Scattered light from close-in extrasolar planets:: Prospects of detection with the MOST satellite

The ultraprecise photometric space satellite MOST (Microvariability and Oscillations of STars) will provide the first opportunity to measure the albedos and scattered light curves from known short-period extrasolar planets. Because of the changing phases of an extrasolar planet as it orbits its parent star, the combined light of the planet-star system will vary on the order of tens of micromagnitudes. The amplitude and shape of the resulting light curve is sensitive to the planet's radius and orbital inclination, as well as the composition and size distribution of the scattering particles in the planet's atmosphere. To predict the capabilities of MOST and other planned space missions, we have constructed a series of models of such light curves, improving upon earlier work by incorporating more realistic details such as limb darkening of the star, intrinsic granulation noise in the star itself, tidal distortion and back-heating, higher angular resolution of the light scattering from the planet, and exploration of the significance of the angular size of the star as seen from the planet. We use photometric performance simulations of the MOST satellite, with the light-curve models as inputs, for one of the mission's primary targets, tau Bootis. These simulations demonstrate that, even adopting a very conservative signal detection limit of 4.2 mumag in amplitude ( not power), we will be able to either detect the tau Bootis planet light curve or put severe constraints on possible extrasolar planet atmospheric models.