Trappist-1 is one of the most intriguing extrasolar systems known to date: its 7 planets orbit in a planar, compact configuration, where the period ratios of nearby pairs are close to the 8:5, 5:3, 3:2, 3:2, 4:3, and 3:2 ratios, moving away from the star. The corresponding Laplace angles associated to triplets of neighbouring planets are observed to be librating, proving the resonant nature of the system.
Such a compact, resonant configuration is a manifest sign of orbital migration in the protoplanetary disc in which the planets formed; however, the preferred outcome of such orbital evolution is the establishment of first-order resonances (like the 3:2 or 4:3), not the high-order 8:5 and 5:3 resonances observed in the inner system.
So far, attempts to recover the currently observed state of the Trappist-1 system have invoked fortuitous capture in unlikely resonances, timely disappearance of the disc as soon as the planets come close to the desired configuration, or other ad-hoc synchronisms.
Here we reproduce the observed orbital configuration in a model that does not require coincidental events within a limited parameter space. The conditions that need to be fulfilled are that the inner border of the disc receded in time, that the planets migrated to the inner disc in two steps, and that migration was not too fast as to disrupt the inner 8:5 - 5:3 chain. The second condition is consistent with them forming at two distinct locations, as also inferred by their compositional dichotomy.
Our results have strong implications on the evolution of inner discs, a process that is currently not observable, and on the formation of planets, suggesting they do not form uniformly throughout the disc but at privileged, separated sites.