為何有這麼多無家可歸的行星？（圖）

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，天文學家們報道說：我們星系中的系外行星可能比系內的恆星還要多，數量上的差別幾乎為二比一，而且這些系外行星中有四分之三很可能是「自由漂泊者」，不受任何恆星的束縛。科學家們推測，這些無家可歸的行星中有許多是因為引力不穩定的軌道而被拋出它們的母恆星系統。但是，通過新的計算機模擬發現了更加奇異的原因。其中的一個可能性是，當恆星正常的燃氫階段結束並開始膨脹為紅巨星之後，恆星確實會將行星推到星際空間去。其他的推測包括引力攝動。引力攝動要麼是過往的恆星引起的，要麼是恆星系統進入或退出我們星系中引力密集的旋臂而引起的，甚至可能是跟濃密的分子雲發生相互作用而引起的。然而，研究人員在一篇被《皇家天文學會月刊》接受並等待發表的論文中報道說：最有可能的原因是，這些系外行星就是被引力彈出去的，這種引力來自它們的母恆星——母恆星在恆星密集的星團中到處遭到碰撞時，引力就產生了。研究人員認為，多數恆星就是在這樣的星團中形成的。