1. Department of Physics, University of Central Florida, Orlando, Florida 32816-2385, USA
2. Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853-6801, USA
3. Department of Astronomy, University of California, Berkeley, California 94720-3411, USA
4. Departments of Earth, Atmospheric, and Planetary Sciences and of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
5. Planetary Systems Laboratory, Code 693
6. Exoplanet and Stellar Astrophysics Laboratory, Code 667, NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771-0001, USA

Correspondence to: Joseph Harrington^1,2 Correspondence and requests for materials should be addressed to J.H. (Email: jharring@physics.ucf.edu).

Abstract

Of the over 200 known extrasolar planets, just 14 pass in front of and behind their parent stars as seen from Earth. This fortuitous geometry allows direct determination of many planetary properties¹. Previous reports of planetary thermal emission^{2, 3, 4, 5} give fluxes that are roughly consistent with predictions based on thermal equilibrium with the planets' received radiation, assuming a Bond albedo of 0.3. Here we report direct detection of thermal emission from the smallest known transiting planet, HD 149026b, that indicates a brightness temperature (an expression of flux) of 2,300  200 K at 8 m. The planet's predicted temperature for uniform, spherical, blackbody emission and zero albedo (unprecedented for planets) is 1,741 K. As models with non-zero albedo are cooler, this essentially eliminates uniform blackbody models, and may also require an albedo lower than any measured for a planet, very strong 8 m emission, strong temporal variability, or a heat source other than stellar radiation. On the other hand, an instantaneous re-emission blackbody model, in which each patch of surface area instantly re-emits all received light, matches the data. This planet is known^{6, 7, 8, 9} to be enriched in heavy elements, which may give rise to novel atmospheric properties yet to be investigated.

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