Kepler-22b, just 2.4 times the size of Earth, is the first planet known to comfortably circle in the habitable zone of a sun-like star. Scientists do not yet know if the planet has a rocky, gaseous, or liquid composition. It's possible that the world would have clouds in its atmosphere, as depicted here. NASA/Ames/JPL-Caltech
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Nestled in the Goldilocks zone of a small, sun-like star is a room-temperature world a little more than twice the size of Earth. Scientists do not yet know if it is rocky or gaseous and whether it has water or clouds, but they do know that it’s the right size, and in the right place, for liquid water to exist. If it does exist, it may be one of the best places to look for life outside of our solar system.

The new planet, Kepler-22, is about 600 light-years away and the smallest planet confirmed to exist smack in the middle of the habitable zone of a sun-like star. It’s one of the most stunning announcements from the Kepler Space Telescope, which stares at a field of stars in the constellations Cygnus and Lyra and looks for blips in brightness to find other planets. While Kepler has (as of today) found more than 2,000 possible planets, finding an Earth-like world in a sun-like environment has proved elusive — until now.

“This is a major milestone on the road to finding Earth’s twin,” said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.

On top of this announcement today, the Kepler science team is sharing 1,094 more planet candidates, many of which are also potentially Earth-like and in habitable zones. Now there are 2,326 planet candidates in the sun-orbiting space telescope’s field of view, in a small sliver of the sky.

Kepler-22b’s discovery was first announced last February, when the Kepler team shared its initial treasure trove of planet candidates. Among 1,235 candidate worlds, there were 54 habitable zone candidates, Kepler-22b among them. Now it’s the first of these to be confirmed.

It takes 290 days to orbit around its star, Kepler-22, a G-class star a lot like the one we know best. “It’s almost a solar twin; it is very similar to our sun,” Natalie Batalha, Kepler deputy science team lead at San Jose State University, said in a press conference.

The planet is about 15 percent nearer to its star than Earth is to the sun. But this is OK because the star is cooler (by about 220 degrees), a bit dimmer, and a little smaller than our star. So the planet is in a really analogous Earth-like orbit. The planet’s temperature is even pretty close to Earth’s, said William Borucki, the Kepler mission’s principal investigator at NASA’s Ames Research Center. “If greenhouse warming on this planet was similar (to atmospheric warming on Earth), its surface temperature would be something like 72° F,” he said.

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This diagram compares our own solar system to Kepler-22, a star system containing the first “habitable zone” planet discovered by NASA’s Kepler telescope. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Kepler-22’s star is a bit smaller than our sun, so its habitable zone is slightly closer in. The diagram shows an artist’s rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun.

It will be a mighty nice place to look for signs of life, he added. Jill Tarter, director of the Center for SETI Research at the SETI Institute, said astrobiologists are taking that to heart.

“We will give a higher priority to worlds that our colleagues tell us are not too warm, not too cold, but just right,” she said.

With so many exoplanets, astronomers will need to start making some catalog decisions, ranking planets by their habitability potential. Along with the Earth Similarity Index we covered last month, the Kepler team has their own proposed planetary directory to make this easier. The Habitable Exoplanets Catalog ranks planets by their surface temperature, similarity to Earth, and capacity to sustain organisms at the bottom of the food chain.

Monday’s announcement came during the inaugural Kepler Science Conference in Mountain View, Calif. With the addition of 1,094 new candidate worlds, the number of planet candidates has increased by 89 percent and now totals 2,326. Of these, 207 are approximately Earth-size, 680 are super Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. So since February, the number of Earth-size and super Earth-size candidates has increased by more than 200 and 140 percent, respectively.

That’s a lot of numbers, and here’s another good one: 10. That’s the number of these new candidates that are near-Earth-sized and in the habitable zones of their host stars. Kepler-22b is not among that list, so that makes at least 11 places elsewhere in the galaxy that might look very, very familiar.

There’s still a bit of work to do to quantify just what Kepler-22b looks like. Now that astronomers are convinced it’s a planet and they know where it is, confirming their findings with the Spitzer Space Telescope, they want to find out what it’s made of. Ground-based telescopes like the Keck Observatory will start making some measurements next summer, when the field of sky that Kepler studies is visible from Earth.

And there is still much more to come, according to NASA scientists. Some software improvements have made it a little simpler to sift through Kepler light-curve data and hunt for planet candidates, so there will be at least one more big batch, according to Batalha. Among those, there may be many more Earth-like candidates to join the ranks.

“We are really zeroing in on the true Earth-sized habitable planets,” she said.

NASA