Kernel density estimate of the distribution of the radii in the biased synthetic population obtained with the hydrodynamic escape model. The grey dashed line includes all detectable planet (at all orbital periods). The green, orange, and blue lines include planets with log (period/day) of 0.8 ± 0.1, 1.3 ± 0.1, and 1.8 ± 0.1. One sees how the centre of the valley shifts to smaller radii with increasing orbital period.