  The depth of an exoplanetary transit in the light curve of a distant star is
commonly approximated as the squared planet-to-star radius ratio, (R_p/R_s)^2.
Stellar limb darkening, however, results in significantly deeper transits. Here
we derive analytical solutions to the overshoot of the mid-transit depth caused
by stellar limb darkening compared to the (R_p/R_s)^2 estimate for arbitrary
transit impact parameters. In turn, this allows us to compute the true
planet-to-star radius ratio from the transit depth for a given parameterization
of a limb darkening law and for a known transit impact parameter. We calculate
the maximum emerging specific stellar intensity covered by the planet in
transit and derive analytic solutions for the transit depth overshoot.
Solutions are presented for the linear, quadratic, square-root, logarithmic,
and non-linear stellar limb darkening with arbitrary transit impact parameters.
We also derive formulae to calculate the average intensity along the transit
chord, which allows us to estimate the actual transit depth (and therefore
R_p/R_s) from the mean in-transit flux. The transit depth overshoot of
exoplanets compared to the (R_p/R_s)^2 estimate increases from about 15% for A
main-sequence stars to roughly 20% for sun-like stars and some 30% for K and M
stars. The error in our analytical solutions for R_p/R_s from the small planet
approximation is orders of magnitude smaller than the uncertainties arising
from typical noise in real light curves and from the uncertain limb darkening.
Our equations can be used to predict with high accuracy the expected transit
depth of extrasolar planets. The actual planet radius can be calculated from
the measured transit depth or from the mean in-transit flux if the stellar limb
darkening can be properly parameterized and if the transit impact parameter is
known. Light curve fitting is not required.
