Atmospheric Lensing and Oblateness Effects during an Extrasolar Planetary Transit

Future high precision photometric measurements of transiting extrasolarplanets promise to tell us much about the nature of these systems. We examinehow atmospheric lensing and (projected) planet oblateness/ellipticity modifytransit light curves. The large density gradients expected in planetatmospheres can offset the unfavorably large observer-lens to source-lensdistance ratio, and allow the existence of caustics. Under such conditions,starlight from all points in the planet's shadow is refracted into viewproducing a characteristic slowing down of the dimming at ingress (vice versafor egress). A search over several parameters, the planet radius etc., cannotproduce a nonlensed transit light curve that can mimic a lensed light curve.The fractional change in the diminution of starlight is approximately the ratioof atmospheric scale height to planet radius, expected to be 1 % or less.Planet oblateness induces an asymmetry to the transit light curve about thepoint of minimum flux, depending on the planet orientation. The fractionalasymmetry is around 0.5 % for a projected ellipticity of 10 %, independent oflensing. For favorable ratios of planet to star radius, the above effects arepotentially observable with future space-based missions. This will allow us toconstrain the planet shape, and its atmospheric scale height, density andrefractive coefficient, providing information on its rotation, temperature andcomposition. For HD 209458b, the only currently known transiting extrasolarplanet, caustics are absent because of the very small lens-source separation.Finally, we provide estimates of other variations to transit light curves thatcould be of comparable importance, including rings, satellites, stellaroscillations, star spots, and weather.Comment: 35 pages, 13 figures, submitted to Ap