  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.
