DIX Planetary Science Seminar

Trappist-1 is one of the most intriguing extrasolar planetary systems. It hosts 7 planets where the period ratios of neighbouring pairs are close to the 8:5, 5:3, 3:2, 3:2, 4:3, and 3:2 ratios in increasing distance from the star. The Laplace angles associated with neighbouring triplets are observed to be librating, proving the resonant nature of the system. This compact, resonant configuration is a manifest sign of disc-driven migration; however, the preferred outcome of such evolution is the establishment of first-order resonances, not the high-order resonances observed in the inner system. This fine discrepancy between qualitative agreement (a resonant state) and quantitative inconsistency offers a unique opportunity to shed light on the other processes that shape the architecture of planetary systems together with disc-driven migration.
We explain the observed orbital configuration of the Trappist-1 system in a model that is largely independent on disc migration and damping parameters, but instead pivots on physical processes that are expected to occur at the inner edge of protoplantary discs. The two key elements of our model are: i) the inner edge of the disc receded with time; and ii) the system was initially separated in two sub-systems. The inner b, c d and e planets were initially placed in a first order 3:2 resonance chain and then evolved to a 8:5 -- 3:2 resonant configuration among planets b, c and d under the effect of the recession of the inner edge of the disc, before the outer planets migrated to the inner edge to establish the full chain we observe today. Our results pivot on the dynamical role of the presently unobservable recession of the inner edge of protoplanetary discs, and reveal the role of recurring phases of convergent migration followed by resonant repulsion with associated orbital circularisation when resonant chains interact with migration barriers.