Formation and evolution pathways of planets. I. Comparison between theory and observations

Abstract

Discoveries of numerous exoplanets by various methods enable detailed characterization including bulk density. Formation and evolution pathways of planets can thus be probed in the mass-radius and mass-density diagrams. We develop a framework to identify dominant processes shaping parameter space in these diagrams by integrating previous studies. These include interior structure models, gas accretion/retention recipes, and photoevaporative and collisional mass losses. We find that the distribution of planets in the diagrams is diversified by two evolution processes: photoevaporative and collisional mass losses, and the properties of planets before experiencing these processes are consistent with predictions of standard core accretion. In particular, collisional mass growth and loss move planets to the parameter space, which is otherwise occupied by water-dominated (i.e., nearly pure water) planets, gathering non-necessity of invoking such planets. A potentially high abundance of water-rich planets are possible with the ice-to-rock ratio capped at $1/3$, similar to solar system comets. We propose a new classification scheme and apply to observed exoplanets. The classification scheme recovers four canonical planet types widely used in the literature and is expended to eight classes in total due to evolution processes. We divide formation and evolution pathways into four stages (core formation, gas accretion, collisional mass growth and loss, and photoevaporation) and trace how planets populate in the mass-radius and mass-density diagrams with time. We apply the framework to habitable zone planets and discuss possible predictions. This work emphasizes the importance of precise mass and radius measurements, especially for small-sized, potentially habitable planets.