1. Spitzer Science Center, 1200 East California Boulevard, Pasadena, California 91125, USA
2. Department of Astrophysical Sciences, Princeton University, Peyton Hall, Ivy Lane, Princeton, New Jersey 08544, USA
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
4. Department of Astronomy, University of Washington, Box 351580, Seattle, Washington 98195, USA
5. Ball Aerospace & Technologies Corporation, PO Box 1062, Boulder, Colorado 80306, USA
6. Michelson Science Center, California Institute of Technology, Mail Stop 100-22, Pasadena, California 91125, USA

Correspondence to: Carl J. Grillmair¹ Correspondence and requests for materials should be addressed to C.J.G. (Email: carl@ipac.caltech.edu).

Abstract

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 spectra^{3, 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 m 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.

1. Spitzer Science Center, 1200 East California Boulevard, Pasadena, California 91125, USA
2. Department of Astrophysical Sciences, Princeton University, Peyton Hall, Ivy Lane, Princeton, New Jersey 08544, USA
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
4. Department of Astronomy, University of Washington, Box 351580, Seattle, Washington 98195, USA
5. Ball Aerospace & Technologies Corporation, PO Box 1062, Boulder, Colorado 80306, USA
6. Michelson Science Center, California Institute of Technology, Mail Stop 100-22, Pasadena, California 91125, USA

Correspondence to: Carl J. Grillmair¹ Correspondence and requests for materials should be addressed to C.J.G. (Email: carl@ipac.caltech.edu).

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