We conduct a pebble-driven planet population synthesis study to investigate the formation of planets around very low-mass stars and brown dwarfs , in the ( sub ) stellar mass range between 0.01 M _ { \odot } and 0.1 M _ { \odot } .
Based on the extrapolation of numerical simulations of planetesimal formation by the streaming instability , we obtain the characterisitic mass of the planetesimals and the initial masses of the protoplanets ( largest bodies from the planetesimal size distributions ) , in either the early self-gravitating phase or the later non-self-gravitating phase of the protoplanetary disk evolution .
We find that the initial protoplanets form with masses that increase with host mass , orbital distance and decrease with age .
Around late M-dwarfs of 0.1 M _ { \odot } , these protoplanets can grow up to Earth-mass planets by pebble accretion .
However , around brown dwarfs of 0.01 M _ { \odot } , planets do not grow larger than Mars mass when the initial protoplanets are born early in self-gravitating disks , and their growth stalls at around 0.01 Earth-mass when they are born late in non-self-gravitating disks .
Around these low mass stars and brown dwarfs we find no channel for gas giant planet formation , because the solid cores remain too small .
When the initial protoplanets form only at the water-ice line , the final planets typically have { \gtrsim } 15 \% water mass fraction .
Alternatively , when the initial protoplanets form log-uniformly distributed over the entire protoplanetary disk , the final planets are either very water-rich ( water mass fraction { \gtrsim } 15 \% ) or entirely rocky ( water mass fraction { \lesssim } 5 \% ) .