Formation of TRAPPIST-1 and other compact systems

TRAPPIST-1 is a nearby 0.08 M_⊙ M-star that was recently found to harbor a planetary system of at least seven Earth-sized planets, all within 0.1 au. The configuration confounds theorists as the planets are not easily explained by either in situ or migration models. In this paper we present a scenario for the formation and orbital architecture of the TRAPPIST-1 system. In our model, planet formation starts at the H₂O iceline, where pebble-sized particles whose origin is the outer disk accumulate to trigger streaming instabilities. After their formation, planetary embryos quickly mature by pebble accretion. Planet growth stalls at Earth masses, where the planet's gravitational feedback on the disk keeps pebbles at bay. Planets are transported by type I migration to the inner disk, where they stall at the magnetospheric cavity and end up in mean motion resonances. During disk dispersal, the cavity radius expands and the innermost planets escape resonance. We argue that the model outlined here can also be applied to other compact systems and that the many close-in super-Earth systems are a scaled-up version of TRAPPIST-1. We also hypothesize that few close-in compact systems harbor giant planets at large distances, since they would have stopped the pebble flux from the outer disk.