Resolving the Surfaces of Extrasolar Planets With Secondary Eclipse Light Curves

We present a method that employs the secondary eclipse light curves of transiting extrasolar planets to probe the spatial variation of their thermal emission. This technique permits an observer to resolve the surface of the planet without needing to isolate its light from that of the central star. We evaluate the feasibility of this technique for the HD 209458 system by first modeling the secondary eclipse light curves that would result from several representations of the planetary emission and then simulating the noise properties of observations of this signal with the Spitzer Space Telescope Infrared Array Camera (IRAC). We consider two representations of the planetary thermal emission: a simple parameterized model, and the results of a dynamical simulation of the planetary atmosphere previously published by Cooper & Showman. We find that observations of the secondary eclipse light curve are most sensitive to the longitudinal contrast in the emission of the planetary hemisphere visible near opposition. We quantify this signal with a parameter, the "uniform time offset," which measures the time delay between the observed secondary eclipse and that of a simulated, uniformly-emitting planet. We compare the predicted amplitude of this parameter for HD 209458b with the precision with which IRAC could measure it. We find that IRAC observations at 3.6 um of a single secondary eclipse should permit sufficient precision to test the Cooper & Showman model of the planetary atmosphere. We quantify the signal-to-noise ratio for this offset in the remaining IRAC bands and find that a modest improvement in photometric precision should permit similarly robust detections