Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation
Jin et al
We use a theoretical end-to-end model that includes planet formation, thermodynamic evolution, and atmospheric escape to investigate how the statistical imprints of evaporation depend on the bulk composition of the planetary cores (rocky vs. icy). We find that the typical population-wide imprints of evaporation like the location of the "evaporation valley" in the distance-radius plane and the corresponding one-dimensional bimodal distribution in planetary radii are clearly different depending on the bulk composition of close-in low-mass planetary cores. Comparison with the observed position of the valley as found recently by Fulton et al. (2017) suggests that Kepler planets in this domain 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 iceline, but still undergoing orbital migration. The core radius becomes visible for planets losing all primordial H/He. For such planets in the "triangle of evaporation" in the distance-radius plane, the degeneracy in possible compositions is reduced. In the observed a-R diagram, we identify a trend to more volatile-rich compositions with increasing planet radius and potentially distance (R/R_earth less than 1.6 rocky; 1.6-3.0 H/He and/or ices; > 3: H/He). Moreover, we find that the mass-density distribution contains important information about planet formation and evolution. Evaporation removes close-in low-mass planets with low density in the mass-density space. This causes density and orbital distance to be anti-correlated for low-mass planets, in contrast to giant planets, where closer planets are less dense, due to inflation mechanisms. The temporal evolution of the statistical properties of the population reported here will be of particular interest for the future PLATO 2.0 mission which will be able to observe the temporal dimension.