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Recently, seven Earth-sized planets were discovered around the M-dwarf starTRAPPIST-1. Thanks to transit-timing variations, the masses and therefore thebulk densities of the planets have been constrained, suggesting that allTRAPPIST-1 planets are consistent with water mass fractions on the order of10%. These water fractions, as well as the similar planet masses within thesystem, constitute strong constraints on the origins of the TRAPPIST-1 system.In a previous work, we outlined a pebble-driven formation scenario. In thispaper we investigate this formation scenario in more detail. We used aLagrangian smooth-particle method to model the growth and drift of pebbles andthe conversion of pebbles to planetesimals through the streaming instability.We used the N-body code \texttt{MERCURY} to follow the composition ofplanetesimals as they grow into protoplanets by merging and accreting pebbles.This code is adapted to account for pebble accretion, type-I migration, and gasdrag. In this way, we modelled the entire planet formation process (pertainingto planet masses and compositions, not dynamical configuration). We find thatplanetesimals form in a single, early phase of streaming instability. Theinitially narrow annulus of planetesimals outside the snowline quickly broadensdue to scattering. Our simulation results confirm that this formation pathwayindeed leads to similarly-sized planets and is highly efficient in turningpebbles into planets ($\sim$50% solids-to-planets conversion efficiency). [...]The water content of planets resulting from our simulations is on the order of10%, and our results predict a `V-shaped' trend in the planet water fractionwith orbital distance: from relatively high (innermost planets) to relativelylow (intermediate planets) to relatively high (outermost planets).

Pebble-driven planet formation for TRAPPIST-1 and other compact systems