  The TRAPPIST-1 system is an iconic planetary system in various aspects (e.g.,
habitability, resonant relation, and multiplicity) and hence has attracted
considerable attention. The mass distribution of the TRAPPIST-1 planets is
characterized by two features: the two inner planets are large, and the masses
of the four planets in the outer orbit increase with orbital distance. The
origin of these features cannot be explained by previous formation models. We
investigate whether the mass distribution of the TRAPPIST-1 system can be
reproduced by a planet formation model using N-body simulations. We used a gas
disk evolution model around a low-mass star constructed by considering disk
winds and followed the growth and orbital migration from planetary embryos with
the isolation mass, which increases with orbital distance. As a result, we find
that from the initial phase, planets in inner orbits undergo rapid orbital
migration, and the coalescence growth near the inner disk edge is enhanced.
This allows the inner planets to grow larger. Meanwhile, compared with the
inner planets, planets in outer orbits migrate more slowly and do not
frequently collide with neighboring planets. Therefore, the trend of increasing
mass toward the outer orbit, called reversed mass ranking, is maintained. The
final mass distribution approximately agrees with the two features of the mass
distribution in the TRAPPIST-1 system. We discover that the mass distribution
in the TRAPPIST-1 system can be reproduced when embryos experience rapid
migration and become trapped near the disk inner edge, and then more massive
embryos undergo slower migration. This migration transition can be achieved
naturally in a disk evolution model with disk winds.
