Why So Many Homeless Planets?
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Last year, astronomers reported that extrasolar planets may outnumber stars in our galaxy by almost a two-to-one margin, and that three-quarters of these worlds are likely to be free-floaters, not bound to any star. Scientists speculated that many of these homeless planets were slung out of their parent solar systems as a result of gravitationally unstable orbits. But new computer simulations blame more exotic causes. One possibility is stars literally pushing the planets into interstellar space after the suns reach the end of their normal hydrogen-burning lives and begin expanding into red giants. Other scenarios involve gravitational perturbations, either caused by passing stars, a solar system entering and exiting our galaxy's gravitationally dense spiral arms, or even via interactions with dense molecular clouds. But the most likely reason, researchers report in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society, is that these extrasolar planets would simply be ejected by the gravitational forces that result when their parent stars get jostled about inside tightly-packed star clusters—the same clusters in which most stars are thought to be formed. |
去年，天文学家们报道说：我们星系中的系外行星可能比系内的恒星还要多，数量上的差别几乎为二比一，而且这些系外行星中有四分之三很可能是“自由漂泊者”，不受任何恒星的束缚。科学家们推测，这些无家可归的行星中有许多是因为引力不稳定的轨道而被抛出它们的母恒星系统。但是，通过新的计算机模拟发现了更加奇异的原因。其中的一个可能性是，当恒星正常的燃氢阶段结束并开始膨胀为红巨星之后，恒星确实会将行星推到星际空间去。其他的推测包括引力摄动。引力摄动要么是过往的恒星引起的，要么是恒星系统进入或退出我们星系中引力密集的旋臂而引起的，甚至可能是跟浓密的分子云发生相互作用而引起的。然而，研究人员在一篇被《皇家天文学会月刊》接受并等待发表的论文中报道说：最有可能的原因是，这些系外行星就是被引力弹出去的，这种引力来自它们的母恒星——母恒星在恒星密集的星团中到处遭到碰撞时，引力就产生了。研究人员认为，多数恒星就是在这样的星团中形成的。 |
译自：美国《科学》杂志网站（17 January 2012, 12:50 PM）
原著：Bruce Dorminey
