Probing Lensing Anomalies Network (PLANET) Collaboration, Institut d'Astrophysique de Paris, Université Pierre & Marie Curie, UMR7095 UPMC-CNRS, 98 bis boulevard Arago, 75014 Paris, France.
Nature. 2012 Jan 11;481(7380):167-9. doi: 10.1038/nature10684.
Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17-30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002-07) that reveals the fraction of bound planets 0.5-10 AU (Sun-Earth distance) from their stars. We find that 17(+6)(-9)% of stars host Jupiter-mass planets (0.3-10 M(J), where M(J) = 318 M(⊕) and M(⊕) is Earth's mass). Cool Neptunes (10-30 M(⊕)) and super-Earths (5-10 M(⊕)) are even more common: their respective abundances per star are 52(+22)(-29)% and 62(+35)(-37)%. We conclude that stars are orbited by planets as a rule, rather than the exception.
大多数已发现的系外行星（exoplanets）都是通过径向速度或凌日法发现的。这两种方法都偏向于探测那些离母恒星相对较近的行星，而且研究表明，大约有 17-30%（参考文献 4、5）的类似太阳的恒星拥有行星。另一方面，引力微引力透镜探测的是离恒星更远的行星。最近，通过微引力透镜发现了一类不被束缚或离恒星非常远的行星。这些行星的数量至少与银河系中的恒星一样多。在这里，我们报告了对微引力透镜数据（收集于 2002-07 年）的统计分析，该分析揭示了距恒星 0.5-10 AU（日地距离）范围内有束缚行星的比例。我们发现，有 17(+6)(-9)%的恒星拥有木星质量的行星（0.3-10 M(J)，其中 M(J) = 318 M(⊕)，M(⊕)是地球的质量）。冰冷的海王星（10-30 M(⊕)）和超级地球（5-10 M(⊕)）更为常见：它们各自的恒星丰度分别为 52(+22)(-29)%和 62(+35)(-37)%。我们的结论是，行星通常绕恒星运行，而不是例外。
