Title: A Theory of Exoplanet Transits with Light Scattering,
Abstract: Exoplanet transit spectroscopy enables the characterization of distant
worlds, and will yield key results for NASA's James Webb Space Telescope.
However, transit spectra models are often simplified, omitting potentially
important processes like refraction and multiple scattering. While the former
process has seen recent development, the effects of light multiple scattering
on exoplanet transit spectra has received little attention. Here, we develop a
detailed theory of exoplanet transit spectroscopy that extends to the full
refracting and multiple scattering case. We explore the importance of
scattering for planet-wide cloud layers, where the relevant parameters are the
slant scattering optical depth, the scattering asymmetry parameter, and the
angular size of the host star. The latter determines the size of the "target"
for a photon that is back-mapped from an observer. We provide results that
straightforwardly indicate the potential importance of multiple scattering for
transit spectra. When the orbital distance is smaller than 10-20 times the
stellar radius, multiple scattering effects for aerosols with asymmetry
parameters larger than 0.8-0.9 can become significant. We provide examples of
the impacts of cloud/haze multiple scattering on transit spectra of a hot
Jupiter-like exoplanet. For cases with a forward and conservatively scattering
cloud/haze, differences due to multiple scattering effects can exceed 200 ppm,
but shrink to zero at wavelength ranges corresponding to strong gas absorption
or when the slant optical depth of the cloud exceeds several tens. We conclude
with a discussion of types of aerosols for which multiple scattering in transit
spectra may be important.