Progress in understanding of giant planet formation has been hampered by a lack of observational constraints to growing protoplanets. Recently, detection of an H{alpha}-emission excess via direct imaging was reported for the protoplanet LkCa15b orbiting the pre-main-sequence star LkCa15. However, the physical mechanism for the H{alpha} emission is poorly understood. According to recent high-resolution three-dimensional hydrodynamic simulations of the flow accreting onto protoplanets, the disk gas flows down almost vertically onto and collides with the surface of a circum-planetary disk at a super-sonic velocity and thus passes through a strong shockwave. The shock-heated gas is hot enough to generate H{alpha} emission. Here we develop a one-dimensional radiative hydrodynamic model of the flow after the shock by detailed calculations of chemical reactions and electron transitions in hydrogen atoms, and quantify the hydrogen line emission in the Lyman-, Balmer-, and Paschen-series from the accreting gas giant system. We then demonstrate that the H{alpha} intensity is strong enough to be detected with current observational technique. Comparing our theoretical H{alpha} intensity with the observed one from LkCa15b, we constrain the protoplanet mass and the disk gas density. Observation of hydrogen line emission from protoplanets is highly encouraged to obtain direct constraints of accreting gas giants, which will be key in understanding the formation of gas giants.
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