Polytropic index for strongly-irradiated planets as a function of M_p at 100 Myr (dotted), 1 Gyr (dashed) and 5 Gyr (solid). As the irradiated atmosphere impedes the radiative cooling of the objects, it retards its contraction. Therefore, the non-monotonous behavior observed at the early ages in the non-irradiated case (Fig. 3) is enhanced, even at a later epoch. The bump at the high mass end of the 100 Myr curve is caused by deuterium burning (see text). The shaded area represents the uncertainty on the polytropic index for the 5  Gyr case (see text).