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远离故土的家 The existence of the most Earthlike planet yet has just been confirmed Dec 10th 2011 | from the print edition ONE of the more memorable slogans to come out of the climate-change talks in Durban over the past few days is: there is no planet B. But what if there were? Over the past couple of decades astronomers have logged thousands of so-called exoplanetsworlds which orbit stars other than the sun. On December 5th the scientists in charge of Kepler, a space telescope designed to look for such planets, confirmed their instruments discovery of its first Earthlike world. It is dubbed, rather unromantically, Kepler 22b. The existence of this planet, which circles a star 600 light-years away, in the constellation of Lyra, had previously been suspected. Kepler, which belongs to NASA, Americas space agency, works by observing dips in a stars brightness as a planet passes in front of it. It flags likely looking reductions as candidate planets, of which Kepler 22b was one. But three passes are needed to confirm a planets existence, and Kepler 22b has now passed this test. Crucially, it orbits well within its stars Goldilocks zone: neither too close nor too far away for liquid water to exist on its surface. It joins two other Earthlike planetsGliese 581d and HD 85512 bdiscovered by another instrument within the past few years. In truth, the term Earthlike is a stretch. Kepler 22b has a radius 2.4 times that of Earth, and if it is made from roughly the same stuff its surface gravity will also be about 2.4 times as strong. But NASAs astronomers remain unsure 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 Goldilocks zones, Kepler 22b orbits comfortably within its own. NASAs researchers reckon its surface temperature is about 22C, compared with 15C on Earth. Its parent star is similar to the sun, again unlike those of the other two candidates, both of which orbit cooler, dimmer stars. Indeed Gliese 581ds parent is a red dwarfthe tiniest stellar species. That means its Goldilocks zone is so close 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 while the other experienced unending darkness. These three potentially habitable exoplanets may soon be joined by many more. In the two and a half years since its launch, Kepler has spotted 2,326 candidate planets. About 650 others have been discovered by other instruments. That plethora allows astronomers to start drawing conclusions about how common various sorts of planets are. Of Keplers haul, 9% seem to be of a similar size to Earth ; a further 29% are Super Earthsplanets substantially larger than Earth that are nevertheless rocky. Forty-eight of Keplers unconfirmed candidates look as if they orbit within their stars habitable zones; of those, ten seem to be Earth-sized. The ultimate goal, of course, is to let astronomers make a plausible estimate of the total number of planets in the galaxy, of the number that could conceivably support life, and of the fraction of those that could sustain human colonists. If only a few of Keplers possible Earthlike planets turn out to be real, that third number is likely to be in the millions. Such knowledge will mark an historic transition, says Chris Lintott, an astronomer at Oxford University who is giving the Kepler team a hand with the data analysis, since the uncertainties around the question of whether life exists elsewhere will cease to be astronomical and become purely biological . Based on the preliminary data, it looks as if there are numerous suitable planets. The science of exobiology may soon cease to be an oxymoron. | 
