Focusing mechanism for the mirror on the Kepler spacecraft.
Courtesy of Ball Aerospace
Baglin has little patience for impatience or for the pressure on her to announce discoveries quickly. “Finding these things isn’t like finding the nose on your face!” she exclaimed, shortly before leaving the café last November to head for the dentist. The Swiss astronomer Mayor gathered data for 20 years, she pointed out, before announcing his first exoplanet. “So when people tell me, ‘You haven’t got any results,’ when we’ve only been in orbit for a year, I say, ‘Stop! Have mercy!’ In three years we’ll have results indicating how common small planets are. Big planets we know we’re going to find. We’re looking for the little ones. Are they there, or aren’t they?”
What about figuring out if one is inhabited? “To me, that’s not the big question,” Baglin said. “Understanding the universe in its totality interests me more than looking for life on a planet like Earth around a star like the sun, which is the declared goal of our competitors. That’s not uninteresting, but it’s not what excites me. I find that very anthropomorphic.”
Corot’s competition is Kepler, and Baglin’s is an astrophysicist named William Borucki at NASA’s Ames Research Center in Mountain View, California. In 1984 Borucki published his first description of the transit technique they would both end up using. At that time Baglin was having her first glimmerings of what would become Corot but had no notion of looking for planets. Nonetheless, Baglin, the planet hunter in spite of herself, beat Borucki into space with it. Now he is nipping at her heels. Kepler’s bureaucratic history was even more tortured than Corot’s, but the spacecraft is headed for launch on April 10, 2009, and astronomers are counting on it to settle the question of just how common Earths are—a result that will guide the whole future search for life in the universe.
I want to write the paper that says, “this is how many earths there are.”
Borucki well remembers the effect he had with that 1984 paper, published in the journal Icarus. “There was no effect,” he says. “It was pretty much ignored.” At that time most researchers thought the way to look for other planets was through astrometry. Borucki was convinced that looking for planetary transits through photometry would be simpler and cheaper. Measuring the brightness of a star over time, he reasoned, would require a much smaller space telescope than trying to take a picture sharp enough to resolve a planet or a tiny loop in the star’s trajectory.
Borucki’s peers were skeptical, though—first, that a transit of an Earth would even be distinguishable against the background noise of the star’s fluctuating light, and second, that it was possible to monitor 5,000 sunlike stars at once, as he proposed to do. Research on the sun during the 1980s laid the first objection to rest; starlight turned out to be a lot less noisy than astronomers had thought. Over periods of days, which is how long it would take an Earth to pass in front of it, the sun’s output varies by only around 10 parts per million, whereas the passage of an Earth would dim it by 84 parts per million.
So such a transit was in principle detectable, but Borucki’s initial idea for a detector still raised eyebrows. He wanted to drill 5,000 holes, one for each star, into a metal template and put it near the focal plane of the telescope, with an individual photodiode and integrated circuit behind each hole. “People in the industry refused to even talk to us about that design,” recalls David Koch, the deputy principal investigator, whom Borucki roped in to his quest in 1992. Borucki’s basic concept was rescued by the emergence of the charge-coupled device, or CCD—the light-sensing chip that was then new and is now in hundreds of millions of digital cameras. A CCD can record the brightness of many stars at once, thus eliminating the need for thousands of photodiodes.
Borucki and Koch first proposed their mission to NASA in 1994; they called it FRESIP, for “Frequency of Earth-Size Inner Planets.” They proposed it again in 1996, 1998, and 2000. “Each time they came back with a list of reasons why we weren’t selected,” Koch says. “It won’t work because of this, it won’t work because of that, they said. And we went back and worked on it until we eliminated every reason they couldn’t select us.” The researchers made sure, for instance, that the minute vibrations of the spinning gyroscopes that kept the telescope pointed at its target would not drown out the signal from a planet. The name of the spacecraft was an easier sell. After the first proposal, Koch suggested naming it Kepler, after the discoverer of the laws of planetary motion. In 2001 NASA finally approved the mission. Over in Europe Corot was getting the go-ahead at around the same time, albeit at a much lower budget.
Though the French won the race into orbit, Kepler will have a telescope measuring 95 centimeters (37.4 inches), 3.5 times the diameter of Corot’s, with a field of view more than 10 times as large. Above all, it will be in orbit around the sun, trailing behind Earth, whereas Corot is in low Earth orbit over the poles. To avoid looking too close either to the sun or to Earth, Corot has to turn 180 degrees twice a year, which is why it can’t look longer than five months at one set of stars. As it orbits the sun, Kepler will stare at the same spot for its entire mission—which will allow it to detect true Earth analogues, those circling stars like the sun in orbits lasting about a year. “The French will find the first terrestrial-size planet,” Borucki predicts. “But those planets won’t be in the habitable zone. We’ll find the first one in the habitable zone.”
Actually, he expects to find several hundred. Kepler’s field of view covers 100 square degrees, around 0.25 percent of the sky, or as Koch puts it, “about two scoops of the Big Dipper.” Koch chose the most star-rich patch he could find in the U.S. Naval Observatory star catalog. It lies just out of the plane of the Milky Way, between the bright stars Vega and Deneb. Of the 13 million stars cataloged in that field of view, Kepler will monitor a subset that are most like the sun in size, mass, and age—stars that are quiet and sedate like our own. Its 42 CCD detectors lie just outside the telescope’s focus. “You don’t have to have a sharp image,” Borucki explains. “In fact, a fuzzy image is better”—the starlight is spread over more pixels, which keeps them from saturating as quickly. Because it has CCDs, Kepler will be able to monitor 100,000 stars simultaneously. It will spot variations as small as 10 parts per million in their light output.
When Kepler lifts off from Cape Canaveral on April 10, Borucki will have spent 17 years on his idea. “You don’t spend this long and then quit 90 percent of the way through,” he says. “I want to see the answers. I want to hold the data in my hand. I want to write the paper that says, ‘This is how many Earths there are.’” He may find none—which would be the most surprising result of all. Neither Borucki nor anyone in the planet-hunting business expects or wants that. It would mean Earths are extremely rare at best. It would mean we really might be alone in the galaxy, if not the universe.
By the time Kepler reports its first results, the exoplanet landscape will no doubt have changed yet again. Astronomers all over the world are hunting a second Earth. In May Drake Deming of NASA was collecting data he hoped might reveal a super-Earth in the habitable zone of a red dwarf (a small and relatively cool star) called Gliese 436; NASA had allowed him to use a spacecraft called Epoxi, which is on its way to a rendezvous with a comet, to observe several stars that are already known to have planets. Also, last May Debra Fischer of San Francisco State University began training a small telescope in Chile on Alpha Centauri, a pair of sunlike stars that are the closest ones to Earth. Following up on an idea of Greg Laughlin’s, she believes that by observing it steadily for several years, she might detect not only an Earth but a Mercury and a Venus as well. “That’s what we’re going for—not just the first Earth but the first terrestrial planetary system,” she says. “It’s a pretty bold and crazy plan.”
