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  3. Billions of years before the Sun was born, the Milky Way galaxy flicked out

Billions of years before the Sun was born, the Milky Way galaxy flicked out

游客2024-05-04  24
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. [br] What might be the reason that the new planet wasn’t gulped down by HIP 13044?
选项 A、Its orbit was once very far from HIP 13044.
B、It was born after HIP 13044 shrank back.
C、It originally occupied Jupiter’s orbit.
D、It will be gulped down in the future.
答案 A
解析 事实细节题。第四段说这颗行星没有被HIP 13044吞噬，也许是因为这颗木星大小的星球原本占据的是木星般的轨道，它到恒星的距离比地球和太阳的更远，故[A]项正确。原文说这颗行星在母星的剧变后幸存了下来，说明它在剧变前就存在，故[B]项错误。原文只是用这颗行星类比木星，不是说它占据了木星的轨道，[C]项是对原文的错误理解，故排除。原文没有提到这颗行星将被吞噬，排除[D]项。
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