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Kepler planet
An artist’s conception highlights the worlds detected by NASA’s Kepler probe. (NASA / JPL-Caltech Illustration)

Astronomers say they’ve used NASA’s Kepler space telescope to discover 10 Earth-sized planet candidates where life could lurk.

The 10 prospects are a part of a larger pool of 219 exoplanet candidates announced today.

One of the newly detected candidates, KOI 7711, appears to be a world only 30 percent wider than Earth with an orbit that lasts about one Earth year.

“7711 is the closest to the Earth in terms of our current measurements of its size and how far away it is from its star. … However, there’s a lot we don’t know about this planet, and as a result, it’s hard to say whether it’s really an Earth twin,” Susan Mullally, a Kepler science team member at the SETI Institute, said today during a news conference.

Today’s additions to the planetary lineup aren’t the only new revelations from the Kepler mission.

Caltech researchers also laid out evidence that the family tree for relatively small planets splits into two distinct branches: Earth-sized planets and gaseous mini-Neptunes. The study equates the discovery to biologists finding out that mammals and reptiles are two different branches of the animal kingdom.

“Astronomers like to put things in buckets,” lead author B.J. Fulton said in a Caltech news release. “In this case, we have found two very distinct buckets for the majority of the Kepler planets.”

Researchers found that most planets discovered by Kepler are the size of super Earths of mini-Neptunes. (NASA / ARC / Caltech / University of Hawaii / B.J. Fulton)

Fulton and his colleagues said that when giants like Jupiter are excluded, the two main types of planets detected by Kepler are worlds that are about 75 percent bigger than Earth and worlds that are just smaller than Neptune (or almost four times bigger than Earth).

Eric Petigura, a co-author of the Caltech study, says almost all stars have planets bigger than Earth and smaller than Neptune orbiting around them, but our star is different.

“We’d really like to know what these mysterious planets are like, and why we don’t have them in our own solar system,” he said.

Despite the discovery of the new candidates, Proxima Centauri b still reigns as the closest potentially habitable exoplanet to Earth, orbiting a red dwarf star just 4.3 light-years away.

This catalog of exoplanets brings Kepler’s tally to 4,034 identified candidates. So far, a little more than half have been confirmed to be planets, including 30 Earth-sized planets in the habitable zone, NASA said in a news release.

The NASA Kepler space telescope discovers planet candidates, including 10 in the habitable zone. (Photo courtesy of NASA/Ames Research Center/Wendy Stenzel)

Kepler identifies planet candidates by looking for momentary blips in the brightness of stars. If a star’s brightness drops, it could be because an orbiting planet crossed in front of it.

Astronomers have to go back and check Kepler’s findings against different types of measurements to make sure the blip was caused by a planetary transit rather than a false signal – for example, the gyrations of a binary star system.

For the first four years of its life in space, Kepler had its eyes on Cygnus constellation. This catalog is the most detailed to date, but it’s also the last for this patch of sky. Kepler broadened its search to other areas for its extended K2 mission, which began in 2014.

Update for 2:25 p.m. PT June 20:  So how close are the newly detected habitable-zone planet candidates? GeekWire asked, and the SETI Institute’s Susan Mullally answered in an email:

“The closest of the 10 new HZ candidates is 45 parsecs away, it is KOI 8012.01. The star is so small and cool that it has to be pretty nearby (45 parsecs is about 145 light-years) in order for it to be bright enough for Kepler to observe it.  The closest new candidate around a more sunlike star is KOI  8174.01.  It is 166 parsecs (540 light-years) away.”

KOI 7711, the candidate that received most of the attention as a potential Earth analog, is even farther out: about 1,700 light-years away.

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