  We study the formation of the TRAPPIST-1 (T1) planets starting shortly after
Moon-sized bodies form just exterior to the ice line. Our model includes mass
growth from pebble accretion and mergers, fragmentation, type-I migration, and
eccentricity and inclination dampening from gas drag. We follow the composition
evolution of the planets fed by a dust condensation code that tracks how
various dust species condense out of the disc as it cools. We use the final
planet compositions to calculate the resulting radii of the planets using a new
planet interior structure code and explore various interior structure models.
Our model reproduces the broader architecture of the T1 system and constrains
the initial water mass fraction of the early embryos and the final relative
abundances of the major refractory elements. We find that the inner two planets
likely experienced giant impacts and fragments from collisions between
planetary embryos often seed the small planets that subsequently grow through
pebble accretion. Using our composition constraints we find solutions for a
two-layer model, a planet comprised of only a core and mantle, that match
observed bulk densities for the two inner planets b and c. This, along with the
high number of giant impacts the inner planets experienced, is consistent with
recent observations that these planets are likely dessicated. However,
two-layer models seem unlikely for most of the remaining outer planets which
suggests that these planets have a primordial hydrosphere. Our composition
constraints also indicate that no planets are consistent with a core-free
interior structure.
