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Astronomers Suggest Some Exoplanet Signals Are False Alarms

Astronomers Suggest Some Exoplanet Signals Are False Alarms
From Wired - April 12, 2018

Until recently, Fergal Mullaly worked for the science office of the Kepler space telescope, the planet-hunting satellite that has verified more than 2,600 planets so far. We have this really strong emotional desire to be able to point to a place in the sky and say, That star there has a planet around it, he says.

For the four years of its main mission, from 2009 to 2013, Kepler fixed its gaze on one region of the sky and watched the light from its stars. If a planet, endlessly ellipsing, passed in front of a star, that light would dim a bit. Kepler's most exciting discoveries have been exoplanets around the same size as Earth, ones that could plausibly have liquid water.

Mullaly just left astronomy to work at an image-analysis company called Orbital Insight. "I felt I'd given astronomy a long inning, and I was ready to try something else," he says. But right before that, he wrote his farewell paper, along with four still-scientists. In the article, they pointed out a potential problem with a portion of Keplers discoveries: Some of the potentially Earth-like planets arent necessarily there at all. Given the data that we have, you can say, Theres definitely planets out there, says Mullaly, 'but exactly which ones are which, its hard to know.

Those planetary transits that Kepler monitors? Frustratingly, other phenomena mimic themanother star passing in front of the star, for instance, or instrumental effects from Kepler itself, adding up in just the right way. Astronomers have to rule out those false positives to declare a planet a planet. The instrumental issue, claims Mullaly's farewell paper, is significantand insufficiently consideredfor small planets that take 200+ days to orbit their stars. For, in other words, worlds like ours.

Fergal would say, These planets are all junk, says astronomer Chris Burke, also on the paper. And I would say, No, theyre fine. But as the team started to dig into the data, Burkes conviction started to wobble. Unfortunately," says Burke, "I had to come to the realization that he was probably more right than I was." Astronomers have been searching the cosmos for a (probably funhouse!) mirror image of Earth, but this new result could put a crack in that glass.

Let's be clear: Kepler has done a fantastic job. Any performance review would say the telescope met or exceeded expectations in its primary mission, and its work continues today, on a secondary mission called K2. It changed our view of the stars to one of solar systems. But each of Keplers detections needs confirmation to move beyond "planet candidate" status, a level-up that can come when another telescope takes a direct image of a planet or when another telescope sees the star wobble periodically, tugged around by the the planet's gravity.

But following up on candidates that way is resource-gobbling. And it's not very feasible for small planets around faint stars, especially the ones with long orbits. So astronomers have turned to a different means of confirmation: statistical validation." If there's a 99+ percent chance that a Kepler candidate is not a false positive, astronomers consider it confirmed. That kind of analysis they can do in bulk.

In their new paper, though, Mullaly, Burke, and their colleagues argue that the current stats methods, which do not account for instrumental contamination, do not cut it for planets that have years longer than 200 days and dont pop decisively out of the noise. A transit signal looks kind of like this: ___. A killer detection would have a big dip, but often, planets with characteristics like ours have much more modest indicators, easily muffled by noise. If Kepler were an alien telescope orbiting a distant alien planet, Earth could just as easily be an instrumental glitch in the data as a planet.

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