  The luminosity of young giant planets can inform about their formation and
accretion history. The directly imaged planets detected so far are consistent
with the "hot-start" scenario of high entropy and luminosity. If nebular gas
passes through a shock front before being accreted into a protoplanet, the
entropy can be substantially altered. To investigate this, we present high
resolution, 3D radiative hydrodynamic simulations of accreting giant planets.
The accreted gas is found to fall with supersonic speed in the gap from the
circumstellar disk's upper layers onto the surface of the circumplanetary disk
and polar region of the protoplanet. There it shocks, creating an extended hot
supercritical shock surface. This shock front is optically thick, therefore, it
can conceal the planet's intrinsic luminosity beneath. The gas in the vertical
influx has high entropy which when passing through the shock front decreases
significantly while the gas becomes part of the disk and protoplanet. This
shows that circumplanetary disks play a key role in regulating a planet's
thermodynamic state. Our simulations furthermore indicate that around the shock
surface extended regions of atomic - sometimes ionized - hydrogen develop.
Therefore circumplanetary disk shock surfaces could influence significantly the
observational appearance of forming gas-giants.
