Reports of the death of our principal planet hunter have been greatly exaggerated. The prolific Kepler space telescope may instead be entering early retirement, spending its golden years seeking out planets with a gravitational magnifying glass.
Since its launch in 2009, NASA's Kepler mission has discovered 132 exoplanets and thousands of other possible worlds, making it one of the most celebrated exoplanet missions.
To catch sight of far-off worlds, Kepler must stare at stars with an unwavering eye, looking for tiny dips in starlight when a planet transits, or crosses in front of, its host star. To do this, the craft needs at least three orientation-controlling reaction wheels to stabilise its vision. Two of its four wheels have now failed.
Shaky eyesight doesn't have to mean curtains for Kepler, says Keith Horne of the University of St Andrews, UK. He and Andrew Gould at Ohio State University in Columbus suggest that the hobbled telescope can use its gear to take up microlensing, an alternative way to spot planets.
When two stars align in our line of sight, the gravitational pull of the closer star bends and magnifies the light of the further star. If the nearer star has orbiting planets, their gravity provides added magnification.
"The signals from planets are quite large in this case, sometimes even a 100 per cent change of brightness of the star, so it's relatively easy to see these things," says Horne.
The pair estimates that Kepler could find a few dozen exoplanets a year with microlensing. These would be very different from its previous quarry. With transits, it is easier to spot planets that orbit close to their stars – crucial for finding worlds warm enough to host liquid water, and maybe life.
Microlensing is better for finding planets further from their stars – past a region dubbed the snow line – where it would be too cold for liquid water. Studying both extremes would help us understand the borders of the habitable "Goldilocks" zone in between, says Horne.
Although ground-based observatories can use microlensing to discover planets, Kepler is in a unique position. It orbits the sun, trailing Earth by millions of kilometres, so it can watch for stellar alignments that we can't see from our angle and it could spot previously undetectable exoplanets.
Combining Kepler's microlensing data with simultaneous views of systems taken from Earth would also provide better estimates of the distances to these planets, just as having two widely spaced viewpoints of a distant mountain allows you to triangulate its distance better.
Not dead yet
Kepler was not designed for microlensing though, so it might be more cost-effective to use funds for a dedicated mission, says Geoff Marcy of the University of California, Berkeley. NASA is considering one such proposal called the Wide Field Infrared Survey Telescope (WFIRST), which may be built from one of the two spy telescopes the agency received from the US Department of Defence last year.
WFIRST isn't due to launch for at least a decade, though, says Horne. "We think it is better to do microlensing with Kepler now," he says. "You could learn so much that you might change the design of the future mission."
Meanwhile, NASA engineers are still trying to coax one of Kepler's reaction wheels back into service, letting it continue its original mission. "They're working on the patient, not planting tulips on its grave," says Marcy.