Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark.
Nature. 2012 Jun 13;486(7403):375-7. doi: 10.1038/nature11121.
The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.
类似于太阳的恒星的光球层中重元素（金属丰度）的丰富程度为初始原行星盘中的化学组成提供了“化石”记录。富含金属的恒星更有可能拥有气态巨行星，这支持了行星通过尘埃和冰粒积累形成的模型。最近的地面调查表明，对于海王星大小的行星，这种相关性减弱了。然而，大小和金属丰度之间的关系如何延伸到地球大小的系外行星的范围尚不清楚。在这里，我们报告了美国宇航局开普勒任务发现的 226 个小型系外行星候选体的宿主恒星的光谱金属丰度，其中包括与太阳系内的类地行星大小相当的天体。我们发现，半径小于四个地球半径的行星形成于宿主恒星的金属丰度范围很广（但平均金属丰度接近太阳），而大型行星则优先形成于金属丰度较高的恒星周围。这一观测结果表明，类地行星在银河系的盘中可能很普遍，它们的形成不需要特别高的金属丰度。
