures and pressures. And so we have to go and study them here on Earth. We’ve been learning a lot about the spectral signatures of water and methane, motivated by these exoplanet studies. Those molecules are crucial to us here on Earth. You’d think we would know everything there is to know about water, but that’s not true.
Did you ever worry that you wouldn’t find anything when you began searching for planets? I was very nervous at the time that we wouldn’t find any of these planets, or that it wouldn’t turn into a very rich field. It’s given me a great sense of delight to see that those risks were rewarded many times beyond my expectations. But it’s only with accepting a level of risk that there’s the possibility of a truly novel discovery. If you do a very conservative project that you know will be productive, in the sense that it’s going to yield some results, there’s a limit on what is the most exciting thing that could happen within that project. If that thing isn’t something that would really keep you up at night, then why are you going through the motions?
How has planet hunting changed since you started? Maybe 10, certainly 20, years ago—if you talked about looking for life on other planets, then you were kind of nutty, right? It was probably a very dangerous thing to do if you were a junior faculty who might be looking for tenure, let’s say. And that’s completely changed—I think now there’s this huge sense that we are really going to make this work, and we’re going to figure out how to actually study the atmospheres of these planets that we’re detecting and look for the chemical signatures of biological activity. So that has gone from being a kind of crazy scientific idea that could never be tested to something that’s really at the heart of the big funding agencies, in particular NASA.
Why search for distant planets when there’s still so much we don’t know about our own Earth? Look back through history and you can find writings from the Greeks that talk about life on planets orbiting other stars. I think there’s been this abiding human question about whether we are alone in the universe. And I think that strikes at the very soul of humanity—of how we picture ourselves in the cosmos. We’ve learned in the last hundred years of the incredible physical size and age of the universe. And now the question is, as it has always been, are we truly alone? And I think that everybody is willing to put in a little bit of money to actually get at the answer to that question.
