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  3. Billions of years before the Sun was born, the Milky Way galaxy flicked out its gravitational tongue and swallow down a tiny nei

Billions of years before the Sun was born, the Milky Way galaxy flicked out its gravitational tongue and swallow down a tiny nei

admin2013-07-02  21
问题     Billions of years before the Sun was born, the Milky Way galaxy flicked out its gravitational tongue and swallow down a tiny neighboring galaxy that had ventured too close. The evidence for that ancient act of cosmic cannibalism(同类相食)is the still-digesting remains of the meal: a handful of relatively nearby stars known as the Helmi Stream, whose weird orbits are a tipoff to their weird origin.

    Now one of those stars has a second claim to fame. HIP 13044, as it’s unglamor-ously known, has a planet whirling around it — the first planet ever found from outside the Milky Way. Aside from its extra-galactic origin, the planet itself isn’t especially remarkable. It’s a bit bigger than Jupiter and orbits its parent star in about 16 days — a "year" so short it would once have been considered impossible for so giant a planet, until multiple discoveries of many similar worlds proved such a revolution rate to be pretty common.
    It’s the star itself that makes the discovery of a planet surprising, for a couple of reasons. For one thing, its age — perhaps 7 or 8 billion years — means that while HIP 13044 was once much like the Sun, it’s gone through a dramatic change of life. As it burned through its supply of hydrogen(氢), the star would have swelled to become a so-called red giant, tens, or even hundreds of times its original size. When that happens to our Sun billions of years from now, Earth will probably be destroyed. Indeed, there’s some circumstantial evidence that HIP 13044 may have gulped down a few planets itself, says the paper’s lead author Johny Setiawan, of Germany’s Max Planck Institute for Astronomy, in Heidelberg.
    But the new planet, called HIP 13044b, survived the disaster. That’s probably because the Jupiter-size world originally occupied a Jupiter-like orbit, much farther from its star than Earth is from the Sun. It spiraled in to its present orbit only after HIP 13044 shrank back to a more dignified size — another common stage of life for stars, which return to their original dimensions when they start burning the helium(氦)in their core.
    The other thing that makes the star unusual is its composition. The Sun is mostly hydrogen and helium, but it also has significant traces of heavier elements like oxygen, carbon and iron, a quality astronomers call "metallicity" despite the non-metallic nature of some of those elements. "In the Milky Way, " says Setiawan, "the more metals a star has , the more likely it is to have planets. "
    Dwarf galaxies like the one in which HIP 13044 was born, however, have stars that are notably metal-poor. It was unclear until now whether that meant they’d also be planet-poor. The fact that Setiawan and his colleague Rainer Klement found one so easily suggests this isn’t the case. "Either they were incredibly lucky, " says Eric Ford, a planet-searcher at the University of Florida, "or planets aren’t uncommon around stars like these."
    Whatever the answer, HIP 13044b is clearly a very different world from any we’ve seen before, one that — without the aid of celestial metals — formed in a very different way. And that in turn suggests that the field of planetary science, which seemed so tidy and settled as recently as the 1990s, is still full of surprises.
What’s the relationship between the stars’ metallicity and its planets’ number in dwarf galaxies?
选项 A、The more metals a star has, the more likely it is to have planets.
B、Stars that are metal-poor can be judged to be planet-poor.
C、Planets are common around metal-poor stars in dwarf galaxies.
D、Stars that are metal-poor in dwarf galaxies can own planets.
答案D
解析 推理判断题。第六段提到塞蒂亚万和他的同事莱纳·克里门特这么容易就发现了一个行星，这说明缺乏金属的恒星未必就缺少行星，可以推断出HIP13044是缺少金属的，而文中指出它具有行星HIP 13044b，故[D]项正确。原文说的是银河系中，恒星所含金属越多，就越有可能拥有行星，并没有说在矮星系中也是如此，故[A]项错误。第六段提到直至现在也不清楚恒星缺乏金属是否意味着他们也缺少行星，故[B]项错误。“缺乏金属的恒星周围的行星并不罕见”是埃里克·福特推测的一种可能，不是经证实的事实，故排除[C]项。
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