Planet in sweet spot of Goldilocks zone for life

Kepler-22b is a planet known to com­fort­ably cir­cle in the hab­it­able zone of a sun-like star. It is the first planet that NASA’s Kepler mis­sion has con­firmed to orbit in a star’s hab­it­able zone — the region around a star where liq­uid water, a require­ment for life on Earth, could per­sist. NASA has found the new planet out­side our solar sys­tem that’s eerily sim­i­lar to Earth in key aspects. Sci­en­tists say the tem­per­a­ture on the sur­face of the planet is about a comfy 72 degrees. Its star could almost be a twin of our sun. It likely has water and land. (AP Photo/NASA/Ames/JPL-Caltech)

SETH BORENSTEIN

AP Sci­ence Writer

WASHINGTON (AP) — A newly dis­cov­ered planet is eerily sim­i­lar to Earth and is sit­ting out­side our solar sys­tem in what seems to be the ideal place for life, except for one hitch. It’s a bit too big.

The planet is smack in the mid­dle of what astronomers call the Goldilocks zone, that hard to find place that’s not too hot, not too cold, where water, which is essen­tial for life, doesn’t freeze or boil. And it has a shop­ping mall-like sur­face tem­per­a­ture of near 72 degrees, sci­en­tists say.

The planet’s con­fir­ma­tion was announced Mon­day by NASA along with other dis­cov­er­ies by its Kepler tele­scope, which was launched on a planet-hunting mis­sion in 2009.

That’s the first planet con­firmed in the hab­it­able zone for Kepler, which had already found Earth-like rocky plan­ets else­where. Twice before astronomers have announced a planet found in that zone, but nei­ther have been as promising.

“This is a phe­nom­e­nal dis­cov­ery in the course of human his­tory,” Geoff Marcy of Uni­ver­sity of Cal­i­for­nia, Berke­ley, one of the pio­neers of planet-hunting out­side our solar sys­tem, said in an email. “This dis­cov­ery shows that we Homo sapi­ens are strain­ing our reach into the uni­verse to find plan­ets that remind us of home. We are almost there.”

The new planet — named Kepler-22b — has key aspects it shares with Earth. It cir­cles a star that could be the twin of our sun and at just about the same dis­tance. The planet’s year of 290 days is even close to ours. It likely has water and rock.

The only trou­ble is the planet’s a bit big for life to exist on the sur­face. The planet is about 2.4 times the size of Earth. It could be more like the gas-and-liquid Nep­tune with only a rocky core and mostly ocean.

“It’s so excit­ing to imag­ine the pos­si­bil­i­ties,” said Natalie Batalha, the Kepler deputy sci­ence chief.

Float­ing on that “world com­pletely cov­ered in water” could be like being on an Earth ocean and “it’s not beyond the realm of pos­si­bil­ity that life could exist in such an ocean,” Batalha said in a phone interview.

Kepler can’t find life itself, just where the con­di­tions might be right for it to thrive. And when astronomers look for life else­where they’re talk­ing about every­thing rang­ing from microbes to advanced intel­li­gence that can be look­ing back at us.

So far the Kepler tele­scope has spot­ted 2,326 can­di­date plan­ets out­side our solar sys­tem with 139 of them poten­tially hab­it­able ones. Even though the con­firmed Kepler-22b is a bit big, it is still smaller than most of the other can­di­dates. It is clos­est to Earth in size, tem­per­a­ture and star than either of the two pre­vi­ously announced plan­ets in the zone.

The con­fir­ma­tion of one of two, though, has been dis­puted. The lat­est dis­cov­ery has been con­firmed sev­eral ways, includ­ing by two other telescopes.

Ear­lier this year, a Euro­pean team of astronomers said they had con­firmed another planet in the hab­it­able zone, but that one was hot and barely on the inside edge of the hab­it­able zone.

For Marcy, who is on the Kepler team, the newest planet is a smidgen too large. But, “that smidgen makes all the dif­fer­ence,” he said.

Because its size implies that it’s closer to Nep­tune in com­po­si­tion than Earth, “I would bet my tele­scope that there is no hard, rocky sur­face to walk on,” Marcy said.

Chief Kepler sci­en­tist William Borucki said he thinks the planet is some­where between Earth and gas-and-liquid Nep­tune, but that it has a lot of rocky mate­r­ial. It’s in a size range that sci­en­tists don’t really know any­thing about. Mea­sure­ments next sum­mer may help astronomers have a bet­ter idea of its makeup, he said.

The planet is 600 light years away. Each light year is 5.9 tril­lion miles. It would take a space shut­tle about 22 mil­lion years to get there.

Kepler spots a planet when it passes in front of its star. NASA requires three of those sight­ings before it begins to con­firm it as a planet. Borucki said the third sight­ing for 22b hap­pened a year ago, just before the tele­scope shut down for a while. It took sev­eral months to fin­ish the confirmation.

“It’s a great gift,” Borucki said. “We con­sider this sort of our Christ­mas planet.”

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