Swain Mark R, Vasisht Gautam, Tinetti Giovanna
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109-8099, USA.
Nature. 2008 Mar 20;452(7185):329-31. doi: 10.1038/nature06823.
Molecules present in the atmospheres of extrasolar planets are expected to influence strongly the balance of atmospheric radiation, to trace dynamical and chemical processes, and to indicate the presence of disequilibrium effects. As molecules have the potential to reveal atmospheric conditions and chemistry, searching for them is a high priority. The rotational-vibrational transition bands of water, carbon monoxide and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jupiter planets. As these bands can overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy. Here we report a near-infrared transmission spectrum of the planet HD 189733b that shows the presence of methane. Additionally, a resolved water vapour band at 1.9 mum confirms the recent claim of water in this object. On thermochemical grounds, carbon monoxide is expected to be abundant in the upper atmosphere of hot-Jupiter planets, but is not identifiable here; therefore the detection of methane rather than carbon monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanism that leads to an enhancement of methane.
预计系外行星大气中存在的分子会强烈影响大气辐射平衡，追踪动力学和化学过程，并表明不平衡效应的存在。由于分子有揭示大气条件和化学性质的潜力，寻找它们是当务之急。水、一氧化碳和甲烷的转动 - 振动跃迁带预计是热木星行星中非连续不透明度的主要来源。由于这些谱带在波长上可能重叠，且来自它们的相应信号较弱，决定性的识别需要精确的红外光谱学。在此，我们报告了行星HD 189733b的近红外透射光谱，该光谱显示存在甲烷。此外，在1.9微米处分辨出的水汽谱带证实了最近关于该天体存在水的说法。基于热化学原理，预计一氧化碳在热木星行星的高层大气中含量丰富，但在此处无法识别；因此，在这样一颗热行星中检测到甲烷而非一氧化碳，可能表明远离永久昼侧存在水平化学梯度，或者可能意味着存在一种尚未完全理解的光化学机制导致甲烷增加。
