The effects of collisional fragmentation on the formation of super-Earths

At least 30% of the discovered exoplanets are represented by super-Earths generally found in compact orbits close-in to their stars. Compact planetary systems are also often found to have planets near to mean motion resonance (MMR) configurations. These features are well reproduced by planet formation models that account for pebble accretion and migration, where planets can grow to several Earth masses by accreting large quantities of inward drifting pebbles (mm- to cm-sized particles) and migrate due to their interaction with gas in the protoplanetary disk. Once reaching the gas disk inner edge, planets stop migrating. The sequential growth and migration of several planets naturally builds up a resonant configuration. After gas dispersal from the protoplanetary nebula, such close packed resonant chain of planets can undergo orbital instabilities. That cause the planets to break out resonance and achieve a new configuration with orbits just close to their MMRs, as observed. However, during this instability phase many planets collide with each other. Previous results were solely based on modelling of perfect accretion during planetary collisions. Here we focus on the effects that more realistic and imperfect accretion have on the final systems formed. We performed N-body simulations of super-Earth growth from planetary embryos while accounting for pebble accretion and migration due to gas disk interactions followed by a phase of long-term gas-free dynamical/instability evolution. We compared our set of simulations considering imperfect accretion with simulations where only perfect accretion was at play. We find that fragmentation events are very common. However, we also find that the total amount of fragmented mass generated by collisions is only about 10% of the system’s total mass during a typical instability phase. In the end, this total amount of mass proved to be insufficient to alter the system’s final orbital architecture, planet masses and multiplicity. Our results indicate that the overall dynamical properties of the final systems formed are not impacted by treating collisions as imperfect accretion.