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The TRAPPIST-1 system is an iconic planetary system in various aspects (e.g.,habitability, resonant relation, and multiplicity) and hence has attractedconsiderable attention. The mass distribution of the TRAPPIST-1 planets ischaracterized by two features: the two inner planets are large, and the massesof the four planets in the outer orbit increase with orbital distance. Theorigin of these features cannot be explained by previous formation models. Weinvestigate whether the mass distribution of the TRAPPIST-1 system can bereproduced by a planet formation model using N-body simulations. We used a gasdisk evolution model around a low-mass star constructed by considering diskwinds and followed the growth and orbital migration from planetary embryos withthe isolation mass, which increases with orbital distance. As a result, we findthat from the initial phase, planets in inner orbits undergo rapid orbitalmigration, and the coalescence growth near the inner disk edge is enhanced.This allows the inner planets to grow larger. Meanwhile, compared with theinner planets, planets in outer orbits migrate more slowly and do notfrequently collide with neighboring planets. Therefore, the trend of increasingmass toward the outer orbit, called reversed mass ranking, is maintained. Thefinal mass distribution approximately agrees with the two features of the massdistribution in the TRAPPIST-1 system. We discover that the mass distributionin the TRAPPIST-1 system can be reproduced when embryos experience rapidmigration and become trapped near the disk inner edge, and then more massiveembryos undergo slower migration. This migration transition can be achievednaturally in a disk evolution model with disk winds.

Rapid-then-slow migration reproduces mass distribution of TRAPPIST-1 system