The dynamics of the TRAPPIST-1 system in the context of its formation.

TRAPPIST-1 is an 0.09 $M_{\odot}$ star, which harbours a system of seven Earth-sized planets. Two main features stand out: (i) all planets have similar radii, masses, and compositions; and (ii) all planets are in resonance. Previous works have outlined a pebble-driven formation scenario where planets of similar composition form sequentially at the H$_2$O snowline (${\sim}0.1$ au for this low-mass star). It was hypothesized that the subsequent formation and migration led to the current resonant configuration. Here, we investigate whether the sequential planet formation model is indeed capable to produce the present-day resonant configuration, characterized by its two-body and three-body mean motion resonances structure. We carry out N-body simulations, accounting for type-I migration, stellar tidal damping, disc eccentricity damping, and featuring a migration barrier located at the disc's inner edge. We demonstrate that the present-day dynamical configuration of the TRAPPIST-1 system is in line with the sequential formation/migration model. First, a chain of first-order resonances was formed at the disc inner edge by convergent migration. We argue that TRAPPIST-1b and c marched across the migration barrier, into the gas-free cavity, during the dispersal of the gas disc. The dispersing disc then pushed planets b and c inward until they settled in a configuration close to the observed resonances. Thereafter, the stellar tidal torque also attributed towards a modest separation of the inner system. We argue that our scenario is also applicable to other compact resonant planet systems.