  Recent observations of the extrasolar planet HD 189733b did not reveal the
presence of water in the emission spectrum of the planet. Yet models of such
'Hot Jupiter' planets predict an abundance of atmospheric water vapour.
Validating and constraining these models is crucial for 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, where the planet's atmosphere
selectively absorbs the light of the parent star, and in broadband photometry.
Here we report on 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 microns 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 structures
might alter the emission spectrum over time. Models that match the observed
spectrum and the broadband photometry suggest that heat distribution from the
dayside to the night side is weak. Reconciling this with the high night side
temperature will require a better understanding of atmospheric circulation or
possible additional energy sources.
