  It has been suggested that hot stellar wind gas in a bow shock around an
exoplanet is sufficiently opaque to absorb stellar photons and give rise to an
observable transit depth at optical and UV wavelengths. In the first part of
this paper, we use the CLOUDY plasma simulation code to model the absorption
from X-ray to radio wavelengths by 1-D slabs of gas in coronal equilibrium with
varying densities ($10^{4}-10^{8} \, {\rm cm^{-3}}$) and temperatures
($2000-10^{6} \ {\rm K}$) illuminated by a solar spectrum. For slabs at coronal
temperatures ($10^{6} \ {\rm K}$) and densities even orders of magnitude larger
than expected for the compressed stellar wind ($10^{4}-10^{5} \, {\rm
cm^{-3}}$), we find optical depths orders of magnitude too small ($>
3\times10^{-7}$) to explain the $\sim3\%$ UV transit depths seen with Hubble.
Using this result and our modeling of slabs with lower temperatures ($2000-10^4
{\rm K}$), the conclusion is that the UV transits of WASP-12b and HD 189733b
are likely due to atoms originating in the planet, as the stellar wind is too
highly ionized. A corollary of this result is that transport of neutral atoms
from the denser planetary atmosphere outward must be a primary consideration
when constructing physical models. In the second part of this paper, additional
calculations using CLOUDY are carried out to model a slab of planetary gas in
radiative and thermal equilibrium with the stellar radiation field. Promising
sources of opacity from the X-ray to radio wavelengths are discussed, some of
which are not yet observed.
