Rapid heating of the atmosphere of an extrasolar planet

Near- infrared observations of more than a dozen 'hot-Jupiter' extrasolar planets have now been reported(1-5). 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 flow(6). Here we report 8-mu 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- mm brightness temperature increased from 800 K to 1,500K over a six- hour period. We also detected a secondary eclipse for the planet, which implies an orbital inclination of i approximate to 90 degrees, 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- mu m photosphere is similar to 4.5 h, in comparison with 3 - 5 days in Earth's stratosphere(7).