Planet Population Synthesis: the Cradle of the TRAPPIST-1 Multiplanet System

Planet Population Synthesis is a statistical approach that serves as a bridge between theoretical planet formation and the observed population of exoplanets. It has led to testable predictions, such as the now- confirmed minimum in the planetary mass distribution between a few Earth masses and 40 Earth masses. In order to apply this technique to low-mass host stars, we have extended the Bern model of planet formation (Mordasini et al. 2009) to the different conditions in their protoplanetary disks. Changes to the original setup include a smaller inner disk radius and a down-scaled disk mass distribution. We present a population of systems with a host star mass of 0.1 Solar masses which we compare to observables of the TRAPPIST-1 multi-planet system (Gillon et al. 2017). We find that most of its features can be robustly reproduced. Using the mean planetary mass as a metric, we find a domain in initial disk solid mass and disk extent favorable for the formation of similar systems. The fact that a well-established formation model can produce similar systems with little additional assumptions suggests that TRAPPIST-1 is not an exotic outlier but a rather typical outcome for very-low-mass systems. This raises important implications for exoplanet demographics at the limit of detectability.