1. UCO/Lick Observatory, University of California, Santa Cruz, California 95064, USA
2. NASA/Goddard Space Flight Center, Planetary Systems Branch, Code 693, Greenbelt, Maryland 20771, USA
3. Carnegie Institute of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington DC 20015, USA

Correspondence to: Gregory Laughlin¹ Correspondence and requests for materials should be addressed to G.L. (Email: laughlin@ucolick.org).

Near-infrared observations of more than a dozen 'hot-Jupiter' extrasolar planets have now been reported^{1, 2, 3, 4, 5}. These planets display a wide diversity of properties, yet all are believed to have had their spin periods tidally spin-synchronized with their orbital periods, resulting in permanent star-facing hemispheres and surface flow patterns that are most likely in equilibrium. Planets in significantly eccentric orbits can enable direct measurements of global heating that are largely independent of the details of the hydrodynamic flow⁶. Here we report 8-m photometric observations of the planet HD 80606b during a 30-hour interval bracketing the periastron passage of its extremely eccentric 111.4-day orbit. As the planet received its strongest irradiation (828 times larger than the flux received at apastron) its maximum 8-m brightness temperature increased from 800 K to 1,500 K over a six-hour period. We also detected a secondary eclipse for the planet, which implies an orbital inclination of i  90°, fixes the planetary mass at four times the mass of Jupiter, and constrains the planet's tidal luminosity. Our measurement of the global heating rate indicates that the radiative time constant at the planet's 8-m photosphere is 4.5 h, in comparison with 3–5 days in Earth's stratosphere⁷.

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