2,325 and counting: Kepler doubles its haul in largest exoplanet discovery ever

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Kepler has now discovered 2,325 exoplanets since it began operations in March 2009(Credit: NASA/W. Stenzel)

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Since it launched six years years ago, NASA's Kepler space telescope has provided a guiding light in our search for extraterrestrial life, scanning the sky for potentially habitable Earth-size planets. Today the agency has announced the discovery of almost 1,300 new exoplanets, doubling the craft's previous tally and giving the chances of finding another world just like ours a healthy little boost.

The announcement was made after researchers sifted through Kepler's July 2015 planet catalogue, which included a total of 4,302 exoplanet candidates. For these candidates to be verified, the probability of them actually being a planet has to be greater than 99 percent. The analysis found that 1,284 of the candidates met this criteria.

"This announcement more than doubles the number of confirmed planets from Kepler," says Ellen Stofan, chief scientist at NASA Headquarters in Washington. "This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth."

Of the 1,284 new planets, NASA says 550 could be rocky like our very own Earth, an estimate based on their size. Furthermore, nine of the 550 orbit their sun in the habitable zone, a region where temperatures will enable liquid water to gather in pools. This latest discovery brings the total number of exoplanets we know to be resting in the habitable zone to 21.

The total number of verified exoplanets now sits at more than 3,200. Kepler has discovered 2,325 exoplanets of these since it began operations in March 2009. It does so by searching for changes in brightness when a planet passes in front of its star and blocks its light, in the same way us Earthlings were able to observe Mercury's transit of the Sun earlier in the week.

But this approach to planet-hunting has involved a painstaking process that requires follow up observations and researchers to verify candidates one at a time. The latest finding, which is the biggest single discovery of planets ever, came about through a new way of analyzing data from Kepler that offers more of a blanket approach.

Timothy Morton, an astrophysicist at Princeton University, developed a statistical validation method to more easily determine whether these candidates are the real deal. It is based on simulations of the flickering signals produced by already verified exoplanets and those found to be false candidates, along with simulations of how common these imposters are expected to be in the Milky Way galaxy. Through these simulations, the researchers say they can gain a statistical probability of a flickering signal being a bona-fide planet without requiring the labor-intensive follow-up observations.

And Kepler's collection of exoplanet scalps may continue to climb. Its July 2015 catalogue also offered up another 1,327 candidates that are more likely than not to be real planets, but do not yet satisfy the 99 percent threshold. The researchers say that 984 of the candidates had already been verified by other techniques, and the other 707 are likely some other celestial bodies.

"Before the Kepler space telescope launched, we did not know whether exoplanets were rare or common in the galaxy," says Paul Hertz, Astrophysics Division director at NASA Headquarters. "Thanks to Kepler and the research community, we now know there could be more planets than stars. This knowledge informs the future missions that are needed to take us ever-closer to finding out whether we are alone in the universe."

The research was published in The Astrophysical Journal.

Source: NASA

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