Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions


Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits1,2, with reasonable mass estimates coming from transit timing variations2,3 and dynamical modelling4. The low bulk densities of the TRAPPIST-1 planets demand substantial volatile content. Here we show, using mass–radius–composition models, that TRAPPIST-1f and g probably contain substantial (≥50 wt%) water/ice, with TRAPPIST-1 b and c being significantly drier (≤15 wt%). We propose that this gradient of water mass fractions implies that planets f and g formed outside the primordial snow line whereas b and c formed within it. We find that, compared with planets in our Solar System that also formed within the snow line, TRAPPIST-1b and c contain hundreds more oceans of water. We demonstrate that the extent and timescale of migration in the TRAPPIST-1 system depends on how rapidly the planets formed and the relative location of the primordial snow line. This work provides a framework for understanding the differences between the protoplanetary disks of our Solar System versus M dwarfs. Our results provide key insights into the volatile budgets, timescales of planet formation and migration history of M dwarf systems, probably the most common type of planetary host in the Galaxy.

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Fig. 1: Modelled χ2 goodness of fit for the masses of the TRAPPIST-1 planets as a function of the planet's radius and relative H2O mass fraction in wt% added to the system.
Fig. 2: The orbital radius of our modelled water-ice snow line (see Methods) as a function of time of planet formation, assuming the condensation temperature of water-ice at 170 K (blue) and 212 K (red).
Fig. 3: Phase diagram with depth as modelled with the ExoPlex mass–radius–composition calculator for the best-fit interiors of TRAPPIST-1f.


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C.T.U. acknowledges the support of Arizona State University through the SESE Exploration fellowship. The results reported herein benefited from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. N.R.H. would like to thank CHW3 and acknowledges the support of the Vanderbilt Office of the Provost through the Vanderbilt Initiative in Data-intensive Astrophysics (VIDA) fellowship.

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C.T.U. and S.J.D. conceived the project and wrote the manuscript. C.T.U. performed the mass–radius–composition calculations. S.J.D. constructed the snow line model and performed the atmospheric retention calculations. N.R.H. supplied the input stellar data and helped to prepare the manuscript. C.T.U. and A.L. wrote the ExoPlex mass–radius–composition calculator.

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Correspondence to Cayman T. Unterborn.

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Unterborn, C.T., Desch, S.J., Hinkel, N.R. et al. Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions. Nat Astron 2, 297–302 (2018).

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