Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
Nature. 2010 Dec 2;468(7324):669-72. doi: 10.1038/nature09596.
In contrast to planets with masses similar to that of Jupiter and higher, the bulk compositions of planets in the so-called super-Earth regime (masses 2-10 times that of the Earth) cannot be uniquely determined from a measurement of mass and radius alone. For these planets, there is a degeneracy between the mass and composition of both the interior and a possible atmosphere in theoretical models. The recently discovered transiting super-Earth exoplanet GJ 1214b is one example of this problem. Three distinct models for the planet that are consistent with its mass and radius have been suggested. Breaking the degeneracy between these models requires obtaining constraints on the planet's atmospheric composition. Here we report a ground-based measurement of the transmission spectrum of GJ 1214b between wavelengths of 780 and 1,000 nm. The lack of features in this spectrum rules out (at 4.9σ confidence) cloud-free atmospheres composed primarily of hydrogen. If the planet's atmosphere is hydrogen-dominated, then it must contain clouds or hazes that are optically thick at the observed wavelengths at pressures less than 200 mbar. Alternatively, the featureless transmission spectrum is also consistent with the presence of a dense, water vapour atmosphere.
与木星和更大质量的行星相比，所谓的超级地球（质量是地球的 2 到 10 倍）的行星的总体成分不能仅通过质量和半径的测量来唯一确定。对于这些行星，在理论模型中，内部和可能的大气的质量和组成之间存在一种简并性。最近发现的过境超级地球系外行星 GJ 1214b 就是这个问题的一个例子。已经提出了三个与该行星的质量和半径一致的行星的不同模型。打破这些模型之间的简并性需要获得对行星大气组成的约束。在这里，我们报告了对 GJ 1214b 在 780 到 1000nm 波长之间的透射光谱的地基测量。该光谱中没有特征排除了（在 4.9σ置信度下）主要由氢组成的无云大气。如果行星的大气以氢为主，那么它必须在观测到的波长处包含在压力小于 200mbar 时光学厚度大的云或霾。或者，无特征的透射光谱也与存在密集的水蒸气大气一致。
