  We calculated hydrogen recombination line luminosities (H-$\alpha$,
Paschen-$\beta$ and Brackett-$\gamma$) from three dimensional
thermo-hydrodynamical simulations of forming planets from 1 to 10
Jupiter-masses. We explored various opacities to estimate the line emissions
with extinction in each cases assuming boundary layer accretion. When realistic
opacities are considered, only lines from planets $\ge$10 Jupiter-mass can be
detected with current instrumentation, highlighting that from most planets one
cannot expect detectable emission. This might explain the very low detection
rate of H-$\alpha$ from forming planets from observations. While the line
emission comes from both the forming planet and its circumplanetary disk, we
found that only the disk component could be detected due to extinction. We
examined the line variability as well, and found that it is higher for higher
mass planets. Furthermore, we determine for the first time, the parametric
relationship between the mass of the planet and the luminosity of the hydrogen
recombination lines, as well as the equation between the accretion luminosity
and hydrogen recombination line luminosities.
