Nature2014-06-27 2:42 PM

太阳系外的“双星记” A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b

论文摘要 

本期Nature上的两篇论文报告了“哈勃太空望远镜”对两个分开的、比木星小一些的太阳系外行星的观测结果。Heather Knutson等人观测到质量与海王星相似的行星GJ 436b的四次“凌日”;Laura Kreidberg等人观测到较小的“超级地球”GJ 1214b的15次“凌日”。穿过这些行星大气层的星光的透射光谱应能对它们各自大气层的性质提供一个很好指示,而对这两个行星来说从“哈勃太空望远镜”的“Wide Field Camera 3”获得的光谱都是几乎没有任何特征。Knutson等人认为,他们的数据表明,GJ 436b上要么存在一个压力为0.1–10 mbar的高云盖(cloud deck),要么存在一个氢含量低的大气层。Kreidberg等人的结论是,他们的近红外光谱表明,GJ 1214b存在高空云,这些云掩蔽了该行星的低层。

Abstract 

GJ 436b is a warm—approximately 800 kelvin—exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 105 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures. A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium6. Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48σ. The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition.

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