Forming giant planets around M dwarfs via pebble accretion: the role of fragmentation

The scarcity of giant planets around M dwarfs raises important questions about the physical processes that regulate planet formation around the least massive stars. In this work, we investigate planet formation around very low-mass stars using N-body simulations that incorporate pebble accretion, gas accretion, planet-disk interactions, star-planet tidal interactions, and stellar evolution during the pre-main-sequence phase. We explore how disk properties influence planetary growth and migration by assuming a compact dust disk  of 30 au.

Our simulations show that low-viscosity disks naturally favor the formation of close-in super-Earths, while the scarcity of larger planets around M dwarfs emerges as a natural outcome of the planet formation process. In cases where planet-planet collisions efficiently assemble cores of 2–3 Earth masses, close-in mini-Neptunes and Neptune-mass planets may form. Moreover, if planetary cores reach masses of ~5 Earth masses within the first ~1 Myr, cold giant planets with masses between Saturn and Jupiter can form at distances of  1–5 au, provided that the disks are massive (~10% of the stellar mass), weakly turbulent (α_t = 10^-4), and long-lived (10 Myr).

We further investigate the role of fragmentation in the formation of planetary cores by introducing an evolving pebble flux linked to different fragmentation velocities associated with distinct grain compositions across the disk. These results provide new insights into the diversity of planetary systems around M dwarfs and the conditions required for giant planet formation in the low-stellar-mass regime.