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arXiv 1009.4196astro-ph.EPastro-ph.SR
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飞越对行星系的影响

The effects of fly-bys on planetary systems

Daniel Malmberg, Melvyn B. Davies, Douglas C. Heggie

备注：
22 pages, 15 figures, 5 tables. Accepted for publication in MNRAS
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摘要：

大多数观测到的太阳系外行星都位于紧密且通常偏心的轨道上。行星形成模型并不容易解释这种高偏心率，因为该模型预测行星应该在相当圆的轨道上。在这里，我们探讨是否涉及行星系的性质类似于那些在太阳系的气体巨星飞，可以产生行星的性质类似于观察到的行星。使用数值模拟，我们表明，飞掠可以导致行星的立即喷射，有时也会导致入侵者捕获一个或多个行星。然而，更常见的是，飞越只会扰乱行星的轨道，有时会使系统处于不稳定状态。在飞越后几百万到几亿年的时间尺度上，这种扰动可以触发行星与行星的散射，导致一个或多个行星的弹射。例如，在太阳系的四颗气态巨行星中，至少有一颗行星被抛出的系统比例在飞越后的10^8年内增加了一倍多。剩下的行星通常会留在更偏心的轨道上，类似于观测到的太阳系外行星的偏心率。我们结合了我们的研究结果，飞掠如何影响太阳系样的行星系，在年轻的星团中遇到的发生率。例如，我们测量了飞越对太阳系中四颗气体巨星的影响。我们发现，对于这样的系统，在典型的疏散星团中，有5%到15%的系统在飞越后的10^8年内遭受行星的喷射。因此，在年轻的星团中的相遇可以显著地改变任何围绕星团中恒星运行的行星的性质。由于居住在太阳附近的大部分恒星形成在星团中，遭遇会显着影响所观察到的太阳系外行星的性质。

英文摘要：

Most of the observed extrasolar planets are found on tight and often eccentric orbits. The high eccentricities are not easily explained by planet-formation models, which predict that planets should be on rather circular orbits. Here we explore whether fly-bys involving planetary systems with properties similar to those of the gas giants in the solar system, can produce planets with properties similar to the observed planets. Using numerical simulations, we show that fly-bys can cause the immediate ejection of planets, and sometimes also lead to the capture of one or more planets by the intruder. More common, however, is that fly-bys only perturb the orbits of planets, sometimes leaving the system in an unstable state. Over time-scales of a few million to several hundred million years after the fly-by, this perturbation can trigger planet-planet scatterings, leading to the ejection of one or more planets. For example, in the case of the four gas giants of the solar system, the fraction of systems from which at least one planet is ejected more than doubles in 10^8 years after the fly-by. The remaining planets are often left on more eccentric orbits, similar to the eccentricities of the observed extrasolar planets. We combine our results of how fly-bys effect solar-system-like planetary systems, with the rate at which encounters in young stellar clusters occur. For example, we measure the effects of fly-bys on the four gas giants in the solar system. We find, that for such systems, between 5 and 15 per cent suffer ejections of planets in 10^8 years after fly-bys in typical open clusters. Thus, encounters in young stellar clusters can significantly alter the properties of any planets orbiting stars in clusters. As a large fraction of stars which populate the solar neighbourhood form in stellar clusters, encounters can significantly affect the properties of the observed extrasolar planets.

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