A Palette of Water Inventories Assembled across Nascent TRAPPIST-1 Analog Planets

Compact multiplanet systems such as TRAPPIST-1 are attractive candidates for planet detection and follow-up atmospheric characterization efforts. By examining the evolution and fates of the neighboring worlds, these systems offer exciting opportunities for comparative exoplanetology. From a planetary habitability standpoint, it is still unclear whether such planets can accrete large quantities of volatiles, grow to sufficent mass, and maintain robust atmospheres and hydrospheres. Previous work predicting the possible water contents in M-dwarf planetary systems used either population synthesis or atmospheric-interior evolution models, but excluded the combined importance of different impact histories and impact ablation of light species. Here, we account for the simultaneous effects of impact delivery, impact erosion, and mantle-atmosphere exchange during secular collision events that are crucial in setting the mass and composition of the growing parent body. Specifically, we use a suite of N-body accretion integrations and a volatile growth model to calculate the accretion timeseries of all seven TRAPPIST-1 planets and their respective gaseous reservoirs. In contrast to previous work that neglected planetesimal impacts, we find much lower surface water inventories for the outer planets (planet e, f, g, h analogs) for both relatively volatile-depleted materials (i.e., down to $10^{-3}-0.1\%$ water mass fraction) and volatile-enriched (i.e., up to 5\%). Interestingly, we find that not all inner planets (planet b, c, d analogs) would remain dry, as around 30\% accretion histories give rise to inner planets with at least 1-10\% of Earth's total oceanic water budget. More generally, our multi-stage modeling framework for tracking volatile accretion can be used to inform exoplanet interior models and to set the initial compositions of exoplanet atmosphere models. Our results, in conjunction with near-term observations of TRAPPIST-1 planets, may be able to help decipher the makeup and formation histories of their supposed planetary building blocks.