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Most models used to predict or fit exoplanet transmission spectra do notinclude all the effects of atmospheric refraction. Namely, the angular size ofthe star with respect to the planet can limit the lowest altitude, or highestdensity and pressure, probed during primary eclipses, as no rays passing belowthis critical altitude can reach the observer. We discuss this geometricaleffect of refraction for all exoplanets, and tabulate the critical altitude,density and pressure for an exoplanet identical to Earth with a 1 bar N2/O2atmosphere, as a function of both the incident stellar flux (Venus, Earth, andMars-like) at the top of the atmosphere, and the spectral type (O5-M9) of thehost star. We show that such a habitable exo-Earth can be probed to a surfacepressure of 1 bar only around the coolest stars. We present 0.4-5.0 micronmodel transmission spectra of Earth's atmosphere viewed as a transitingexoplanet, and show how atmospheric refraction modifies the transmissionspectrum depending on the spectral type of the host star. We demonstrate thatrefraction is another phenomenon that can potentially explain flat transmissionspectra over some spectral regions.

Impact of atmospheric refraction: How deeply can we probe exo-Earth's atmospheres during primary eclipse observations?