  The role of radiation pressure in shaping exoplanet photoevaporation remains
a topic of contention. Radiation pressure from the exoplanet's host star has
been proposed as a mechanism to drive the escaping atmosphere into a "cometary"
tail and explain the high velocities observed in systems where mass loss is
occurring. In this paper we present results from high-resolution 3-D
hydrodynamic simulations of a planet similar to HD 209458b. We
self-consistently launch a wind flowing outward from the planet by calculating
the ionization and heating resulting from incident high-energy radiation, and
account for radiation pressure. We first present a simplified calculation,
setting a limit on the Lyman-$\alpha$ flux required to drive the
photo-evaporated planetary material to larger radii and line-of-sight
velocities. We then present the results of our simulations, which confirm the
limits determined by our analytic calculation. We thus demonstrate that, within
the limits of our hydrodynamic simulation and for the Lyman-$\alpha$ fluxes
expected for HD 209458, radiation pressure is unlikely to significantly affect
photoevaporative winds or to explain the high velocities at which wind material
is observed, though further possibilities remain to be investigated.
