Although Kepler and Corot are focusing on sunlike stars that could support true analogues of Earth, much of the action at ground-based telescopes is concentrating on red dwarf stars, for the simple reason that planets are easier to find there. An Earth-like planet would cause a bigger wobble and a darker transit in a red dwarf than in a sun, and the effect would be even more pronounced if the planet were in the habitable zone—because the habitable zone, where liquid water can exist, lies closer to a cool red dwarf. In the fall of 2007 David Charbonneau of Harvard began deploying a network of small telescopes in Arizona that will be focused on detecting transiting super-Earths in the habitable zones of red dwarf stars. The Swiss and California teams think they can do the same with the wobble technique. Of the super-Earths they’ve discovered so far, some—including the one around Gliese 876—orbit red dwarfs, though none lie in the habitable zone.
Is anything living out there? A hint of an answer could come in the next few years.
No one knows for sure whether a rocky planet in a red dwarf’s habitable zone would truly be habitable. Until recently, in fact, astronomers assumed it wouldn’t be. A planet so close to a star might have been blasted and sterilized, early in its existence, by flares from the star. It might be gravitationally locked, with one face always pointing toward the star, the way the moon always points the same face toward Earth. In that case the whole atmosphere might have frozen and snowed out on the dark side, leaving the planet airless and barren. But after having some of their preconceptions shattered by the discovery of Jupiter-size planets orbiting their stars in less than two days, planet hunters are no longer so confident of the others. Life might emerge on a red dwarf planet, some now think, after the star has aged and its flares have settled down; winds on the planet might transport heat from one hemisphere to the other, keeping the atmosphere from freezing. After a workshop on red dwarfs in 2005, Jill Tarter of the SETI Institute—a leading thinker on alien life—and her colleagues published an analysis that convinced many researchers that red dwarfs are worthy targets for Earth hunters. That’s a happy conclusion, given that red dwarfs are the most common stars in the galaxy and also the easiest targets for ground-based telescopes.
But even if a habitable Earth-like world is found first from the ground, it will most likely take a space observatory to search for the chemical signals that tell us what we really want to know: Is anything living out there? If the planet is one that can be observed transiting, it just might be possible to provide a hint of an answer in the next few years. As a transiting planet passes in front of its star, some starlight passes through the planet’s atmosphere and continues on toward Earth—minus certain spectral frequencies that have been absorbed by molecules in the atmosphere. In 2001, using Hubble, Charbonneau and his colleagues detected the first exoplanetary atmosphere that way; it belonged to a hot Jupiter called HD 209458 b, and it contained sodium, they said. Three years later Charbonneau found himself locked in a race with Deming to be the first to detect the flip side of a planetary transit—the moment, called secondary eclipse, when a planet passes behind its star. This time it was Deming who was observing HD 209458 b, with the Spitzer Space Telescope, an orbiting infrared observatory. Charbonneau, he knew, had collected data on a different hot Jupiter a month earlier. “We didn’t want to be second,” Deming recalls. “I was analyzing data while I was eating Christmas dinner. I had to catch Dave.” In the end they published papers simultaneously and held a joint press conference.
What each had done for the first time was detect an exoplanet’s photons. No telescope yet can spatially distinguish an exoplanet from its star; the distance between them is too small and the brightness contrast too large. A Jupiter adds about a billionth to the visible light of a sunlike star, and about a ten-thousandth to the star’s infrared glow (planets give off more heat than they do reflected starlight). By observing the combined infrared radiation of star and planet with Spitzer and then subtracting the radiation recorded from the star alone when it hid the planet, Deming and Charbonneau had detected the heat of the planet itself. From that they could calculate its temperature; Charbonneau’s team has since been able to create a crude weather map of their exoplanet, HD 189733 b, which showed that fierce winds must be spreading heat around its surface. Others using the secondary eclipse technique have detected evidence for water vapor and methane in the atmosphere of HD 189733 b.
These findings are trials for the far tougher task of picking apart the light of an Earth-like planet, much smaller and farther from its star (and thus far dimmer) than the hot Jupiters studied to date. Spitzer wasn’t designed to measure the spectrum of hot Jupiters, but it did. And the James Webb Space Telescope, which is slated to replace Spitzer in 2013, has not been designed to detect the spectrum of Earth-like exoplanets—but with its 6.5-meter (21.3-foot) mirror, nearly eight times the diameter of Spitzer’s, Deming and Charbonneau think it might. Other astronomers are more cautious. “I certainly wouldn’t claim JWST is going to prove habitability, because it’s not,” says Mark Clampin, project scientist for the observatory.
Sara Seager of MIT, who collaborates with Deming, is trying to figure out which spectral signatures in a planet’s atmosphere would provide the best evidence for signs of life. Water vapor is indicative of liquid surface water, which is necessary but not sufficient for life as we know it. Oxygen, which would quickly react out of Earth’s atmosphere if it weren’t continually produced by plants, is closer to a smoking gun, especially if it were seen together with methane. Then there is what Seager has dubbed “vegetation’s red edge”: At wavelengths of 700 to 750 nanometers, at the red end of the visible range, the reflectance of leafy green plants sharply increases, to four or five times what it is even at green wavelengths.
Whether the Webb can detect such a signature is not yet known, and even if it can, the data probably won’t be definitive. Deming and Charbonneau’s secondary eclipse technique, ingenious as it is, lacks the power to distinguish between life and something else. Making that distinction will require a new kind of space telescope. That’s where Corot and especially Kepler come in. They won’t provide targets for that future space telescope, unfortunately. To monitor many stars and maximize its chances of finding Earths, Kepler is forced to monitor distant ones; any Earths it finds will most likely be about 300 light-years away, too far for any currently imaginable space telescopes to take a spectrum from. What Kepler will do is tell astronomers—and NASA and ESA—what sort of space telescope it will take. If nearly every sunlike star has an Earth, we might find life around a relatively nearby star, with a relatively small telescope. If Earths are rare, the next telescope will have to be big.
Eventually such a telescope will get built—and if the pace of discovery remains as rapid as it is now, that day will come sooner rather than later. Finding convincing evidence for extraterrestrial life may take decades, but that is not a long time given the stakes. “Throughout recorded history we’ve had this question: Are we alone?” Tarter says. “For millennia, all we could do was ask the philosophers. Suddenly we have a way of looking for an answer that is not based on a belief system. I live in the first generation of humans that is able to do that. I think that’s extremely exciting.”
Other astronomers too feel acutely the historic nature of their quest. One of the last things to be mounted on Kepler this fall, before it makes the journey to Canaveral for the launch, will be a metal plaque engraved with the names of all 2,000 scientists, engineers, and managers who contributed to its mission, which will run through 2012. Kepler will follow a 53-week orbit around the sun, meaning that it will steadily drift farther behind Earth. “It loses a week a year,” Borucki says. “So 53 years after launch, it will come back to Earth. At that point, I expect, people will go up and pick up the spacecraft and put it in the Smithsonian. I know that sounds far-fetched. But I really think it will happen.”
