Recent observations of the extrasolar planet HD 189733b did not reveal the presence of water in the emission spectrum of the planet1. Yet models of such ‘hot-Jupiter’ planets predict an abundance of atmospheric water vapour2. Validating and constraining these models is crucial to understanding the physics and chemistry of planetary atmospheres in extreme environments. Indications of the presence of water in the atmosphere of HD 189733b have recently been found in transmission spectra3,4, where the planet’s atmosphere selectively absorbs the light of the parent star, and in broadband photometry5. Here we report the detection of strong water absorption in a high-signal-to-noise, mid-infrared emission spectrum of the planet itself. We find both a strong downturn in the flux ratio below 10 µm and discrete spectral features that are characteristic of strong absorption by water vapour. The differences between these and previous observations are significant and admit the possibility that predicted planetary-scale dynamical weather structures6 may alter the emission spectrum over time. Models that match the observed spectrum and the broadband photometry suggest that heat redistribution from the dayside to the nightside is weak. Reconciling this with the high nightside temperature7 will require a better understanding of atmospheric circulation or possible additional energy sources.
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This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech), under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This study was supported in part by NASA (grant NNGO4GL22G).
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Grillmair, C., Burrows, A., Charbonneau, D. et al. Strong water absorption in the dayside emission spectrum of the planet HD 189733b. Nature 456, 767–769 (2008). https://doi.org/10.1038/nature07574
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