专八
The dark star--named Nemesis by astronomers--is thought to be a brown dwarf that spins round the Sun inan orbit so large that it is measured in light years, the distance light travels in a year, equivalent to about 6,000billion miles.
The research suggests that, every 26m years, the star's eccentric orbit brings it within one light year of the solarsystem. There it causes havoc in the Oort Cloud, a huge region surrounding the solar system that contains billionsof bits of cosmic rubble left over from the formation of planets.
Of the millions of rocks it throws out of orbit at each visit, some hurtle earthwards--and have several times nearly wiped out life on Earth.
Astronomers have long wondered if the Sun has a smaller partner. Recently, two independent groups of
researchers have found evidence of one.
One group, led by John Matese, professor of physics at the University of Louisianaat Lafayette, confirms thenotion that it is most likely to be a brown dwarf, a star that never accumulated enough mass to ignite and which hassimply sat in space smoldering for billions of years.
Matese studied 82 comets from the Oort Cloud and found common elements in the shape of their orbits thatcould only be explained if they had been influenced by the gravitational pull of an object severn1 times the size ofJupiter and existing about 25,000 times farther from the Sun than the Earth.
Matese said: "A companion to the Sun orbiting at these distances would have little effect on the planets. But itwould play a big role in the way comets 'made their way, from their birth places in the planetary disc out to the: ~,
Oort Cloud and in how they can return to the inner solar system."
Further research was published by Richard Muller, professor of physics at the University of California,
Berkeley, following analyses of moon rock samples brought back to Earth by Apollo 14.
The absence of a protective atmosphere means the moon has been subjected to intense bombardment in its 4.5 billion-year existence.
Muller's breakthrough was to find a way to date how long ago any particle was melted---meaning he could
build up a picture of whether the moon gets a constant barrage or suffers spells of intensebombardment.
He said: "The evidence clearly shows that the moon has gone through spells of relatively fiequent impacts andothers of reduced intensity. I believe it is likely that this is because the Oort Cloud is being disturbed by a massivebody that is throwing comets out of stable orbits, a small fraction of which could reach the Earth."
Muller and others believe that the dark star probably takes about 26m years to complete an orbit around the stm.
Other scientists have already noted that mass extinctions of life on Earth seem to occur in a pattem with gaps equivalent to multiples of 26m, suggesting some regular event is causing the comets to come Earth's way.
The best-known such event was the one that wiped out the dinosaurs 65m years ago, but that was not the worst; the planet has suffered several such large mass extinctions.
Astronomers have found the first Earth-sized planet orbiting another star. The discovery raises the chances of finding planets that could support life as we know it.
根据以上内容，回答题。
Which of the following is NOT a feature of the Nemesis?
A．It contributes to showering of comets on the earth.
B．It is a smaller partner of the Sun.
C．It has a large orbit.
D．It contains bits of cosmic rubble.
第一段指出文章叙述的主要事件：科学家发现，一颗和太阳相似的暗星可能是导致彗星和陨石定期造访地球的原因。
第二段介绍了这颗行星Nemesis的基本特征，如颜色、大小、运行方式等。
第三和第四段指出：研究表明Nemesis的偏心轨道使奥尔特星云遭受了巨大的破坏，每次造访太阳系时，那些偏离轨道的碎石都足以毁灭地球万物。
第五段到第十三段介绍了JohnMatese和RichardMuller两位物理学教授带领的团队对Nemesis的研究发现以及他们各自的观点。
第十四段和十五段介绍了其他科学家的观点：他们认为这颗暗星可能与地球上生物的大规模灭亡有关。最后一段总结全文指出：这颗和地球大小相同的行星的发现增加了发现其他可容人类生存的星球的可能性。
【试题解析】
本题的出题点在文章细节处。本题针对Nemesis特征设题。由第一段第二句中的whose eccentric orbitis responsible for periodically showering the Earth with comets and meteorites可知A说法正确，故排除；第五段指出，天文学家们一直想知道太阳是否有一个比自己小一点的行星伙伴，最近两个研究小组找到了它存在的证据，即前面所说的Nemesis，故排除8；由第二段可知，Nemesis的运行轨道非常大，故排除C。由第三段可知，contains billions ofbits ofcosmic rubble是OortCloud(奥尔特星云)的特点，而不是Nemesis的，故选D。
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