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The existence of the most Earthlike planet yet has just beenconfirmed

Dec 10th 2011 | from the print edition

ONE of the more memorable slogans to come out of the climate-changetalks in Durban over the past few days is: “there is no planet B”. But whatif there were? Over the past couple of decades astronomers have loggedthousands of so-called “exoplanets”—worlds which orbit stars other than the sun. On December 5th thescientists in charge of Kepler, a space telescope designed to look for suchplanets, confirmed their instrument’s discovery of itsfirst Earthlike world. It is dubbed, rather unromantically, Kepler 22b.

The existence of this planet, which circles a star 600 light-yearsaway, in the constellation of Lyra, had previously been suspected. Kepler,which belongs to NASA, America’s space agency, works by observing dips in a star’s brightnessas a planet passes in front of it. It flags likely looking reductions as “candidateplanets”, of which Kepler 22b was one. But three passes are needed to confirma planet’s existence, and Kepler 22b has now passed this test. Crucially, itorbits well within its star’s “Goldilocks zone”: neither too close nor too far away for liquid water (andtherefore, perhaps, life) to exist on its surface.

It joins two other Earthlike planets—Gliese 581d and HD85512 b—discovered by another instrument within the past few years. Intruth, the term “Earthlike” is a stretch. Kepler 22b has a radius 2.4 times that of Earth, andif it is made from roughly the same stuff its surface gravity will also beabout 2.4 times as strong. But NASA’s astronomers remainunsure whether it is predominantly gaseous, liquid or solid.

Nevertheless, Kepler 22b is the most promising exoplanet yet found.Unlike the others, which skirt the edges of their stars’ Goldilockszones, Kepler 22b orbits comfortably within its own. NASA’sresearchers reckon its surface temperature is about 22°C, comparedwith 15°C (at least for now) on Earth. Its parent star is similar to thesun, again unlike those of the other two candidates, both of which orbitcooler, dimmer stars. Indeed Gliese 581d’s parent is a reddwarf—the tiniest stellar species. That means its Goldilocks zone is soclose to it that the planet may be tidally locked, as the moon is to the Earth.If that were the case, one side of Gliese 581d would be permanently lit (andheated) while the other experienced unending darkness.

These three potentially habitable exoplanets may soon be joined bymany more. In the two and a half years since its launch, Kepler has spotted2,326 candidate planets. About 650 others have been discovered by otherinstruments. That plethora allows astronomers to start drawing conclusionsabout how common various sorts of planets are. Of Kepler’s haul, 9%seem to be of a similar size to Earth (though not all are in the Goldilockszone of their star); a further 29% are Super Earths—planetssubstantially larger than Earth that are nevertheless rocky. Forty-eight ofKepler’s unconfirmed candidates look as if they orbit within their stars’ habitablezones; of those, ten seem to be Earth-sized.

The ultimate goal, of course, is to let astronomers make a plausibleestimate of the total number of planets in the galaxy, of the number that couldconceivably support life, and of the fraction of those that could (at least intheory) sustain human colonists. If only a few of Kepler’s possibleEarthlike planets turn out to be real, that third number is likely to be in themillions.

Such knowledge will mark an historic transition, says Chris Lintott,an astronomer at Oxford University who is giving the Kepler team a hand withthe data analysis, since the uncertainties around the question of whether lifeexists elsewhere will cease to be astronomical (how many suitable planets arethere?) and become purely biological (how easy is it for life to get going, andhow easy is it for it to become intelligent?). Based on the preliminary data,it looks as if there are numerous suitable planets. The science of exobiologymay soon cease to be an oxymoron.

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