Formation of TRAPPIST-1 and other compact systems

TRAPPIST-1 is a nearby 0.08 M⊙ M-star that was recentlyfound to harbor a planetary system of at least seven Earth-sizedplanets, all within 0.1 au. The configuration confounds theorists as theplanets are not easily explained by either in situ or migration models.In this paper we present a scenario for the formation and orbitalarchitecture of the TRAPPIST-1 system. In our model, planet formationstarts at the H₂O iceline, where pebble-sized particles whoseorigin is the outer disk accumulate to trigger streaming instabilities.After their formation, planetary embryos quickly mature by pebbleaccretion. Planet growth stalls at Earth masses, where the planet'sgravitational feedback on the disk keeps pebbles at bay. Planets aretransported by type I migration to the inner disk, where they stall atthe magnetospheric cavity and end up in mean motion resonances. Duringdisk dispersal, the cavity radius expands and the innermost planetsescape resonance. We argue that the model outlined here can also beapplied to other compact systems and that the many close-in super-Earthsystems are a scaled-up version of TRAPPIST-1. We also hypothesize thatfew close-in compact systems harbor giant planets at large distances,since they would have stopped the pebble flux from the outer disk.