1. Exoplanets and Stellar Astrophysics Laboratory, Mail Code 667
2. Planetary Systems Laboratory, Mail Code 693, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3. Department of Physics and Space Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA
4. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd, NW, Washington DC 20015, USA
5. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
6. Department of Physics, University of Central Florida, Orlando, Florida 32816, USA

Correspondence to: L. Jeremy Richardson¹ Correspondence and requests for materials should be addressed to L.J.R. (Email: lee.richardson@colorado.edu).

Abstract

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 atmospheres^{1, 2}. One such technique^{3, 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 m) 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 m that we attribute to emission by silicate clouds. We also find a narrow, unidentified emission feature at 7.78 m. Models of these 'hot Jupiter'⁵ planets predict a flux peak^{6, 7, 8, 9} near 10 m, 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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