Measuring the albedo of an extrasolar planet provides insights into its atmospheric composition and its global thermal properties , including heat dissipation and weather patterns .
Such a measurement requires very precise photometry of a transiting system sampling fully many phases of the secondary eclipse .
Spacebased optical photometry of the transiting system HD 209458 from the MOST ( Microvariablity and Oscillations of STars ) satellite , spanning 14 and 44 days in 2004 and 2005 respectively , allows us to set a sensitive limit on the optical eclipse of the hot exosolar giant planet in this system .
Our best fit to the observations yields a flux ratio of the planet and star of 7 \pm 9 ppm ( parts per million ) , which corresponds to a geometric albedo through the MOST 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 of 0.17 .
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 bright reflective clouds in this exoplanet ’ s atmosphere .
We determine refined parameters for the star and exoplanet in the HD 209458 system based on a model fit to the MOST light curve .