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Measuring the albedo of an extrasolar planet provides insights into itsatmospheric composition and its global thermal properties, including heatdissipation and weather patterns. Such a measurement requires very precisephotometry of a transiting system sampling fully many phases of the secondaryeclipse. Spacebased optical photometry of the transiting system HD 209458 fromthe MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 and44 days in 2004 and 2005 respectively, allows us to set a sensitive limit onthe optical eclipse of the hot exosolar giant planet in this system. Our bestfit to the observations yields a flux ratio of the planet and star of 7 $\pm$ 9ppm (parts per million), which corresponds to a geometric albedo through theMOST bandpass (400-700 nm) of $A_g$ = 0.038 $\pm$ 0.045. This gives a 1$\sigma$upper limit of 0.08 for the geometric albedo and a 3$\sigma$ upper limit of0.17. HD 209458b is significantly less reflective than Jupiter (for which $A_g$would be about 0.5). This low geometric albedo rules out the presence of brightreflective clouds in this exoplanet's atmosphere. We determine refinedparameters for the star and exoplanet in the HD 209458 system based on a modelfit to the MOST light curve.

The Very Low Albedo of an Extrasolar Planet: MOST Spacebased Photometry of HD 209458