The TRAPPIST-1 system presents many challenges to planet formation theory. It is an ultra-compact system where the seven planets are all very similar (around 1 Earth radius) with a composition that tends to be intermediate between "dry" and "water rich", and where planet formation was obviously efficient. We have previously argued that its properties are hard to understand from classical planet formation concepts, and sketched a new pebble-driven growth model (Ormel 2017), which we have now quantitatively investigated. The crucial feature of our scenario is that planetesimals form only in a narrow annulus just outside the water snowline, due to outward diffusion and condensation of water vapor. Once a planetesimal has grown large by pebble accretion and mergers, it starts migrating inwards and the process may repeat itself. Our results indicate that this formation mechanism is able to explain the moderate water fractions of the TRAPPIST-1 planets as well as their similar sizes.
