Some astronomers are questioning the existence of what might be the most Earth-like planet yet found outside the solar system, based on a reexamination of archival data.

Kepler 452 b was discovered by NASA’s Kepler space telescope and announced in 2015. At the time it seemed like everything astronomers had hoped for in an Earth analogue: slightly larger and more massive than our planet, and in a habitable 385-day orbit around a star remarkably similar to our sun.

But at about 1,000 light-years away, Kepler 452 b is far too faint for easy follow-up studies. Its apparent existence is based solely on data gathered during Kepler’s primary mission, which ran from 2009 to 2013 before being cut short by equipment malfunctions. During this period the spacecraft stared continuously at a single patch of sky, waiting for any of the stars there to almost imperceptibly dim from the shadows of planets passing across their faces. Such “transits” are how Kepler found the vast majority of its planets; but many things besides planets can cause stars to slightly dim, leading to far more false alarms than discoveries of new worlds. For any candidate planet to be confirmed as genuine, it would have to be observed transiting at least three times. Due to its long orbital period, Kepler 452 b barely met that minimal criterion before the telescope’s primary mission ended—but a host of other, more technical tests convinced the Kepler team the planet had a 99 percent chance of being real.

In a new analysis, reported last month in a paper accepted to The Astrophysical Journal, researchers take an extremely statistical approach to considering any given planetary candidate—averaging out errors from the entire span of the Kepler mission, and from every instrument in aggregate. In the process, they say they learned to better distinguish a true planetary signal from astrophysical false alarms or instrumental noise. Armed with a deeper understanding of Kepler’s quirks, the astronomers argue they can more easily flag where and how the spacecraft’s minor defects could compromise data. The authors used this approach to re-vet Kepler’s data from more than 100,000 stars, hoping to find ways to more rapidly confirm strong planetary candidates and boost the odds of validating borderline ones. Their bulk reanalysis showed worlds with orbits of less than 200 days were very easy to confirm, because these planets’ transits repeated enough to display a clear trend outside any background instrumental or astrophysical noise. But the authors found that confirming small, relatively Earth-size planets with longer periods proved tougher due to the premature end of Kepler’s main mission. With this in mind, they set their sights on one of the most marginal targets under these constraints: Kepler 452 b.

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