Formation and Evolution of Close-In Small Exoplanets: Linking Theory to Observed Trends

In this talk, I will review recent theoretical advances on the formation and dynamical evolution of close-in super-Earths and mini-Neptunes. In the “breaking the chain model”, planets form at a range of orbital distances and migrate inward due to planet-disk gravitational interactions. As they converge toward the inner edge of the protoplanetary disk, they typically become trapped in compact resonant chains. The subsequent dynamical evolution of these chains often leads to dynamical instabilities and giant impacts, producing planetary system architectures that are broadly consistent with key observed trends, including the period ratio distribution, intra-system radius uniformity, and the bimodal radius distribution with a dip near ~1.8 Earth’s radii, known as the “radius valley.” Only a small fraction of systems remain in resonance, consistent with iconic observed examples such as TRAPPIST-1, Kepler-223, and TOI-178. I will also discuss recent observational findings indicating an excess of orbital eccentricities among planets within the radius valley (1.5 < R < 2 Earth's radii), and explore possible evolutionary pathways that may explain its origin. Finally, I will conclude by discussing how emerging observational evidence seems to support the “breaking the chain” evolutionary pathway.