Invited Talk - Splinter Exoplanets

Friday, 17 September 2021, 14:00   (virtual Exo)

Super-Earths, mini-Neptunes, and the Radius Valley: linking observations with theory

J. Venturini, O. Guilera, J. Haldemann, P. Ronco, C. Mordasini
International Space Science Institute

The existence of a radius valley in the size distribution of exoplanets, which separates super-Earths from mini-Neptunes, stands as one of the most important observational constraints to understand the origin and composition of exoplanets with sizes below Neptune. In this presentation, I review the main mechanisms proposed to explain the origin of the Radius Valley. I highlight the inconsistency between pure evolution models that assume an arbitrary initial core mass distribution with one single composition, and the results from formation models which predict that planets with both dry and wet cores should exist at short orbital periods. I present new theoretical efforts developed with the aim to bridge such inconsistency, by combining formation and evolution calculations. Our simulations include the growth of dust into pebbles by coagulation, drift and fragmentation; and the evolution of the gaseous disc by viscous accretion and photoevaporation. A planet grows from a moon-mass embryo by either silicate or icy pebble accretion, depending on its position with respect to the ice line. We include gas accretion, type-I/II migration and photoevaporation driven mass-loss after formation. We find that while the first peak of the size distribution is undoubtedly populated by bare rocky cores, the second peak tends to be composed by half-rock/half-water planets with thin or non-existent H-He atmospheres. Finally, I discuss further observational constraints for theoretical modelling, such as the dependence of the valley location on the mass of the star and orbital period; as well as particular cases where the compositional degeneracy of mini-Neptunes could be lifted.