Transit surveys show that approximately half of sun-like stars have planetary systems with exoplanets smaller than Neptune within 1 au. Surprisingly, these planetary systems are even more common around low mass M dwarfs, where a reduced planet occurrence rate would be expected based on lower protoplanetary disk masses.
To solve this M dwarf riddle, I present a planet formation model based on pebble accretion. In this model, giant planet cores forming at larger distances from the star block the radial drift of pebbles into the inner regions of the protoplanetary disk, suppressing the growth of planet cores close to the star. Because the distant, cold giant exoplanets form more readily around more massive stars --- as observed in radial velocity surveys --- this suppression mechanism is more efficient for sun-like stars than for M dwarfs.
With this model we explain the elevated planet occurrence rates of M dwarfs. It also predicts that when super-earths and giant planets occur in the same system, their masses should be anti-correlated.