  We have made a comprehensive transit search for exoplanets down to about 2
Earth radii in the HD 209458 system, based on nearly uninterrupted broadband
optical photometry obtained with the MOST (Microvariability and Oscillations of
Stars) satellite, spanning 14 days in 2004 and 44 days in 2005. We have
searched these data for limb-darkened transits at periods other than that of
the known giant planet, from about 0.5 days to 2 weeks. Monte Carlo statistical
tests of the data with synthetic transits inserted allow us to rule out
additional close-in exoplanets with sizes ranging from about 0.20-0.36 RJ
(Jupiter radii), or 2.2-4.0 RE (Earth radii) on orbits whose planes are near
that of HD 209458b. These null results constrain theories that invoke lower
mass planets in orbits similar to HD 209458b to explain its anomalously large
radius, and those that predict "hot Earths" due to the inward migration of HD
209458b.
