Gabriele Pichierri


Formation of the Trappist-1 system

Its 7 planets orbit in a planar, compact configuration, where the period ratios of nearby pairs are locked in the 8:5, 5:3, 3:2, 3:2, 4:3, and 3:2 commensurabilities, moving inward to outward along the chain. The corresponding Laplace angles associated to triplets of neighbouring planets are observed to be librating, proving the resonant nature of the system.
These compact, resonant configurations are typically a 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. Tidal evolution after the formation of a first-order 7-planet resonance chain cannot alone explain the configuration observed today.
In this work, we show how the 8:5 and 5:3 resonances can instead be established in a systematic way. Starting from the currently observed planetary masses, we consider a model including disc-planet interactions and tidal evolution to explain the current orbital features of the Trappist-1 system that can account for its formation and subsequent evolution after the disappearance of the protoplanetary disc.

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Formation of the Trappist-1 system
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