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

Cayman T. Unterborn, Steven Desch, Natalie R. Hinkel, Alejandro Lorenzo

Research output: Contribution to journalArticlepeer-review

87 Scopus citations


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 transits 1,2, with reasonable mass estimates coming from transit timing variations 2,3 and dynamical modelling 4 . 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.

Original languageEnglish (US)
Pages (from-to)297-302
Number of pages6
JournalNature Astronomy
Issue number4
StatePublished - Apr 1 2018

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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