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Though ~10 Earth mass rocky/icy cores are commonly held as a prerequisite forthe formation of gas giants, theoretical models still struggle to explain howthese embryos can form within the lifetimes of gaseous circumstellar disks. Inrecent years, aerodynamic-aided accretion of "pebbles," objects ranging fromcentimeters to meters in size, has been suggested as a potential solution tothis long-standing problem. While pebble accretion has been demonstrated to beextremely effective in local simulations that look at the detailed behavior ofthese pebbles in the vicinity of a single planetary embryo, to date there havebeen no global simulations demonstrating the effectiveness of pebble accretionin a more complicated, multi-planet environment. Therefore, we haveincorporated the aerodynamic-aided accretion physics into LIPAD, a Lagrangiancode that can follow the collisional / accretional / dynamical evolution of aprotoplanetary system, to investigate how pebble accretion manifests itself inthe larger planet formation picture. We find that under generic circumstances,pebble accretion naturally leads to an "oligarchic" type of growth in which alarge number of planetesimals grow to similar-sized planets. In particular, oursimulations tend to form hundreds of Mars- and Earth-mass objects between 4 and10 AU. While the merging of some oligarchs may allow them to grow massiveenough to form giant planet cores, leftover oligarchs lead to planetary systemsthat cannot be consistent with our own solar system. We investigate variousideas presented in the literature (including evaporation fronts and planettraps) and find that none easily overcome this tendency toward oligarchicgrowth.

Challenges in Forming the Solar System's Giant Planet Cores via Pebble Accretion