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Infrared radiation from an extrasolar planet


A class of extrasolar giant planets—the so-called ‘hot Jupiters’ (ref. 1)—orbit within 0.05 au of their primary stars (1 au is the Sun–Earth distance). These planets should be hot and so emit detectable infrared radiation2. The planet HD 209458b (refs 3, 4) is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter5), which may be the result of ongoing tidal dissipation6, but this explanation requires a non-zero orbital eccentricity ( 0.03; refs 6, 7), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 µm) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24-µm flux is 55 ± 10 µJy (1σ), with a brightness temperature of 1,130 ± 150 K, confirming the predicted heating by stellar irradiation2,8. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within ± 7 min, 1σ), which means that a dynamically significant orbital eccentricity is unlikely.

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Figure 1: Observations showing our detection of the secondary eclipse in HD 209458.
Figure 2: Amplitude of the secondary eclipse versus assumed central phase, with confidence intervals for both.
Figure 3: Flux from a model atmosphere shown in comparison to our measured infrared flux at 24 µm.


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We thank G. Laughlin for communicating the latest orbital eccentricity solutions from the Doppler data and for his evaluation of their status. We acknowledge informative conversations with D. Charbonneau, G. Marcy, B. Hansen, K. Menou and J. Cho. 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 directly by NASA, and by its Origins of Solar Systems programme and Astrobiology Institute. We thank all the personnel of the Spitzer telescope and the MIPS instrument, who ultimately made these measurements possible. L.J.R. is a National Research Council Associate at NASA's Goddard Space Flight Center.

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Correspondence to Drake Deming.

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Supplementary information

Supplementary Figure S1

Sample flat-fielded and flux-calibrated MIPS image of HD 209458 produced by the Spitzer S11.0 data processing pipeline. (PDF 95 kb)

Supplementary Figure S2

A tabulation of the temporal deviations in normalized intensity for all detector pixels not containing the star, with Fourier power spectra, for a sample data block. Shows gaussian white noise properties. (PDF 70 kb)

Supplementary Figure S3

A tabulation of the temporal deviations in normalized intensity for all detector pixels within ± 10 pixels of the the star, for a sample data block. Shows gaussian property of the noise. (PDF 47 kb)

Supplementary Figure Legends

Legends to accompany Supplementary Figures S1-S3. (DOC 23 kb)

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Deming, D., Seager, S., Richardson, L. et al. Infrared radiation from an extrasolar planet. Nature 434, 740–743 (2005).

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