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A spectrum of an extrasolar planet


Of the over 200 known extrasolar planets, 14 exhibit transits in front of their parent stars as seen from Earth. Spectroscopic observations of the transiting planets can probe the physical conditions of their atmospheres1,2. One such technique3,4 can be used to derive the planetary spectrum by subtracting the stellar spectrum measured during eclipse (planet hidden behind star) from the combined-light spectrum measured outside eclipse (star + planet). Although several attempts have been made from Earth-based observatories, no spectrum has yet been measured for any of the established extrasolar planets. Here we report a measurement of the infrared spectrum (7.5–13.2 µm) of the transiting extrasolar planet HD 209458b. Our observations reveal a hot thermal continuum for the planetary spectrum, with an approximately constant ratio to the stellar flux over this wavelength range. Superposed on this continuum is a broad emission peak centred near 9.65 µm that we attribute to emission by silicate clouds. We also find a narrow, unidentified emission feature at 7.78 µm. Models of these ‘hot Jupiter’5 planets predict a flux peak6,7,8,9 near 10 µm, where thermal emission from the deep atmosphere emerges relatively unimpeded by water absorption, but models dominated by water fit the observed spectrum poorly.

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Figure 1: Wavelength-integrated flux as a function of orbital phase, showing the detection of the secondary eclipse, centred at phase 0.5.
Figure 2: The spectrum of HD 209458b from 7.5 μm to 13.2 µm.
Figure 3: Separate analysis to confirm the unidentified emission feature near 7.8 µm.


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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 a contract with NASA. We acknowledge cooperation with D. Charbonneau, C. Grillmair and H. Knutson. Our understanding of the long-term telescope pointing drift was derived from H. Knutson’s study of the effect in their 30 h programme to measure the light curve of HD 189733b. D. Charbonneau provided his measurement of the eclipse depth of HD 209485b, which was key to casting our results in terms of contrast, rather than differential contrast. We also thank M. Swain and A. Mainzer for discussions regarding the telescope pointing oscillation. We thank the teams that designed, built, operate and support the Spitzer Space Telescope and the IRS. We also thank the NASA Astrobiology Institute, which has centres at both NASA Goddard and the Carnegie Institution of Washington. L.J.R. acknowledges support as a NASA Postdoctoral Fellow at NASA Goddard (formerly the NRC Research Associateship Program). K.H. performed the high-pass filtering analysis as part of her participation in the Summer Undergraduate Internship in Astrobiology, funded by the Goddard Center for Astrobiology. S.S. thanks the Spitzer Theory Program and the Carnegie Institution of Washington for support.

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Correspondence to L. Jeremy Richardson.

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Supplementary information

Supplementary Methods

This file contains Supplementary Methods with detailed description of methods used to reduce the data, including Supplementary Figures 1-4 and additional references. (PDF 417 kb)

Supplementary Data

This file contains Supplementary Data with the actual spectra derived from our data for both eclipse events. Columns are wavelength in microns, contrast, and 1-σ uncertainty (TXT 7 kb)

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Richardson, L., Deming, D., Horning, K. et al. A spectrum of an extrasolar planet. Nature 445, 892–895 (2007).

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