How Flow Isolation May Set the Mass Scale for Super-Earth Planets

Abstract: Much recent work on planet formation has focused on the growth of planets by accretion of grains whose aerodynamic properties make them marginally coupled to the nebular gas, a theory commonly referred to as "pebble accretion". While pebble accretion can ameliorate some of the issues presented by growth by purely gravitational processes, it has other issues when compared with observations of exoplanetary systems. A particular concern is the preponderance of planets that end their growth as "super-Earths" or "sub-Neptunes", with masses in the range 2-10 $M_\oplus$. Once planets reach this mass scale, timescales for growth by pebble accretion are so rapid that ubiquitously ending growth here is difficult. In this work, we highlight this issue in detail using our previously published model of pebble accretion, and also propose a possible solution: feedback between the growing planet's atmosphere and the gas disk inhibits accretion of smaller particle sizes by forcing them to flow around the growing planet instead of being accreted. For reasonable fiducial disk parameters this "flow isolation" will inhibit accretion of all available particle sizes once the planet reaches super-Earth masses. We also demonstrate that the characteristics of this "flow isolation mass" agree with previously published trends identified in the \textit{Kepler} planets.