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Planetary Transits with the ALMA Radio Interferometer

Planetary transits are commonly observed at visible wavelengths. Here we investigate the shape of a planetary transit observed at radio wavelengths. Solar maps of the Sun at 17 GHz are used as a proxy for the stellar eclipse by several sizes of planets from Super-Earths to Hot Jupiters. The relative depth at mid transit is the same as observed at visible wavelengths, but the limb brightening of the stellar disk at 17 GHz is clearly seen in the shape of the transit light curve. Moreover, when the planet occults an active region the depth of the transit decreases even further, depending on the brightness of the active region relative to the surrounding disk. For intense active region, with 50 times the brightness temperature of the surrounding disk, the decrease can supersede the unperturbed transit depth depending on the size of the eclipsing planet. For a Super-Earth (R_p=0.02 R_s) crossing, the decrease in intensity is 0.04%, increasing to 0.86% in the case when a strong active region is present. On the other hand, for a hot Jupiter with R_p=0.17 R_s, the unperturbed transit depth is 3% increasing to 4.7% when covering this strong active region. This kind of behavior can be verified with observation of planetary transits with the ALMA radio interferometer.