  Several short-period Jupiter-mass planets have been discovered around nearby
solar-type stars. During the circularization of their orbits, the dissipation
of tidal disturbance by their host stars heats the interior and inflates the
sizes of these planets. Based on a series of internal structure calculations
for giant planets, we examine the physical processes which determine their
luminosity-radius relation. In order for young or intensely heated gas giant
planets to attain quasi-hydrostatic equilibria, with sizes comparable to or
larger than two Jupiter radii, their interiors must have sufficiently high
temperature and low density such that degeneracy effects are relatively weak
compared to those in a mature and compact Jupiter. Consequently, the polytropic
index monotonically increases whereas the central temperature increases and
then decreases with the planets' size. These effects, along with a
temperature-sensitive opacity for the radiative surface layers of giant
planets, cause the power index of the luminosity's dependence on radius to
decrease with increasing radius. For planets larger than twice Jupiter's
radius, this index is sufficiently small that they become unstable to tidal
inflation. We make comparisons between cases of uniform heating and cases in
which the heating is concentrated in various locations within the giant planet.
Based on these results we suggest that accurate measurement of the sizes of
close-in young Jupiters can be used to probe their internal structure under the
influence of tidal heating.
