Planetary population synthesis model for validation of planet formation theory

speaker: Tadahiro Kimura (NAOJ)


Exoplanet exploration has discovered more than 5000 exoplanets and revealed statistical characteristics of their distribution. These observed distributions are key to validating current theories and advancing our understanding of planet formation processes. For this purpose, we need a model that combines all known formation processes and predicts the current distribution of exoplanets. We have developed a new such model called the planetary population synthesis model and statistically compared its predictions with observed exoplanet distributions. The model deals in detail with the formation and evolution of primordial atmospheres, and by using analytical or empirical models for each elementary process, it can predict observables such as planetary mass, radius and orbital period at a quite low computational cost. This allows comparisons to be made with observations for many model parameter sets. In this study we focus on the distribution of radii and orbital periods of so-called ‘super-Earths’ and ‘sub-Neptunes’ with radii of about 1-4 Earth radii, and investigate whether the observed distribution can be explained within the framework of our model. The results show that the distribution can be reproduced by changing the distribution of initial planetesimals, the disk gas viscosity and the composition of the primordial atmosphere in particular. We predict that both super-Earths and sub-Neptunes have rocky solid cores, and that sub-Neptunes retain their primordial atmospheres enriched with water vapour to this day. These predictions are expected to be verified by future observations of exoplanet atmospheres.