An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the<i>MOST</i>Satellite

Abstract

We present space-based photometry of the transiting exoplanetary system HD\n209458 obtained with the MOST (Microvariablity and Oscillations of STars)\nsatellite, spanning 14 days and covering 4 transits and 4 secondary eclipses.\nThe HD 209458 photometry was obtained in MOST's lower-precision Direct Imaging\nmode, which is used for targets in the brightness range $6.5 &lt; V &lt; 13$. We\ndescribe the photometric reduction techniques for this mode of observing, in\nparticular the corrections for stray Earthshine. We do not detect the secondary\neclipse in the MOST data, to a limit in depth of 0.053 mmag (1 \\sigma). We set\na 1 \\sigma upper limit on the planet-star flux ratio of 4.88 x 10^-5\ncorresponding to a geometric albedo upper limit in the MOST bandpass (400 to\n700 nm) of 0.25. The corresponding numbers at the 3 \\sigma level are 1.34 x\n10^-4 and 0.68 respectively. HD 209458b is half as bright as Jupiter in the\nMOST bandpass. This low geometric albedo value is an important constraint for\ntheoretical models of the HD209458b atmosphere, in particular ruling out the\npresence of reflective clouds. A second MOST campaign on HD 209458 is expected\nto be sensitive to an exoplanet albedo as low as 0.13 (1 sigma), if the star\ndoes not become more intrinsically variable in the meantime.\n