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Scientists say they may have solved a far-out mystery: how Uranus and Neptune came to exist at the very edges of the solar system. A new study says the two icy planets may have been born much closer to the sun than previously thought, and ended up in their current orbits after gravitational forces from Jupiter violently hurled them away. That would explain how the two planetary giants—each more than 10 times the mass of the Earth—could exist at the far edge of the solar system, where there was not enough gas and dust to make a planet eons ago.
The study is based on computer simulations conducted by Martin Duncan of Queen‟s University in Kingston, Ontario, and colleagues. It was published in Thursday‟s issue of the journal Nature. All of the planets in the solar system are believed to have evolved through the accumulation of a large number of small bodies that circulated in a huge disk around the sun. The researchers theorize that Uranus and Neptune formed their cores near the orbits of Jupiter and Saturn,within a ring of about 5 to 10 astronomical units from the sun. One astronomical unit equals the distance from the sun to the Earth. (Earth, however, is thought to have formed much later than the big planets.
Previous estimates of 10 to 20 AU have been given for the birthplaces of Uranus and Neptune, which now orbit at19 and 30 AU, respectively. Duncan said Jupiter grew fastest because it was closest to the sun, where the planet-forming disk was the densest, and then exerted gravitational forces on its smaller planetary siblings. Saturn may have helped eject Uranus and Neptune.
Renu Malhotra, a scientist at the Lunar and Planetary Institute in Houston, said the study does not explain why Uranus and Neptune did not accumulate gas like Jupiter and Saturn, since the four planets formed at roughly the same time and in roughly the same place.
Malhotra said the evidence shows that Uranus and Neptune were formed perhaps 30% closer to the sun than their present locations—but not as close as Duncan proposes. The planets then may have gently migrated out to their current locations, she said. He also said that gravity and friction from gas that surrounded Uranus and Neptune could have prevented them from being hurled out. Alan Boss, an astrophysicist at the Caregie Institution in Washington, said more research is needed on Duncan‟s theory. “It‟s a radical idea.” He said, “but since we‟re in a stalemate on Neptune and Uranus, maybe we need a radical idea.”
Which of the following is NOT true?
文章第一段第二句提到“the two icy planets may have been born much closer to the sun than previously thought”（他们形成时离太阳的位置可能比我们最初预想的近） 而不是“形成时离太阳很近”， 选项 A 错误。 根据文章第一段第二句， 选项 B 正确。 根据文章第一段最后一句“each more than 10 times the mass of the Earth …not enough gas and dust to make a planet eons ago”， 天王星和海王星的体积大约是地球的 10 倍， 而且在很久以前在太阳系的边缘没有足够的气体和尘埃来形成一颗行星， 选项 C 与选项 D 正确。 故选 A。
Duncan‟s study indicates________.
文章反复提到， 天王星和海王星形成时的位置可能比现在的位置要离太阳近。 其他几项并未在文中阐明。故选 C。
Malhotra‟s view differs from that of Duncan‟s in that ________.
根据文章最后一段第一句， “Malhotra said the evidence shows that … not as close as Duncan proposes.”Malhotra 认为证据表明天王星和海王星形成时离太阳的距离可能比现在位置的距离近 30%左右， 但并没有Duncan 所说的那么近。 故选 D。
According to the passage, the reason why Uranus and Neptune were not hurled far out as a result of ________.
根据文章最后一段第三句“gravity and friction from gas … could have prevented them from being hurled out”，天王星和海王星周围气体的重力和摩擦力可以阻止它们被抛出。 故选 C。
The author‟s attitude toward the proposed solutions to the mystery is ________.
根据文章可知， 作者在叙述的过程中保持客观的态度。 impartial 不偏不倚的。 radical 激进的。 uncertain 不确定的。 funny 滑稽的。 故选 B。