  Most models used to predict or fit exoplanet transmission spectra do not
include all the effects of atmospheric refraction. Namely, the angular size of
the star with respect to the planet can limit the lowest altitude, or highest
density and pressure, probed during primary eclipses, as no rays passing below
this critical altitude can reach the observer. We discuss this geometrical
effect of refraction for all exoplanets, and tabulate the critical altitude,
density and pressure for an exoplanet identical to Earth with a 1 bar N2/O2
atmosphere, as a function of both the incident stellar flux (Venus, Earth, and
Mars-like) at the top of the atmosphere, and the spectral type (O5-M9) of the
host star. We show that such a habitable exo-Earth can be probed to a surface
pressure of 1 bar only around the coolest stars. We present 0.4-5.0 micron
model transmission spectra of Earth's atmosphere viewed as a transiting
exoplanet, and show how atmospheric refraction modifies the transmission
spectrum depending on the spectral type of the host star. We demonstrate that
refraction is another phenomenon that can potentially explain flat transmission
spectra over some spectral regions.
