Enhanced formation efficiency of super-Earths around M dwarfs

Yayaati Chachan

Close-in super-Earths are the most common planets around Sun-like stars but they are even more prevalent around M stars. Here, we develop a picture of super-Earth formation that accounts for the stellar mass dependence of their occurrence rates. We show that pebble accretion can produce chains of pebble isolated cores that merge to form the observed super-Earths in actively heated inner regions of the disk. Given the steep dependence of temperature on orbital distance in such disks and the inverse dependence of pebble Stokes number on temperature, we find that simultaneous planetesimal accretion is necessary to allow the inner cores in the chain to reach pebble isolation mass. The total pebble mass required and available to form these chains of super-Earths is ultimately set by the pebble accretion efficiency of the outermost core in the chain. The pebble accretion efficiency increases super-linearly with decreasing stellar mass while disk mass is only expected to be linearly dependent on the stellar mass. As a result, a larger fraction of disks around M stars are capable of spawning super-Earths, in agreement with observations. Since the pebble isolation mass has a roughly linear dependence on the stellar mass in actively heated regions of the disk, we expect the characteristic super-Earth mass to decline around lower mass stars. For ultracool dwarfs, the inner disk is significantly colder and transitions to being passively heated, leading to smaller pebble isolation masses and a stronger gradient in mass with orbital period. Consequently, super-Earths do not form around ultracool dwarfs for fiducial disk parameters, which may explain why TRAPPIST-1 like systems are extremely rare.