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Planet hunters plot course for habitable worlds

December 29, 2014 This article courtesy of Nature News.

Researchers aim to set aside differences in search for life on distant spheres.

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Scott Gaudi is tired of the fighting. An astronomer at Ohio State University in Columbus, he specializes in the notoriously fractious field of exoplanet research, in which battles have included bitter fights over data access and epic rifts between teams searching for planets outside our Solar System.

On 4 January in Seattle, Washington, Gaudi will take a tentative first step towards corralling this rowdy bunch. As chair of NASA’s Exoplanet Exploration Program Analysis Group, he will try to nudge a roomful of US exoplanet scientists into generating a coherent, specific vision for where the field should go.

The time is right. Researchers have almost finished combing through the thousands of leads that were produced by NASA’s planet-hunting Kepler spacecraft between 2009 and 2013, and are squeezing some more data out of the craft’s limited ‘K2’ mission extension (see Nature 514, 414–415; 2014). By the mid-2020s, budgets permitting, astronomers expect to have a satellite called the Wide-Field Infrared Survey Telescope (WFIRST) busy cataloguing planets that are too far away from their host stars for Kepler to have spotted them.

Together, Kepler and WFIRST will produce a rough census of how many planets there are in our Galaxy. But NASA has yet to work out how to tackle the next, more crucial questions: could anything actually live on any of these planets? And what will it take to understand a given world’s chances of being habitable?

“The big thing we’re wondering now is: what is it that we want to do after WFIRST?” says Gaudi.

He and others say that it is not too early to start worrying. NASA prioritizes its missions according to community surveys that happen every ten years. Exoplanet science fared badly in the 2010 survey, partly because the community could not agree on a unified vision.

“We live in a time where, for the first time in history, we can potentially answer whether we are alone in the Universe,” says Lisa Kaltenegger, an astronomer at Cornell University in Ithaca, New York. “It would be such a shame if we don’t get to it.”

Nearly everyone agrees that the next big step would be a space telescope that could observe alien worlds directly. (Kepler uses indirect methods to infer the existence of extrasolar planets.) Back in 2000, NASA even started planning such a spacecraft. But those dreams foundered on the potential cost of the mission, and on the lack of technologies to make it happen.

A direct-imaging telescope would use one of two methods to block out the light of host stars and let it detect much fainter planets in orbit around them. One approach is a coronagraph, a disc that sits inside a space telescope and blocks the light of the central star so that the planet pops into view. WFIRST is currently planned to have a coronagraph that would enable it to image exoplanets directly, but the instrument may be cut for budgetary reasons. Another option is a starshade, an orbiting piece of opaque material that would position itself at some distance from a space tele­scope and block the star’s light from there.

“We’d all be very disappointed if there wasn’t some kind of imaging mission in the next decade, given how exciting and vibrant this field is,” says Suvrath Mahadevan, an exoplanet researcher at Pennsylvania State University in University Park.

For now, the European Space Agency is planning PLATO (Planetary Transits and Oscillations of Stars), a 2024 exoplanet mission that would not image planets directly. And US astronomers will keep trying to pick up more bits of information about exoplanets. The Transiting Exoplanet Survey Satellite, a NASA mission scheduled for 2017, will hunt for planets crossing the faces of half a million nearby bright stars. And the James Webb Space Telescope, due to launch in 2018, will explore clouds and atmospheres on relatively small exoplanets — although it is unlikely to see anything as small as Earth.

Still, those upcoming missions do not really address the question of whether humans are alone in the cosmos, says Aki Roberge, an astrophysicist at the Goddard Space Flight Center in Greenbelt, Maryland. “I believe the time is near,” she says, “when we should really try to tackle it with a mission capable of finding habitable conditions on nearby Earth-like worlds and seeing if they might support life.”

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