  We use an end-to-end model of planet formation, thermodynamic evolution, and
atmospheric escape to investigate how the statistical imprints of evaporation
depend on the bulk composition of planetary cores (rocky vs. icy). We find that
the population-wide imprints like the location of the "evaporation valley" in
the distance-radius plane and the corresponding bimodal radius distribution
clearly differ depending on the bulk composition of the cores. Comparison with
the observed position of the valley (Fulton et al. 2017) suggests that close-in
low-mass Kepler planets have a predominately Earth-like rocky composition.
Combined with the excess of period ratios outside of MMR, this suggests that
low-mass Kepler planets formed inside of the water iceline, but still
undergoing orbital migration. The core radius becomes visible for planets
losing all primordial H/He. For planets in this "triangle of evaporation" in
the distance-radius plane, the degeneracy in compositions is reduced. In the
observed diagram, we identify a trend to more volatile-rich compositions with
increasing radius (R/R_Earth<1.6 rocky; 1.6-3.0 ices and/or H/He; >3: H/He).
The mass-density diagram contains important information about formation and
evolution. Its characteristic broken V-shape reveals the transitions from solid
planets to low-mass core-dominated planets with H/He and finally to
gas-dominated giants. Evaporation causes density and orbital distance to be
anti-correlated for low-mass planets, in contrast to giants, where closer-in
planets are less dense, likely due to inflation. The temporal evolution of the
statistical properties reported here will be of interest for the PLATO 2.0
mission which will observe the temporal dimension.
