Temporal evolution of the luminosity and the radius of a close-in, super-Earth planet with a tenuous H₂/He envelope (M_Z = 4.2   M_⊕, M_XY = 0.045   M_⊕, i.e., about 1%). Left panel: intrinsic luminosity as a function of time. The red solid line is the total intrinsic luminosity, while the blue dotted line is L_radio. The left axis gives L in units of present-day intrinsic Jovian luminosities. The right panel shows the total radius (red solid line) and the radius of the solid core (blue dotted line). The inset figure shows the total radius at late times. Note the delayed contraction between about 0.2 and 2 Gyr due to radiogenic heating. Atmospheric mass loss and outgassing are neglected, so that one directly sees the effect of L_radio. These simplifications could, however, also mean that the evolution is in reality more complex than shown here.