考研阅读精选：远离故土的家
Home away from homeThe existence of the most Earthlike planet yet has just been confirmed
Dec 10th 2011 | from the print edition
memorable slogans to come out of the climate-changetalks in Durban over the past few days is: “there is no planet B”. Butwhat if there were? Over the past couple of decades astronomers havelogged thousands of so-called “exoplanets”—worlds which orbit starsother than the sun. On December 5th the scientists in charge of Kepler, aspace telescope designed to look for such planets, confirmed theirinstrument’s 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, America’sspace agency, works by observing dips in a star’s brightness as aplanet passes in front of it. It flags likely looking reductions as“candidate planets”, of which Kepler 22b was one. But three passes areneeded to confirm a planet’s existence, and Kepler 22b has now passedthis test. Crucially, it orbits well within its star’s “Goldilockszone”: neither too close nor too far away for liquid water (andtherefore, perhaps, life) to exist on its surface.
It joins twoother Earthlike planets—Gliese 581d and HD 85512 b—discovered by anotherinstrument within the past few years. In truth, the term “Earthlike” isa stretch. Kepler 22b has a radius 2.4 times that of Earth, and if itis made from roughly the same stuff its surface gravity will also beabout 2.4 times as strong. But NASA’s astronomers remain unsure whetherit 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 22borbits comfortably within its own. NASA’s researchers reckon its surfacetemperature is about 22°C, compared with 15°C (at least for now) onEarth. Its parent star is similar to the sun, again unlike those of theother two candidates, both of which orbit cooler, dimmer stars. IndeedGliese 581d’s parent is a red dwarf—the tiniest stellar species. Thatmeans its Goldilocks zone is so close to it that the planet may betidally locked, as the moon is to the Earth. If that were the case, oneside of Gliese 581d would be permanently lit (and heated) while theother experienced unending darkness.
These three potentiallyhabitable exoplanets may soon be joined by many more. In the two and ahalf years since its launch, Kepler has spotted 2,326 candidate planets.About 650 others have been discovered by other instruments. Thatplethora allows astronomers to start drawing conclusions about howcommon various sorts of planets are. Of Kepler’s haul, 9% seem to be of asimilar size to Earth (though not all are in the Goldilocks zone oftheir star); a further 29% are Super Earths—planets substantially largerthan Earth that are nevertheless rocky. Forty-eight of Kepler’sunconfirmed candidates look as if they orbit within their stars’habitable zones; of those, ten seem to be Earth-sized.
Theultimate goal, of course, is to let astronomers make a plausibleestimate of the total number of planets in the galaxy, of the numberthat could conceivably support life, and of the fraction of those thatcould (at least in theory) sustain human colonists. If only a few ofKepler’s possible Earthlike planets turn out to be real, that thirdnumber is likely to be in the millions.
Such knowledge will markan historic transition, says Chris Lintott, an astronomer at OxfordUniversity who is giving the Kepler team a hand with the data analysis,since the uncertainties around the question of whether life existselsewhere will cease to be astronomical (how many suitable planets arethere?) and become purely biological (how easy is it for life to getgoing, and how easy is it for it to become intelligent?). Based on thepreliminary data, it looks as if there are numerous suitable planets.The science of exobiology may soon cease to be an oxymoron.
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