The terrestrial planet formation around M dwarfs: insitu, inward migration, or reversed migration

Terrestrial planets are commonly observed to orbit M dwarfs with close-in trajectories. In this work, we extensively perform N-body simulations of planetesimal accretion with three models of in situ, inward migration, and reversed migration to explore terrestrial formation in tightly compact systems of M dwarfs. In the simulations, the solid discs are assumed to be 0.01 per cent of the masses of host stars and spread from 0.01 to 0.5 au with the surface density profile scaling with r^-k according to the observations. Our results show that the in-situ scenario may produce $7.77^{+3.23}_{-3.77}$ terrestrial planets with an average mass of $1.23^{+4.01}_{-0.93}\,{\rm M}_{\oplus }$ around M dwarfs. The number of planets tends to increase as the disc slope is steeper or with a larger stellar mass. Moreover, we show that $2.55^{+1.45}_{-1.55}$ planets with a mass of $3.76^{+8.77}_{-3.46} \ {\rm M}_{\oplus }$ are formed in the systems via inward migration, while $2.85^{+1.15}_{-0.85}$ planets with $3.01^{+13.77}_{-2.71} \ {\rm M}_{\oplus }$ are yielded under reversed migration. Migration scenarios can also deliver plentiful water from the exterior of the ice line to the interior due to more efficient accretion. The simulation outcomes of the reversed migration model produce the best match with observations, being suggestive of a likely mechanism for planetary formation around M dwarfs.