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  3. A New Planet Is Discovered [Humans have long been fascinated by the possibility of extra-terrestrial life. Novels, films and

A New Planet Is Discovered [Humans have long been fascinated by the possibility of extra-terrestrial life. Novels, films and

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问题                A New Planet Is Discovered
    [Humans have long been fascinated by the possibility of extra-terrestrial life. Novels, films and folk-tales have encouraged curiosity and speculation about what might be ’out there’ in space. More recently, scientists have joined in on the collective wonderment after the first discovery by astronomers of a new planet—Giles 581g—that could almost certainly support such organisms. In fact, Steven Vogt—one of the two astronomers credited with discovering the planet—has declared that ’the chances of fife on this planet are 100 percent’ and that he has ’ almost no doubt about it’.]
    A．Although we currently know very little about Giles 581g, a couple of its features strongly indicate that this planet might harbor the existence of organisms. Firstly, Giles 581g is located at almost exactly the right distance from its red dwarf parent star (Giles 581 )in order to sustain liquid water, which is the only known criterion for organic formation. Planets that orbit too close to or too far away from their stars do not have a suitable climate; this can only be found in a slender strip of solar space around each star known to astronomers as a ’habitable zone’. Around our star—the Sun—Earth is the only planet that occupies this zone, although it nearly stretches to Venus and Mars. Giles 581g is also the right size for organic life. It is about 3.1 to 4.3 times the size of Earth, and this relatively low mass means it should be made mostly of rock. Planets that grow beyond 10 times the size of Earth tend to become gaseous and uninhabitable, without the solid or liquid infrastructure necessary for organisms.
    B．Giles 581g is part of a string of planets in the Giles 581 solar system. Two of Giles 581g’s siblings—Giles 581e and Giles 581 b—orbit too close to their parent star to support any kind of life on them. Giles 581c skims the near side of the habitable zone, but scientists suggest that it does not have enough of a toehold in this zone to provide a stable infrastructure for organic formations—roughly the same goes for Giles 581d, which has a stronger presence in the habitable zone on its far side, but may not be hot enough for liquid water—opinion is not ’completely settled’ on this matter, says James Casting of Pennsylvania State University. Casting, who has studied the two Giles 581 planets on the outer edges of the habitable zone, suggests thatGliese581gis’smack dab in the middle’ of the zone which, along with its size and composition, makes it the most exciting and realistic prospect for extra-terrestrial life yet.
    C．The comparisons between the Earth and Giles 581g should not be overstated, however. Even if Giles 581 g can sustain organisms, it would be a very different place to live. The main difference is that Giles 581g orbits much closer to its star than the Earth does to the Sun. Because Giles 581 is only one percent as bright as the Sun, it exudes little warmth, and its habitable zone lies much closer than the Sun’s. At this closer distance, planets in the zone get locked into strong gravitational pulls that tend to slow their circular movements over time. Eventually, they become stuck with one side constantly facing the star—just as the Moon always shows the Earth the same face. Because of this, it is likely that Giles 581g experiences permanent daytime on the side facing the star and permanent shadow on the other side. It is estimated therefore that average temperatures on the star side would be about 71 degrees C and average temperatures on the other much chillier: -34℃. Nevertheless, Steven Vogt suggests that Giles 581g probably has a comfortable area along the midpoint, known as the terminator. Any life here would always see the star sitting on the horizon and consequently experience eternal sunrise or sunset.
    D．Even if there is no life on Giles 581g, its discovery reveals that habitable planets are quite common, with around 10 to 20 percent of red dwarves and sun-like stars boasting them. Giles 581 is one of just nine stars at that particular distance which astronomers have searched with high enough precision to uncover a planet in the habitable zone. While the odds may prove to be slightly lower than 10 percent, the evidence of life—or at least, the high possibility of life—is still overwhelming. There are at least a few hundred billion stars in our galaxy, which means that around 20 to 40 billion planets have the potential for the development of organisms.
    E．And so, the search for life continues. Unfortunately, Giles 581g is not particularly amenable to observation. The next step in finding out the chances of life on the planet would be to measure its light spectrum, a process that would reveal molecular oxygen if indeed it exists. The glare from the parent star makes this impossible to do with current instruments, however. Another way to gather information would be to transport a vessel to and from the planet. The technology currently exists to do this, but at 20 light years from earth, it would take 200 years for astronomers to receive the result. Luckily, new information may just be a few years away. Astronomers suggest that many rocky planets are likely to be found in habitable zones during the coming years, and some of these will provide a better platform for research with current instruments.
Questions 14-18
    Reading Passage 2 has five paragraphs, A-E.
    Which paragraph contains the following information?
    Write the correct letter, A-E, in boxes 14-18 on your answer sheet.
A pre-requisite for the beginning of life
选项
答案A
解析
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