A map of the day–night contrast of the extrasolar planet HD 189733b

Abstract

‘Hot Jupiter’ extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 au is the average Sun–Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets1,2,3. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet’s surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems4,5,6, over half an orbital period, from which we can construct a ‘map’ of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 ± 33 K and a maximum brightness temperature of 1,212 ± 11 K at a wavelength of 8 μm, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter7. Our data indicate that the peak hemisphere-integrated brightness occurs 16 ± 6° before opposition, corresponding to a hotspot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 ± 24 s later than predicted, which may indicate a slightly eccentric orbit.

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Figure 1: Observed phase variation for HD 189733b, with transit and secondary eclipse visible.
Figure 2: Time series of the transit and secondary eclipse.
Figure 3: Brightness estimates for 12 longitudinal strips on the surface of the planet.

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Acknowledgements

We thank J. Winn for sharing data from a recent paper describing the behaviour of the spots on the star, and D. Sasselov and E. Miller-Ricci for discussions on the properties of these spots. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. We are grateful to the entire Spitzer team for their assistance throughout this process. H.A.K. was supported by a National Science Foundation Graduate Research Fellowship.

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Correspondence to Heather A. Knutson.

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Knutson, H., Charbonneau, D., Allen, L. et al. A map of the day–night contrast of the extrasolar planet HD 189733b. Nature 447, 183–186 (2007). https://doi.org/10.1038/nature05782

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