Requiem for the World’s Greatest Planet Hunter

The 2,740 planets detected by Kepler as they passed in front of their stars are depicted here as black dots. View the image full size to appreciate the true avolume of data. (Credit: Kepler/NASA)

Kepler’s results show that about one in six stars has one or more Earth-sized planets in a fast orbit. That translates to at least 17 billion such planets in our galaxy, and many more planets in slower, Earth-like orbits. Credit: F. Fressin (CfA)

A bewildering variety of planets have been found by the Kepler spacecraft. This illustration is based on best guesses about what those planets are like. Further study almost guarantees more surprises. (Credit: C. Pulliam & D. Aguilar/CfA)

After more than four years in space, restlessly searching for planets orbiting other stars, NASA’s Kepler space telescope may have met its demise.
The Kepler project is typically described in terms of raw numbers. As of the last official announcement, it had found 2,740 likely new planets–including 1,200 Neptune-size planets, 350 Earth-size planets, and at least 4 planets that orbit within the “habitable zone” where liquid water can exist. All of those numbers are sure to increase, as more observations are confirmed and as mission scientists continue to dig through a trove of archived data. But spirit, not statistics, is what really defines Kepler. It is a modern version of the expedition of Lewis and Clark, or the great voyages of Vasco da Gama and Ferdinand Magellan. It is a headlong plunge into the unknown cosmic territory around us.
Extrapolating from Kepler’s results, astronomers now estimate there are at least 17 billion Earth-size planets in our galaxy. That is another number, yes, but one with a powerful message: Another age of exploration awaits, one that may very well lead to the discovery that humanity is not alone in the universe. 
There could hardly be a sharper contrast between Kepler’s grandiose mission and the humble mechanical failure that threatens to end it. The fault lies in one of the telescope’s gyroscopic reaction wheels, a key component of its pointing system. Kepler needs three reaction wheels running at all times to maintain a stable position. Think of it like a stool: Two legs are just not enough to keep things balanced.
Extreme stability is essential because of the way that Kepler finds planets around other stars. It points a 0.95-meter (3-foot) light-collecting mirror toward the constellation Cygnus, watching approximately 150,000 stars and monitoring the tiny dimming that happens whenever a planet passes in front of one of them. And by tiny I mean tiny; Kepler seeks out brightness changes as slight as 0.01 percent. Since all the stars in Kepler’s field of view are quite close together in the sky, and since the shadows of passing planets are exceedingly subtle, there is little room for error.
Kepler started out with four functional reaction wheels, allowing for one to go bad without harming the mission. Last July, wheel #2 failed and was switched off. On May 12, wheel #4 apparently failed as well, leaving only two working wheels and sending Kepler into an automatic “safe” mode.
That is not necessarily the end of the line for the hardy telescope, which has been operating nonstop for 1,532 days. Scott Hubbard, former director of NASA’s Ames Research Center, notes that it might be possible to restart wheel #2 and see if it is now functional, which is possible if its lubrication shifted in the interim. Kepler has thruster rockets that could be used to stand in for the third reaction wheel, at least for a few months until the propellant runs out. NASA mission planners have also discussed trying to run wheel #4 backward or otherwise getting it somewhat operational again.
So a requiem for Kepler is somewhat premature. It is also not the right tone. If this is the end of the mission, there is far more to celebrate than there is to mourn. The Kepler team just posted an upbeat assessment of what comes next: “Even if data collection were to end, the mission has substantial quantities of data on the ground yet to be fully analyzed, and the string of scientific discoveries is expected to continue for years to come.” William Borucki, the principal investigator for Kepler, is optimistic that more Earth-like planets lurk within that trove.
Last week, an international team used Kepler data to identify a new planet, nicknamed “Einstein’s planet,” using a novel technique based on the effects of relativity. Multiple ground-based efforts will carry on the search for planets around other stars, and NASA recently approved a successor mission, TESS, that will specifically seek nearby Earthlike worlds.
This is also a good time to recall how extraordinary it is that the Kepler mission happened at all. Last year Borucki gave an in-depth interview with Andrew Grant, an associate editor at DISCOVER, describing the hurdles that he and his collaborators overcame to make the mission happen–a process that at times seemed more Sisyphean than Herculean.
In the 1970s, when Borucki began thinking about searching for extrasolar planets, nobody knew if such planets even existed. In the 1980s, when Borucki began sketching out the starlight-dimming search technique that Kepler uses, most of his colleagues scoffed. “The reviewers quickly rejected it,” he recalls. “They said there were no detectors that could make such precise measurements.”
I clearly remember the mood at that time, because I too was obsessed with the idea of planets around other stars back when it was more science fiction than fact. In the 1950s an 1960s, Dutch astronomer Peter van de Kamp reported evidence of planets around Barnard’s Star and 61 Cygni–but these claims were later debunked. In his book The Cosmic Connection, Carl Sagan showcased some elegant simulations of what other planetary systems might look like–but these were only theoretical models.
For years, Canadian astronomer Bruce Campbell championed the idea of watching the back-and-forth motions of stars to search for the gravitational pull of unseen planets. That technique eventually turned out to be spectacularly successful, but it required a level of precision beyond what Campbell could attain.
Campbell is a forgotten hero in the quest to find to find new worlds beyond our solar system. His pioneering work made possible the series of spectacular planetary finds that began with the discovery of a searing-hot, Jupiter-size planet around 51 Pegasi in 1995. Seven years before that, Campbell (along with Gordon Walker and Stephenson Yang) published a paper presenting evidence for a planet around the star Gamma Cephei. He admitted that the detection was tentative, since it was right at the limits of what his equipment could do.
The report received little attention, and Campbell’s name is missing from most stories about the discovery of exoplanets. He barely registers in a Google search (even after you weed out the other Bruce Campbell, the one who stars in Burn Notice). But the Gamma Cephei planet was eventually confirmed, and the 1988 paper by Campbell, Walker, and Stephenson is the very first verified detection of a planet around another star.
Whether or not this is the end of the Kepler spacecraft, Kepler’s mission–and the mission of Campbell, Borucki, and the many many others who made it possible–goes on. The next great moment in the age of planetary discovery lies just ahead.-NASA