Level 4 Reading 12A

Reading 12A

阅读12A

PLANET HUNTERS It took humans thousands of years to explore our own planet, and centuries to comprehend our neighboring planets. Nowadays, though, new worlds are being discovered every week. To date, astronomers have identified more than 3,900 "exoplanets"—worlds orbiting stars other than the sun. There's a "hot Saturn" 260 light-years from Earth that orbits its parent star so rapidly that a year there lasts less than three days. Circling another star 150 light-years out is a "hot Jupiter," whose upper atmosphere is being blown away by the star's solar winds. Astronomers have also found five planets orbiting a pulsar— the remains of a once mighty star shrunk into an atomic city-size nucleus that spins. Some worlds have fallen into their suns. Others have been thrown out of their systems to become "floaters" that wander in eternal darkness. Among all these, astronomers are eager to find a hint of the familiar: planets that resemble Earth. That is, they are looking for planets that orbit their stars at just the right distance—neither too hot nor too cold—to support life. However, we have not yet found planets that are quite like our own. To see a planet as small and dim as ours amid the glare of its star is like trying to see a firefly in a fireworks display. Yet by pushing technology to the limits, astronomers are rapidly approaching the day when they can find another Earth. In Search of Other Earths The most direct approach to finding a planet is to take a picture of it with a telescope. Astronomers have detected more than half of the confirmed exoplanets this way. All of them are big and bright and conveniently far away from their stars. A more effective way to detect an exoplanet, though, is to use a method known as the Doppler technique. This involves analyzing starlight for evidence that the star's movement is affected by the gravitational pull of a planet. In recent years, astronomers have refined the technique. They can now tell when a planet is pulling its star by only one meter a second—about human walking speed. That's enough to detect a giant planet in a big orbit, or a small planet if it's close to its star. Another approach is to watch a star for a slight dip in its brightness. This occurs when an orbiting planet passes in front of the star and blocks part of its light. At most, a tenth of all planetary systems are oriented so that these mini-eclipses—called transits—are visible from Earth. So, astronomers have to monitor a lot of stars to capture just a few transits.

行星猎手 人类花了数千年的时间来探索我们自己的星球，花了几个世纪的时间来理解我们的邻近行星。 然而，如今，每周都有新的世界被发现。 迄今为止，天文学家已经发现了3900多颗“系外行星”，即围绕太阳以外的恒星运行的行星。 有一个距离地球260光年的“热土星”，它围绕母恒星运行的速度如此之快，以至于一年的时间不到三天。 绕着150光年外的另一颗恒星旋转的是“热木星” 其高层大气正被恒星的太阳风吹走。 天文学家还发现了五颗围绕脉冲星运行的行星-- 一颗曾经强大的恒星的残骸缩成了一个旋转的原子城大小的原子核。 有些世界已经落入他们的太阳。 其他人则被赶出了他们的系统，成为在永恒黑暗中游荡的“漂浮者”。 在所有这些中，天文学家渴望找到一个熟悉的线索：类似地球的行星。 也就是说，他们正在寻找以合适的距离绕恒星运行的行星——既不太热也不太冷——以支持生命。 然而，我们还没有发现与我们自己的行星非常相似的行星。 看到像我们这样又小又暗的星球 在耀眼的星光下，就像在烟花汇演中试图看到萤火虫。 然而，通过将技术推向极限，天文学家们正迅速接近找到另一个地球的那一天。 寻找其他地球 寻找行星最直接的方法是用望远镜给它拍照。 天文学家已经通过这种方式探测到了一半以上的已确认系外行星。 它们都又大又亮，离星星很远。 不过，探测系外行星的一种更有效的方法是使用多普勒技术。 这涉及分析星光，以寻找恒星运动受到行星引力影响的证据。 近年来，天文学家改进了这项技术。 他们现在可以分辨出一颗行星何时以每秒一米的速度拉动它的恒星——大约是人类的行走速度。 这足以探测到大轨道上的一颗巨型行星，或者一颗靠近恒星的小行星。 另一种方法是观察恒星的亮度是否略有下降。 当一颗绕轨道运行的行星从恒星前方经过并阻挡其部分光线时，就会发生这种情况。 最多有十分之一的行星系统是这样定向的，即这些迷你日食（称为凌日）可以从地球上看到。 因此，天文学家必须监测大量恒星，才能捕捉到几次凌日。

The dream of astronomers is to discover a rocky planet roughly the size of Earth orbiting in a habitable zone— that is, not so close to a star that the planet's water has boiled away, but not so far out that it has frozen into ice. If they succeed, they will have found what biologists believe could be a promising abode for life. The best places to look may be dwarf stars, which are smaller than the sun. Dwarf stars are plentiful; seven of the ten stars nearest to Earth are dwarf stars. They also provide a steady supply of sunlight to any life-bearing planets within their habitable zone. Additionally, dwarf stars are dim, so the habitable zone lies closer in. If the planet is closer to the star, it's easier for astronomers to detect a transit observation. A close-in planet also has a stronger pull on its star. That makes it easier to detect with the Doppler method. Indeed, one of the most promising planets yet found—the "super Earth" Gliese 581 d— orbits in the habitable zone of a red dwarf star only one-third the mass of the sun. Life—But Not as We Know It? If an Earth-like planet is found within a star's habitable zone, a space telescope could be used to look for signs of life. Most likely, scientists will examine the light coming from the planet for possible indications of past or present life, such as atmospheric methane and oxygen. They might also look for the "red edge" produced when chlorophyll-containing plants reflect red light. Directly detecting and analyzing the planet's own light will not be easy. Its light might be just one ten-billionth the light of the star's. But when a planet transits, starlight shining through the atmosphere could reveal clues to its composition that a space telescope might be able to detect. The challenge facing scientists is not just having to perform a chemical analysis of planets they cannot see. They must also keep in mind that life there may be very different from life here at home. The lack of the red edge from an exoplanet, for instance, does not exclude the possibility of life. Life thrived on Earth for billions of years before land plants appeared and populated the continents. The problem is that biological evolution is very unpredictable. It is possible that life originated on an Earth-like planet at the same time it did here. But life on that planet today would almost certainly be very different. As the biologist Jacques Monod once commented, life evolves not only through necessity, but also through chance— the unpredictable intervention of countless accidents. Chance has played a role many times in our own planet's history. The most dramatic examples are the mass extinctions that wiped out millions of species and created room for new life forms to evolve. Some of these accidents appear to have been caused by asteroids or comets colliding with Earth. An impact 66 million years ago, for instance, helped kill off the dinosaurs and opened up opportunities for the distant ancestors of human beings. Hence, scientists look not just for exoplanets identical to modern Earth, but for planets resembling the Earth as it used to be, or that it might have been. It was not easy for earlier pioneers to undertake explorations of the ocean floors, map the farside of the moon, or find evidence of oceans beneath the frozen surfaces of Jupiter's moons. Neither will it be easy to find life on the planets of other stars. But we now have reason to believe that billions of such planets exist. They hold the promise of expanding not only the scope of human knowledge, but also the richness of the human imagination.

天文学家的梦想是发现一颗大约地球大小的岩石行星，在宜居带内绕轨道运行-- 也就是说，不要离恒星太近，以至于行星上的水沸腾了，但也不要离得太远，以至于它结冰了。 如果他们成功了，他们将找到生物学家认为有希望的生命栖息地。 最佳的观赏地点可能是比太阳小的矮星。 矮恒星很多；离地球最近的十颗恒星中有七颗是矮星。 它们还为宜居带内的任何有生命的行星提供稳定的阳光供应。 此外，矮恒星很暗，所以宜居带离它们更近。 如果这颗行星离恒星更近，天文学家就更容易探测到凌日观测。 近地行星对其恒星的引力也更强。 这使得多普勒方法更容易检测到。 事实上，迄今为止发现的最有希望的行星之一——“超级地球”格利泽581d-- 位于红矮星宜居带的轨道只有太阳质量的三分之一。 生活——但不是我们所知道的那样？ 如果在恒星的宜居带内发现了一颗类地行星， 太空望远镜可以用来寻找生命的迹象。 最有可能的是，科学家们将检查来自地球的光线，以寻找过去或现在生命的可能迹象， 例如大气中的甲烷和氧气。 他们还可能寻找含有叶绿素的植物反射红光时产生的“红边”。 直接探测和分析地球自身的光线并不容易。 它的光可能只有恒星光的100亿分之一。 但当一颗行星凌日时， 穿过大气层的星光可以揭示其组成的线索，太空望远镜可能能够探测到这些线索。 科学家们面临的挑战不仅仅是必须对他们看不见的行星进行化学分析。 他们还必须记住，那里的生活可能与家里的生活大不相同。 例如，系外行星没有红色边缘并不排除存在生命的可能性。 在陆地植物出现并在大陆上定居之前，生命在地球上繁衍了数十亿年。 问题是生物进化是非常不可预测的。 生命可能在这里同时起源于一颗类地行星。 但今天那个星球上的生命几乎肯定会大不相同。 正如生物学家雅克·莫诺曾经评论的那样， 生命不仅在必然中进化，也在偶然中进化-- 无数事故的不可预测的干预。 机遇在我们星球的历史上多次发挥了作用。 最引人注目的例子是大灭绝，它消灭了数百万物种，并为新的生命形式的进化创造了空间。 其中一些事故似乎是由小行星或彗星与地球碰撞造成的。 例如，6600万年前的一次撞击， 帮助消灭了恐龙，为人类的远祖开辟了机会。 因此，科学家们不仅要寻找与现代地球相同的系外行星，还要寻找与过去或可能曾经的地球相似的行星。 对于早期的开拓者来说，探索海底、绘制月球背面的地图并不容易， 或者在木星卫星的冰冻表面下发现海洋的证据。 在其他恒星的行星上找到生命也不容易。 但我们现在有理由相信，存在数十亿颗这样的行星。 它们不仅有望扩大人类知识的范围，而且有望丰富人类的想象力。

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