Gliese 876 is a modest star, just one-third the mass of our sun and only 15 light-years away, but it has a history-making planetary system all its own. In 1998 a team led by Geoff Marcy of the University of California at Berkeley detected the first sign of something interesting there: a giant planet, twice the mass of Jupiter, circling Gliese 876 once every two months, its gravity yanking the star back and forth at the speed of a jet plane. Three years later the same group found a second planet, half the mass of Jupiter and closer in, pulling the star around at the speed of a race car. Although the planets are too faint to be seen directly, their motions cause the star’s spectrum to wobble back and forth across the digital detector of an astronomical telescope.
In the past decade, announcements of Jupiter-size planets have become commonplace; about 300 of them have been found so far. In 2005, however, with the help of improved detection software, Marcy’s team turned up something else orbiting Gliese 876—something truly new. This invisible object added one more regular component to the star’s motion, like the third note, faint and high, of a piano chord. It was another planet, orbiting in just two days and pulling on the star much more gently, not at jet plane or race car speeds but at a speed a man could run. This planet, dubbed Gliese 876 d, is clearly no Jupiter, Marcy realized. It is no more than seven or eight times as massive as our own: a “super-Earth.” Until then, all the known exoplanets (planets circling other stars) were big and gaseous, but this one is probably made of rocky materials—the first world like ours found in an alien solar system.
Gliese’s super-Earth lies so close to its star that it has just about no chance of being inhabited. If it has an atmosphere at all, it probably consists of dense steam, says Greg Laughlin of the University of California at Santa Cruz, a member of the discovery team. But if we can find one rocky, Earth-like planet right in our galactic backyard, surely there must be many more. Already, the Swiss astronomers who in 1995 discovered the first Jupiter-like exoplanet—and who are the great rivals of the California group in the exoplanet hunt—said in June that they had identified not one but three super-Earths orbiting a single star 40 light-years away. The smallest is just four times as massive as Earth. “We’ll find an Earth-mass planet by 2010,” Laughlin predicts, “and an Earth-mass planet that’s potentially habitable by 2012.”
And yet we still won’t have found a true second Earth. The hallmark of Earth, after all, is not its mass, nor its rockiness, nor the fact that it is potentially habitable. The hallmark is that it is actually inhabited. These days, nobody doubts that there are other reasonably cool, rocky planets out there among the 100 billion stars in the Milky Way. Everything astronomers have learned about how stars and planets form says there must be. But is there life on any of those rocks, and if so, can we detect it? “That’s not going to happen from Earth,” Laughlin says. “It has to happen from space.”
In space, above our atmosphere, stars do not twinkle; in space a telescope is also beyond day and night and can thus stare at the same star for weeks on end, gradually teasing from its light the barely perceptible but regular flickers caused by a small orbiting planet. A French satellite called Corot, the first space telescope devoted primarily to looking for rocky planets, is in orbit now. An even more capable American mission, Kepler, will be launched in April. It is expected to find hundreds of Earths, including the first ones orbiting stars like the sun at distances like that of our own Earth. Then, in 2013, NASA will launch a giant infrared telescope called the James Webb Space Telescope. An all-purpose observatory, the Webb was not designed to follow up on the discoveries of Corot and Kepler. But if pushed to the limit, it just might be able to provide the first indication of life—a telltale molecule, such as oxygen, in the planet’s atmosphere—on a super-Earth circling another star. By 2014 headlines could be announcing the first tentative evidence of life beyond our solar system.
One rainy Tuesday afternoon last November, Annie Baglin, the chief scientist of Corot, sat at the window of a café near the Place Denfert-Rochereau in Paris, drinking tea. It had not been an easy day. French railway workers, striking over their retirement benefits, had shut down the commuter trains, preventing Baglin from reaching her office on the suburban campus of the Paris Observatory. The railway workers and other civil servants were marching down the Boulevard Montparnasse, near Baglin’s home, brandishing bright red flares that filled the air with chlorine-scented smoke; off the boulevard, platoons of shield-toting, armor-wearing riot police stood nervously at the ready. Arriving late for her rendezvous at the café, wearing the dark pink coat she had said would make her recognizable, Baglin explained that her car had been towed—apparently the traffic wardens were not on strike. From the café Baglin would be proceeding to her dentist to have a tooth extracted. On the plus side, her spacecraft was performing beautifully.
As Baglin launched? into the story of the little spacecraft that could, in principle, find many rocky planets, her high, thin voice sometimes disappeared into the noise of the sirens outside. She is a shortish woman of 70, with close-cropped gray hair and a warm, no-nonsense demeanor—her parents were both schoolteachers. A brief profile of her on the Paris Observatory Web site is titled “Annie Baglin—Never Say Die.” Getting Corot to the launchpad, she explained, was a long, hard slog, marked by bureaucratic near-death experiences.
She never intended to be a planet hunter. In the mid-1980s she and her colleagues proposed a space telescope to do stellar seismology—to study the inner workings of stars from vibrations on their surface, much as seismologists study Earth’s interior by analyzing earthquakes. The French and European space agencies were noncommittal about the idea. Then came 1995 and the announcement of the discovery of the first exoplanet, by Michel Mayor and his colleagues at the Geneva Observatory. Baglin and everyone else immediately realized that a spacecraft designed to detect the light fluctuations caused by starquakes might also be able to detect a planet. Suddenly, Baglin says, “we were very much sought after. In hindsight, one can say that if it hadn’t been for the discovery of exoplanets, we never would have been approved to do Corot. That’s what sold it.”
Launched in December 2006, Corot is thus a 1,300-pound spacecraft that does two very different things. No telescope yet exists that can take a picture of even a giant exoplanet; astronomers compare the task to taking a picture of a firefly next to a searchlight thousands of miles away. Mayor and his colleagues showed instead that it was possible, through a technique called astrometry, to detect the slight wobble in a star’s light caused by the gravitational pull of an orbiting planet. Most of the 300-some exoplanets discovered since have been found that way. But Corot relies on a different technique that has lately come to the fore in ground-based searches as well. Called photometry, it detects the slight but regular dimming in a star’s light when a planet transits in front of it.
What the search for planetary transits has in common with the observation of starquakes is the need to stare at the same stars for a long time—long enough to detect very slow vibrations or to detect at least three transits of a planet. Otherwise, you can’t be sure it was really a starquake or a planet you saw, and not random fluctuations in the starlight. Corot stares at the same spot in the sky for 150 days before switching to another. “Corot is Zen,” Baglin says. “Once we’re set up, we don’t move. We don’t even breathe.”
The spacecraft’s 27-centimeter (10.6-inch) telescope monitors up to 12,000 sunlike stars at once. Getting a big sample is crucial because only one in a hundred of those stars that do have planets will be oriented so that the passage of the planet in front of the star is visible from Earth. The precision of the telescope’s measurements has exceeded its makers’ hopes. “If Corot were to observe the million lightbulbs that shine along the Champs-Elysées at Christmas,” said a press release from the Paris Observatory a few days before Christmas in 2007, “it would be able to detect whether a single bulb was flashing.” That parts-per-million sensitivity should allow Corot to detect the dips in a star’s light caused by a transiting planet with a radius just twice that of Earth—and perhaps an even smaller one, provided its orbit is tighter than Mercury’s, so that the planet completes three transits during the 150-day viewing period.
Not long after the launch, the Corot science team, including Baglin, published a description of the mission. It concluded with this prediction: “The first confirmed terrestrial planets are expected in the spring of 2008.” By last spring, however, the Corot team had announced only two new “hot Jupiters” and one unconfirmed super-Earth, with 40 more candidates in the pipeline. Seeing transits is not enough; periodic dips in a star’s light could be caused by a small companion star too dim to detect directly. To confirm a planet, Corot’s candidates have to be observed from the ground using the wobble technique, which determines the mass of the transiting object; a planet will be much lighter than a companion star. But competition for telescope time on the ground is fierce, especially with so many planet hunters around. “It’s a real bottleneck,” Laughlin says.