Kepler’s dream (Image: T. Pyle/JPL-Caltech/NASA)

“Kepler was my North, my South, my East and West, my working week, no weekend rest, my noon, my midnight, my talks, my song; I thought Kepler would last forever: I was wrong.”

So laments planet hunter Geoff Marcy, with a due nod to W. H. Auden, upon hearing the news that NASA’s Kepler space telescope is probably close to ending its search for extrasolar planets.

Since its launch in 2009, Kepler has discovered 132 exoplanets and identified nearly 3000 more possible planets, including a handful of potentially rocky worlds that may be able to host life. The mission’s plethora of data has transformed our view of the galaxy, showing, for instance, that Earth-sized worlds should be common, and that one may even be lurking a mere 6.5 light years away.


But the pioneering telescope has been hobbled by a damaged reaction wheel, NASA announced at a press conference today. These wheels help Kepler keep its orientation in space, and precision steering is crucial to the mission.

Rest and relaxation

The telescope has been watching some 150,000 stars near the constellation Cygnus for changes in brightness caused by a planet crossing, or transiting, its face as seen from Earth. To do this, it needs to keep the same specific pixels of its light detector trained on given stars for months at a time.

Kepler started out with four reaction wheels – one to control its motion around each axis and one spare. One wheel stopped turning in July last year, leaving the telescope with no backup. In January, a second wheel began misbehaving, so mission managers gave Kepler a small break from planet hunting, in the hopes that a spot of R&R would fix the problem.

Despite the Kepler team’s best efforts, on Sunday the space telescope put itself into safe mode, an automated stop of its science operations that is usually triggered by a system glitch. Follow-up communication from the team showed that the wheel had stopped moving and that it is not responding to attempts to restart it.

“Unfortunately, Kepler is not in a place where I can go up and rescue it,” John Grunsfeld, a former NASA astronaut who now works in the agency’s science directorate, said during the press briefing. The prognosis is not good, but the team is not quite ready to give up the ghost. “We’re looking at the data very carefully to see if it’s possible to get back into science mode,” said Grunsfeld. “I wouldn’t call Kepler down and out just yet.”

Tragic timing

Word of Kepler’s woes was not exactly a surprise. The mission was initially funded for three and a half years and has been running on an extended time frame.

“It was already known that it wasn’t going to go on for decades,” says astronomer Daniel Fabrycky of the University of Chicago. “Its days were numbered.”

Still, if Kepler cannot be brought back into action, the timing will be tragic for scientists hoping to realise the mission’s primary goal: determining what fraction of sun-like stars host Earth-sized planets with temperatures favourable for life.

Astronomers need to see at least three transits to confirm a signal as an orbiting planet. That means planets farther from their stars, in the habitable “Goldilocks zone” – in which a planet’s surface temperature is just right for liquid water to exist – will take longer to find. Kepler needs a few more years’ worth of data to see planets around sun-like stars with orbits that last longer than 200 days, says exoplanet researcher Sara Seager of the Massachusetts Institute of Technology.

“It will mean that the mission won’t confirm as many small planets in the habitable zone as had been hoped,” says Fabrycky. “It’s a real loss.”

Earth twins await

But Kepler team member William Borucki points out that even if Kepler’s main mission is over, the team has about two years of data that have not been fully searched. He is optimistic that the bounty still holds some surprises. “We’re really pretty positive we’ll get Earth-sized planets in the habitable zone around stars like the sun. I’m confident that the data we have will allow us to accomplish that,” he said at the press briefing.

There will be a gap of at least four years before the next exoplanet-chasing telescopes come online. NASA will launch the Transiting Exoplanet Survey Satellite (TESS) in 2017 to scan the entire sky for exoplanets. The European Space Agency also plans to launch the Characterizing Exoplanets Satellite (CHEOPS) in 2017, with the goal of following up on known planetary systems and searching for traces of life.

For Fabrycky, such missions are a sign of Kepler’s lasting legacy. “The people who worked on it will go off and do other things, but in its wake it will have inspired a lot of people to join this field.”