NASA's Kepler Space Telescope Finds Hundreds of New Exoplanets, Boosts Total to 4,034

This story was updated at 2:45 p.m. EDT.

NASA has unveiled the complete set of data from the first four years of the agency's Kepler Space Telescope mission, which stared at a single patch of the sky in the search for alien planets. The result: Kepler has discovered 219 new candidates since NASA's last data unveiling, including 10 near-Earth-size planet candidates in the so-called habitable zone around their stars where the conditions are just right for liquid water to exist on a planet's surface — a key feature in the search for habitable worlds.

That process discovered potential planets like the newly found KOI 7711 (short for Kepler object of interest), an exoplanet that appears very much like Earth — just 1.3 times Earth's radius at an orbit that lets the planet feel about as much radiation as Earth gets from the sun. For KOI 7711 and the other planets, the percent the star dimmed let researchers determine its size, and the frequency of the dimming revealed the orbit.

To determine which dimmings of the 200,000 stars observed by Kepler were likely to be planets, the data went through an intensive vetting process. As Thompson described, about 34,000 signals were found — both transiting planets and noise that could have come from the camera or star itself. After vetting, the total came down to about 4,000 candidates, 50 of which were Earth-size and in the habitable zone.

The researchers then put simulated transits into the data and recorded how many were actually picked up by the software — determining how many transits the process might have missed. And they put noise through the process, too, checking how many were marked as transiting planets — so they knew how many planets were likely to be false alarms. [NASA's Planet-Hunting Kepler Explained (Infographic)]

A research group used the Keck Observatory in Hawaii to gauge the size of 1,300 stars measured by Kepler, which allowed them to more precisely pinpoint the stars' sizes — and therefore the size of their potential planets. They found that while researchers had thought there was a smooth population containing the whole range of sizes between 1 and 4 times that of Earth, there was a much sharper divide.

That sharp divide likely comes from the planet formation process, Fulton said: Planets' rocky cores form from smaller pieces, and then the protoplanet's gravity attracts hydrogen and helium gas. A little bit of gas makes the planet much bigger, putting it on the mini-Neptune side of things. Planets in the middle, Fulton said, can suffer a setback that puts them back on the rocky super-Earth side of things: The newfound atmosphere can be baked away if the star is too close by or there's not enough to start with.

While the Kepler data set provides the best-ever glimpse of exoplanet demographics for one slice of the sky, future telescopes — like NASA's Transiting Exoplanet Survey Satellite set to launch in 2018 — will allow researchers to follow up on these Kepler finds to characterize the planets even more. They may someday even take direct images of exoplanets with tools like Hubble Space Telescope's successor, the James Webb Space Telescope (also set to launch in 2018). Plus, additional data from Kepler's current K2 mission will give researchers a glimpse into what things look like in other parts of the sky, revealing planets around star clusters of different ages, with different iron contents, and many more low-mass stars than Kepler saw the first time around, the researchers said.

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