[Abridged] The transmission of light through a planetary atmosphere can bestudied as a function of altitude and wavelength using stellar or solaroccultations, giving often unique constraints on the atmospheric composition.For exoplanets, a transit yields a limb-integrated, wavelength-dependenttransmission spectrum of an atmosphere. When scattering haze and/or cloudparticles are present in the planetary atmosphere, the amount of transmittedflux not only depends on the total optical thickness of the slant light paththat is probed, but also on the amount of forward-scattering by the scatteringparticles. Here, we present results of calculations with a three-dimensionalMonte Carlo code that simulates the transmitted flux during occultations ortransits. For isotropically scattering particles, like gas molecules, thetransmitted flux appears to be well-described by the total atmospheric opticalthickness. Strongly forward-scattering particles, however, such as commonlyfound in atmospheres of Solar System planets, can increase the transmitted fluxsignificantly. For exoplanets, such added flux can decrease the apparent radiusof the planet by several scale heights, which is comparable to predicted andmeasured features in exoplanet transit spectra. We performed detailedcalculations for Titan's atmosphere between 2.0 and 2.8 micron and show thathaze and gas abundances will be underestimated by about 8% ifforward-scattering is ignored in the retrievals. At shorter wavelengths, errorsin the gas and haze abundances and in the spectral slope of the haze particlescan be several tens of percent, also for other Solar System planetaryatmospheres. We also find that the contribution of forward-scattering can befairly well described by modelling the atmosphere as a plane-parallel slab.
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