1. Planetary Systems Laboratory and Goddard Center for Astrobiology, Code 693;
2. Exoplanet and Stellar Astrophysics Laboratory, Code 667, NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015, USA
4. Center for Radiophysics and Space Research, Cornell University, 326 Space Sciences Bldg, Ithaca, New York 14853-6801, USA

Correspondence to: Drake Deming¹ Correspondence and requests for materials should be addressed to D.D. (Email: Leo.D.Deming@nasa.gov).

Abstract

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 m) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24-m flux is 55 10 Jy (1), with a brightness temperature of 1,130 150 K, confirming the predicted heating by stellar irradiation^{2, 8}. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within 7 min, 1), which means that a dynamically significant orbital eccentricity is unlikely.

1. Planetary Systems Laboratory and Goddard Center for Astrobiology, Code 693;
2. Exoplanet and Stellar Astrophysics Laboratory, Code 667, NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015, USA
4. Center for Radiophysics and Space Research, Cornell University, 326 Space Sciences Bldg, Ithaca, New York 14853-6801, USA

Correspondence to: Drake Deming¹ Correspondence and requests for materials should be addressed to D.D. (Email: Leo.D.Deming@nasa.gov).

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