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Multiple planet systems provide an ideal laboratory for probing exoplanetcomposition, formation history and potential habitability. For the TRAPPIST-1planets, the planetary radii are well established from transits (Gillon et al.,2016, Gillon et al., 2017), with reasonable mass estimates coming from transittiming variations (Gillon et al., 2017, Wang et al., 2017) and dynamicalmodeling (Quarles et al., 2017). The low bulk densities of the TRAPPIST-1planets demand significant volatile content. Here we show usingmass-radius-composition models, that TRAPPIST-1f and g likely containsubstantial ($\geq50$ wt\%) water/ice, with b and c being significantly drier($\leq15$ wt\%). We propose this gradient of water mass fractions impliesplanets f and g formed outside the primordial snow line whereas b and c formedinside. We find that compared to planets in our solar system that also formedwithin the snow line, TRAPPIST-1b and c contain hundreds more oceans worth ofwater. We demonstrate the extent and timescale of migration in the TRAPPIST-1system depends on how rapidly the planets formed and the relative location ofthe primordial snow line. This work provides a framework for understanding thedifferences between the protoplanetary disks of our solar system versus Mdwarfs. Our results provide key insights into the volatile budgets, timescalesof planet formation, and migration history of likely the most common planetaryhost in the Galaxy.

Inward Migration of the TRAPPIST-1 Planets as Inferred From Their Water-Rich Compositions