1] Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA [2] Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark.
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark.
Nature. 2014 May 29;509(7502):593-5. doi: 10.1038/nature13254.
Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods. Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Here we report the metallicities (that is, the abundances of elements heavier than hydrogen and helium) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (∼4.5σ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen-helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates, implying that host star metallicity, which is a proxy for the initial solids inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.
大约一半的半径小于四个地球半径的系外行星（exoplanets）处于短周期轨道中。尽管它们数量众多，但这些行星的组成在很大程度上是未知的。现有证据表明，它们的组成范围从小而高密度的岩石行星到由岩石核心和厚氢氦气体包层组成的低密度行星。在这里，我们报告了超过 400 颗恒星的金属丰度（即比氢和氦重的元素的丰度），这些恒星拥有 600 个系外行星候选体，并发现这些系外行星可以分为三个种群，这些种群由统计上明显不同的（约 4.5σ）金属丰度区域定义。我们将这些区域解释为反映了类地行星（半径小于 1.7 个地球半径）、具有岩石核心和氢氦包层的气态矮行星（半径在 1.7 和 3.9 个地球半径之间）和冰或气态巨行星（半径大于 3.9 个地球半径）的形成机制。这些过渡与通过动力学质量估计推断的过渡非常吻合，这意味着主星金属丰度（即原行星盘初始固体丰度的代理）是调节行星系统结构的关键因素。
