On Kepler‘s retirement

Six years ago, I visited NASA’s Ames Research Center near San Francisco to interview William Borucki, the principal investigator of the Kepler mission. The then-73-year-old had spent a third of his years working to put a planet-hunting telescope into space. My goal was to learn what made him tick.

Did I learn. Over several hours in his office and at lunch, Borucki touched on his childhood, NASA bureaucracy, and his preferred name for the planet-hunting mission: the accurate yet dull FRESIP—Frequency of Earth-Size Inner Planets. But I saw him truly light up only during our extensive discussions of the optics that made Kepler possible. I nodded and scribbled notes as he showed me lenses, CCDs, and other instruments and explained how they worked. It was a bit much for a wide-eyed reporter with little background in optics to process.

Now, as the nearly decadelong mission comes to an end , it’s clear how the passion and determination of Borucki and his talented colleagues translated to scientific success rivaling that of missions several times more expensive. Those attributes also explain why, in the end, despite major setbacks that could have killed the mission years ago, the spacecraft stopped its work only when it ran out of fuel.

Borucki’s love of optics, the root of the Kepler mission, dates back to the Apollo era. In 1962, fresh out of a physics graduate program at the University of Wisconsin, Borucki joined NASA to help develop heat shields for the crewed capsules that, if all went as planned, would soon be returning from the Moon. Borucki’s job was to decode the light emitted by the shock waves created when his colleagues sent model spacecraft plowing into air fired at Mach 15 from old battleship guns. He became a photometry junkie.

That skill came in handy after the Moon missions ended and NASA scientists transitioned to thinking about robotic missions to explore the solar system and beyond. Whereas some researchers wondered whether a telescope could detect the pull of a distant planet on its host star via astrometry, Borucki viewed the problem through a different lens. He explored the idea of looking for subtle dips in the intensity of starlight—shadows—caused by a planet crossing in front of its star. Borucki would spend the next couple of decades convincing NASA that he and his growing team of colleagues could build an instrument capable of reliably detecting brightness changes of 10 ppm, the level of precision required to identify Earth-size planets transiting Sun-size stars.

By the time Kepler rose skyward in March 2009, the astronomy community was more than a dozen years into the exoplanet era. But we still knew very little about the galaxy’s planet population. At that time, the exoplanet catalog contained a few hundred entries, nearly all of them “hot Jupiters” that were sufficiently massive and tightly orbiting to substantially jolt the motion of their host stars.

Nearly a decade later, Kepler has provided a robust survey. It has identified nearly 2700 planets and flagged another 2900 likely candidates, numbers that Borucki says are in line with the team’s pre-mission expectations. Of course, it’s one thing to have strong theories that the formation of planets goes hand in hand with the formation of stars; it’s another to have overwhelming evidence that the galaxy hosts more planets than stars.

For all the success of the mission, we can still play the what-if game. Kepler stared at the same patch of 150 000 stars for more than four years, longer than the time allotted for its primary mission. But in May 2013, the second of four reaction wheels that steer Kepler failed, which prevented the telescope from remaining fixed on a single target. The unplanned end of the initial survey meant that although Kepler captured a sufficient snapshot of short-period planets, it didn’t secure enough data to guarantee the detection of truly Earth-like worlds : relatively small planets with roughly yearlong orbits around Sun-like stars. The fact that mission engineers found a way to stabilize the spacecraft with sunlight is remarkable, and the resulting K2 mission led to discoveries well beyond Kepler‘s original scope . But Earth’s twins may remain shrouded in secrecy for a while longer.

The silver lining is that Kepler rounded up a far more diverse set of worlds than the most optimistic astronomer could have envisioned. Among the potpourri are planets larger than Earth but smaller than Neptune, planets that may be made almost entirely of water, and warm planets orbiting the coolest and most numerous kind of star , M dwarfs. Though astronomers worry about factors like tidal locking and x-ray flares, M-dwarf stars are incredibly long-lived, giving their planets plenty of opportunity to spark and sustain life. In fact, the ultimate legacy of Kepler may be the knowledge that having an Earth-size world in the habitable zone of a Sun-size star is not the only—and perhaps not even the optimal—recipe for life.

Looking back on my 2012 interview , I keep returning to Borucki’s thoughts on where Kepler belongs in the pantheon of NASA endeavors. Just over three years into the mission, he had already ranked it as the most important one NASA had ever flown—ahead of the Hubble Space Telescope and tied with the Apollo voyages he helped bring home.

Even today, many of us less biased observers would dispute that ranking. But our judgment may change if and when the known population of worlds with life surpasses one. Kepler‘s successor, TESS, is already finding planets orbiting nearby stars that are easiest to analyze in detail. If the James Webb Space Telescope, WFIRST, or some other powerhouse space telescope acquires decisive evidence of extraterrestrial biology, we’ll have to point back to the robotic shadow hunter that opened our eyes to our galaxy’s bounty of planets.