|
|
考研英语阅读部分是冲刺收尾阶段重点复习的大项，历年考研英语都是过线容易高分难，所以同学们要想拿高分务必在最后阶段强化英语阅读“得英语阅读，得天下”，太奇考研英语辅导专家王瑾老师收集整理了历年阅读精选和各建议同学们在距离考试时间还剩四周的时间里每天一篇，快速提高阅读能力。
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 “exoplanets”—worlds 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 instrument’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’s space agency, works by observing dips in a star’s 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 planet’s existence, and Kepler 22b has now passed this test. Crucially, it orbits well within its star’s “Goldilocks zone”: neither too close nor too far away for liquid water (and therefore, perhaps, life) to exist on its surface.
It joins two other Earthlike planets—Gliese 581d and HD 85512 b—discovered 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 NASA’s 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. NASA’s researchers reckon its surface temperature is about 22°C, compared with 15°C (at least for now) 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 581d’s parent is a red dwarf—the 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 (and heated) 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 Kepler’s haul, 9% seem to be of a similar size to Earth (though not all are in the Goldilocks zone of their star); a further 29% are Super Earths—planets substantially larger than Earth that are nevertheless rocky. Forty-eight of Kepler’s unconfirmed candidates look as if they orbit within their stars’ habitable zones; of those, ten seem to be Earth-sized.
|
|
发表于 2016-8-9 08:13:57