• Space Exploration

Bill Borucki's Planet Search

Finding another Earth may be easier than the Kepler project's long quest for funding.

• By Andrew Lawler
• Air & Space Magazine, May 01, 2003

[Editor's Update: The Kepler planet-hunting telescope is scheduled to launch from Cape Canaveral on March 5, 2009.]

It should have been a moment of triumph: The maverick prophet finally welcomed into the high temple. After 20 years of dogged and often lonely effort bordering on obsession, Bill Borucki had won approval from NASA to build the first spacecraft designed to find Earth-size planets beyond our solar system. If successful, his mission, named for Johannes Kepler, the astronomer who calculated laws of planetary motion, could rewrite our understanding of solar system formation and single out targets in the search for extraterrestrial civilizations.

But in January 2002, just weeks after the mission’s approval, Borucki’s legendary patience was tested by a meeting held with officials in a windowless room at NASA’s Washington, D.C. headquarters. Agency managers first offered congratulations, only to sheepishly explain they were short on cash and had to delay the project’s launch by a year, to 2007. What’s more, the team would have to turn control of the project over to a rival NASA center. When Borucki informed his colleagues—a close-knit mix of seasoned space veterans and eager postdocs—they were astonished, then furious. “People turned colors,” Borucki recalls with a smile.

After living in the wilderness for so long, Borucki was not about to let small matters such as money or management derail his project. While others on his team continued to rant and fume, Borucki quietly returned to his office at NASA’s Ames Research Center, south of San Francisco, and set about doing what the agency hierarchy requested.

“All I care about is the science,” he says one evening over Chinese food. Coming from most scientists, the statement would sound naive or simply platitudinous, but Borucki leans his spare frame forward, and this time he doesn’t smile. “And when I go home tonight,” he says, “I will work.”

Counting Photons
The search for extrasolar planets is one of the most spirited pursuits in modern astronomy, with potential Nobel Prizes driving physicists as forcefully as innate curiosity. Since a Swiss team located a giant planet in the constellation Pegasus in 1995, researchers have racked up around 100 planetary systems and many more candidates. The race now is to find Earth-size planets; the holy grail is to find one in the habitable zone, where temperatures would allow liquid water—and therefore possibly life—to exist.

The standard method for planet searching, astrometry, involves looking for a regular wobble in the parent star—a sign of its brood’s gravitational tug—but the worlds discovered by this technique are typically larger than Jupiter and often orbit as close to their stars as Mercury does to the sun. Though important for the impact they’ve had on solar system formation theories, the planets are hardly Earth-like. With current technology, astrometry is also limited; at best it can spot terrestrial planets only around the nearest stars. And enormous practical challenges involving precision flying and mirror technology still have to be solved before interferometry and coronagraphy, conventional ground-based approaches, will work from a platform in space.

To find smaller planets at greater distances, other methods are needed; it’s here that Borucki is cutting a new path. His small and relatively simple spacecraft, Kepler, will employ a revolutionary technique called transit photometry, which precisely counts photons from a star’s light to detect periodic dips. Kepler will fix its eye on 100,000 stars in the constellation Cygnus, and during a four-year mission around our sun it will stare relentlessly at those stars for dimming, which could mean planets are passing over the faces of—transiting—their home suns. Three dips of the same duration and degree and at equal intervals would confirm an orbiting entity.

Cygnus, lying 55 degrees above the solar system’s ecliptic plane, is a particularly good target; it’s home to a dense star field and can’t be obscured by asteroids or sunshine. Borucki’s team will study around 135,000 of the constellation’s dwarfs—those similar to our sun—and after a year weed out roughly 25 percent that are too variable for transit spotting. Before budget issues and hardware reliability come into play, Kepler will have the opportunity to see up to four transits by planets in one-year orbits and up to three by those in 1.9-year orbits.

Sweat and Zeal
Borucki’s single-minded zeal is as clear as his youthful blue eyes. Born in Chicago in 1939, he grew up in Delavan, Wisconsin, “between Yerkes Observatory and the Playboy Club on Lake Geneva,” and expressed interest in astronomical matters early on. While in his teens, the town sheriff would close off roads so that Borucki and his buddies could launch 10-foot multi-stage rockets. Borucki’s father, an inspector at a clock factory, procured timing mechanisms for them.

In 1962, a year after President Kennedy challenged the Soviets to a moon race, Borucki, fresh from a physics degree at the University of Wisconsin at Madison, landed at Ames, where he studied the effects of radiation on reentry vehicles—work that was used to design Apollo heat shields. But what ultimately fired his passion was the possibility of discovering other worlds.

He was in the right place. In the 1970s Ames hosted a session on space colonization, and it also was the home of NASA’s Search for Extraterrestrial Intelligence. Borucki got to know many of SETI’s legendary figures, including Carl Sagan and Jill Tarter.

In the summer of 1982, Borucki looked in on an Ames conference on extrasolar planets, a far-out topic at the time. Transit photometry was mentioned only in passing because detectors of the day simply couldn’t measure stellar variability to the degree necessary. “We needed precision of one or two parts in a hundred thousand, and no one knew how to get there,” he recalls.

The idea of transit photometry wasn’t new. Astronomer Frank Rosenblatt speculated in a 1971 paper that the method could prove a valuable tool, but he died shortly thereafter.

Borucki picked up the thread and became increasingly intrigued—some would say infatuated—with the possibilities. He published a couple of papers on the subject, and in 1984, he somehow persuaded the director of Ames to fork over enough money from his discretionary purse to fund a small conference on the subject.

Roughly 20 astronomers attended the meeting, held in San Diego, and decided that it was theoretically possible to build such detectors. Scientists at the federal National Bureau of Standards suggested silicon diodes as quantum-perfect detectors—devices that would spit out a single electron for every photon of light absorbed. For three years Ames’ director used his discretionary fund to pay for Borucki’s development of silicon diode detectors, but Ames managers questioned how well the detectors would operate in space and remained skeptical.

In the late 1980s, Borucki’s team began to look at charge-coupled devices—technology more familiar to the astronomical community. Unlike silicon diodes, which can monitor only one star at a time, CCDs are array detectors that can survey thousands of stars simultaneously and are ideal for digital data gathering. Borucki had tested CCDs years before and had been disappointed with their degree of precision, but his team determined that newer CCDs were capable of the precision necessary to detect minuscule brightness changes. Though they’d never be as precise as silicon diodes, Borucki chose to go with CCDs for credibility.

“I hate CCDs,” he says, chuckling but serious. “The only reason I chose them was because I knew I had to convince the community that we could do the job. It was easier to convince them with CCDs than it was with silicon diodes.” His spacecraft would eventually require dozens—the most recent design calls for 42, compared to the Hubble Space Telescope’s four.

Here to There
In early 1993, Ames called for a review of Borucki’s work. Just as space-qualified CCDs were becoming commercially available, Borucki gave an eight-hour presentation that convinced Ames of his project’s feasibility.

New data from NASA’s Solar Max mission lent a supporting hand. “Solar variability was coming out to about 10 parts per million. At the time scale of a transit, solar variability is much smaller than a transit,” says Dave Koch, Kepler’s deputy principal investigator and Borucki’s right-hand man. “The [review] committee said ‘Yes, this can now be done. It’s not just a cute idea, it really is a practical idea.’ ”

From then on, the center quietly provided a steady stream of money for the project. Ames officials backed the high-risk research for its own sake, but they were also aware of the interferometer work being done by their NASA rivals at the Jet Propulsion Laboratory in Pasadena, and entering the planet race appealed to their competitive instincts.

By 1994, Borucki and his team had closed in on the algorithms necessary to make sense of CCD data, and the project, nicknamed FRESIP (Frequency of Earth-Sized Inner Planets), was ready to compete for a NASA Discovery mission. (Discovery missions were to reflect NASA chief Dan Goldin’s vision of cheaper and more tightly focused projects that could be launched 36 months from selection and cost no more than $299 million from design through launch and data analysis.)

Though NASA liked the concept of FRESIP, it was shot down by reviewers, who said the effort would cost far more than the team anticipated. Several team members believed NASA overestimated costs by simply scaling down the expenses of bigger, more inherently complex missions. “We were put in the same box as the large high-precision astronomical telescopes,” says Larry Webster, Kepler’s project manager and an old NASA hand who has worked closely with Borucki for the past decade.

Two years later, Borucki and his team were ready again. This time they had three groups check out their costing scheme, but reviewers said that the system, renamed Kepler to distance it from FRESIP’s price tag flap, didn’t seem capable of imaging the promised tens of thousands of stars. So the team built a camera with a CCD and began testing it in October 1997 at Lick Observatory in the nearby mountains. Lacking funds to hire anyone to operate the device, called Vulcan, Borucki organized a program in which volunteers would take care of the night shifts required to keep Vulcan functioning.

It wasn’t too hard to convince people to pitch in. Researchers at SETI were eager for the Kepler team’s data so that they could point their giant radio telescopes at systems with Earth-size planets. Astrobiologists at Ames strongly supported the mission too. “We really believed in the potential of the project so we donated our time,” says Ames Integrative Studies Lead Lynn Harper. After many treks up to the observatory, Harper and other volunteers managed to find the money to enable Borucki to hire an operator.

Though the tests weren’t designed to spot terrestrial planets, other discoveries, later verified by spectroscopy, proved that the remotely operated camera was photometrically precise. “He’s shown that he can find eclipsing binaries, which are as hard to find as looking for transits,” says Koch.

Go Prove It
When the team came back with yet another Discovery proposal in 1998, reviewers complained that in space, cosmic rays or noise and jiggles in the spacecraft could interfere with the precision of the detectors. The Kepler team took it in stride. “They always loved the science [but] they always had a technical question about our ability to do the job,” says Larry Webster.

This time NASA granted Borucki a half-million dollars to build a demonstrator that could prove the proposed system would work. It was an unprecedented step for NASA headquarters. Ames agreed to match the amount, and within 88 days—Webster counted them one by one—the team assembled an end-to-end ground system. “We worked seven days a week, and had most of the machine shops in the [San Francisco Bay area] working with us,” Borucki says.

The Kepler Tech Demo, a 10-foot phone-booth-like steel-and-styrofoam frame, surrounded a single CCD, a coolant system, and other hardware. It took six months to get it working, and Borucki grew nervous. “We were spending Ames money like crazy,” he recalls, “and I was waiting for the moment they would say, ‘Hey Bill, great try, why don’t you move on to something else?’ ” The results from the contraption, however, clinched the deal: The Demo detected simulated planetary transits—brightness changes of 100 parts per million—in a mocked-up 1,600-star sky. In December 2001, reviewers ran out of criticisms. Kepler was chosen as a Discovery mission.

For Webster, NASA’s challenge made all the difference. “It was kind of a ‘We don’t quite believe you can do it. Go prove it. When you’re ready, come back,’ ” he recalls. “We did that in spades and came back in the 2000 proposal and there was just nothing left to critique. We were perfect.”

It wasn’t an unalloyed victory, though. Weeks later, following Borucki’s meeting with NASA managers, the Kepler team swallowed hard and turned mission development (everything up to launch) over to JPL—one of the two NASA centers, along with the Goddard Space Flight Center, designated to carry out missions beyond Earth orbit.

Nailing the Numbers
With Kepler’s selection, Borucki’s biggest victory may have been on behalf of Ames in its sibling rivalry. Up to that point, says Kent Cullers, who is in charge of R&D at the now-private SETI, “JPL has had the lion’s share of the R&D funding” for finding Earth-like planets.

JPL managers, once critical of the project, now sing Kepler’s praises. “Kepler will do a great job nailing down the numbers” of terrestrial planets, predicts Charles Beichman, chief scientist for NASA’s Terrestrial Planet Finder program. Its findings may help determine the TPF mission’s design. “We’ve pushed Kepler aggressively in the last four or five years,” he adds, though he admits that in “the mists of time,” questions were raised about Kepler’s efficacy. “Now it is very solid and credible,” Beichman says.

Beichman and others are nevertheless quick to point out Kepler’s limitations; imaging stars mostly at great distances using photometry, Kepler can find evidence of terrestrial worlds but can’t provide more specific characteristics. So while astronomers will learn much about the distribution of such planets, it will be up to later missions to pinpoint what kinds of atmospheres and surfaces such worlds might have—and whether life exists on them.

NASA will seek answers to these questions with its Origins missions, which will include Hubble’s successor, the Next Generation Space Telescope; the Space Interferometry Mission, which will search among wobbling stars for planets; and the still-far-off TPF mission, which will attempt to photograph distant planets and study their sizes, positions, and atmospheres.

The Kepler team’s greatest challenge will be deciphering the reams of data its spacecraft beams back. Gregory Henry, an astronomer at Tennessee State University, says that the tough part will be differentiating between a small planet and stellar variability, or detecting the existence of a nearby binary that is subtly eclipsing the target star. Small dead suns circling active stars also could fool Kepler. And many small planets won’t cross between their sun and the spacecraft.

The way to resolve the problems is by doing intensive ground-based observations to determine with precision the movement of each star, its mass, and its rotation speed to ensure that the dimming effect is in fact caused by a planet. “I see no showstoppers,” says Henry, “but it will be difficult to interpret the results.” Borucki insists that his scientific team, which includes many astronomers with access to large telescopes, is aware of the interpretation tangles. Cullers says that SETI is already excited to use Kepler’s data.

European Competition
If imitation is the sincerest form of flattery, then Kepler should no longer be considered an oddball program run by a maverick. Two ground-based international networks, Transits of Extrasolar Planets and Optical Gravitational Lensing Experiment, have used the transit method to find evidence of planets, though all have been substantially larger than Earth. Last January, OGLE announced the discovery of yet another Jupiter-size planet, this one 5,000 light-years away—the most distant found—with an orbit of just 29 hours and surface temperatures that probably reach 3,000 degrees Fahrenheit.

Borucki is confident there are plenty of smaller, more hospitable worlds in our galaxy, but is mindful that finding extrasolar life is the ultimate goal. “The next step is to go there and join the club of intelligence” or, he believes, if no other terrestrial planets are found, accept the idea that we are likely the lone sentient beings in the Milky Way. “This is like Columbus,” he says of Kepler’s search. “How much would you pay to be part of that expedition—to be first?”

European researchers are laying plans for two photometric space missions as well. The first, called COROT, would be able to spot only planets 10 times larger than Earth. Its 2004 launch date, however, is in question due to budget troubles. The other mission is a more formidable competitor for Kepler. Dubbed Eddington, after British astrophysicist Arthur Eddington, whose measurements of gravitationally bent starlight confirmed Einstein’s general relativity theory, it will also look at Cygnus and study its distant stars for potential terrestrial companions.

Compared to Kepler, Eddington, slated for a 2008 launch, won’t last as long or image as many stars, and those involved are quite aware that Borucki’s team has a jump on them and an excellent chance of finding planets. However, Europe has excelled in extrasolar planetary detection, and researchers there are loathe to give up the early lead. “The Kepler team has put years of effort into ensuring that all aspects are understood,” says Alan Penny, an Eddington team member based at Britain’s Rutherford Appleton Laboratory. He warns, however, that launch dates can change—a polite hint that Europe could try to scoop Kepler.

Borucki’s affable manner vanishes at the mention of a race. “We will beat the Europeans in any competition,” he says forcefully. “We have the expertise and the support.” His eyes quickly soften. It is late. And there’s no doubt he will be working into the night.