For its entire mission, Kepler’s view will remain fixed on one of the spiral arms of our galaxy, on a field of stars in the constellations Cygnus and Lyra. Most of the stars it sees lie between 600 and 3,000 light-years from Earth, but some are as close 30 light-years. The cyclic dimming of more distant stars would be too faint for Kepler to measure. Although Kepler cannot peer nearly as far into the cosmos as the Hubble telescope, its view is far more panoramic, as wide as 27 full moons across the sky. Hubble focuses on a dot of sky no larger than what you would see by looking through a coffee stirrer.
The challenge for Kepler—or more specifically, for Jenkins’s software—is to tease out brightness changes caused by the passage of a planet and to distinguish them from all the normal stellar variations, such as flares and star spots (the stellar equivalent of sunspots) or even nearby eclipsing stars. As with many things in life, timing is crucial. Planets give themselves away by the length of time it takes them to pass across the face of a star—typically a few hours. Star spots, which are embedded in a star’s surface, typically rotate on a scale of days or weeks. So Jenkins’s software searches for dips in brightness lasting up to half a day. If a planet is indeed the cause of the change in brightness, the exact same change should recur days, months, or years later, depending on how long the planet takes to orbit its star. Ideally, the Kepler team waits until the spacecraft has recorded three identical dips in brightness separated by equal intervals before concluding that they have probably found a planet.
Kepler’s onboard computers can store a bit more than two months of data; the data are highly compressed for efficient storage and transmission to Earth, where NASA’s servers can hold a total of about 60 terabytes. Without the compression scheme, the Kepler mission never would have flown. “It would have taken us five times as long to downlink the data and five times as much hardware to store the data on board,” Jenkins says. “Compression was absolutely crucial to the mission.” As it is, the spacecraft has already beamed down more than a terabyte of brightness measurements. Kepler astronomers have enough data to keep them busy for decades.
NASA is planning to release results from Kepler regularly through November 2013, when the mission is scheduled to end. The most sought-after results still lie ahead. After Jenkins and his colleagues have weeded out sunspots and other planet poseurs from the data, Marcy and other astronomers use the Doppler wobble method with terrestrial telescopes to verify that the remaining planet candidates, or “objects of interest,” are indeed planets. The process will demand at least three years to find a completely Earth-like planet: one that is in a yearlong, Earth-like orbit around a star just like the sun.
The grand prize for Kepler would be to find a world just like ours—no bigger, no smaller—orbiting a sunlike star in an orbit the same size as Earth’s. That challenge lies at the very edge of Kepler’s remarkable capabilities. And it is definitely beyond the reach of even the most powerful Earth-bound telescopes.
Doppler measurements have been used to confirm the existence of Earth-size planets discovered by Kepler in habitable zones around stars smaller and dimmer than the sun. Kepler-10 b is one such example. But Doppler measurements won’t be able to confirm a true twin of Earth: A planet’s gravitational effect on its star depends on their relative mass and the distance between them. An Earth-size world close to a lightweight, dim star is much easier to find than a planet more like our own. Marcy laments, “We won’t ever definitely verify or measure the density of true Earth clones that orbit as far as our own Earth does from the sun.”
There is a chance, though, that Kepler will get lucky. If the spacecraft finds an Earth-like planet circling a star orbited by at least one other planet, Kepler’s data alone could be used to determine the masses of the planets without measuring the star’s wobble. The gravitational interactions of the two planets would affect the timing of their orbits, which is something Kepler could measure. And the gravitational interactions, in turn, would reveal the planets’ masses. The Kepler team has already pulled off that feat for two planets orbiting a star called Kepler-9, about 2,000 light-years from Earth. The planets orbiting Kepler-9, astronomers determined, are such low density they would float if placed in a giant bathtub filled with water. Nobody understands the nature of these bloated worlds. “It’s a beautiful, beautiful example of what Kepler can do,” says Natalie Batalha, the mission’s deputy science team leader.
Kepler may well find planets that closely resemble Earth. Or it may continue to upend our notions of what is out there. “All planetary systems were supposed to look like ours,” Borucki reminds me. “The planets we’re finding are in the wrong places! And their orbits are unlike anything anyone predicted. Now, that’s a warning. That tells you we don’t know how to predict what’s out there.”
Kepler has proved that planets are the rule in the galaxy, not the exception, but it won’t tell us if any of those planets actually harbor life. That’s a question for future missions, and NASA has plans for an ambitious one. Called the Terrestrial Planet Finder, it would study the atmospheres of exoplanets and search for the chemical signatures of life: oxygen, water vapor, carbon dioxide. But congressional budget cuts have left the mission in limbo for now. It’s a project sorely in need of its own Bill Borucki.
“If there is no life in the entire galaxy, that would still be pretty profound for mankind,” Borucki says as we sit in his office. “So I think this is the most important problem we can attack. And I’m going to have to leave it to my grandkids.”
Despite Kepler’s jaw-dropping success, Borucki still frets about the mission, and one of my last questions touches a nerve. “By the end of Kepler’s mission, three years from now . . .” I begin to ask, but Borucki doesn’t let me finish.
“No!” he says, leaning forward over his desk, his soft voice rising in volume. “It’s going to go on at least six years. Don’t let people tell you that just because it’s funded for three and a half years it’s going to quit. It is not. We’re going to get the money and continue for six years. The program managers don’t believe that, but I believe it.”
Then, pausing after each word and rapping his knuckles on his desk for emphasis, he says, “You—can’t—shut—the mission—down. It’s working, and as we look longer and longer, we’ll find more and more small planets. Do you want to throw away an opportunity to find a huge number of planets in the galaxy because you didn’t come up with the money? It’s the only mission in the foreseeable future that can do this job and you’re gonna turn it off? Nobody’s gonna turn it off.”