Saturday, October 11, 2014

Problems With Pebble Accretion Model for Forming Gas Giants

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


Kretke et al


Though ~10 Earth mass rocky/icy cores are commonly held as a prerequisite for the formation of gas giants, theoretical models still struggle to explain how these embryos can form within the lifetimes of gaseous circumstellar disks. In recent years, aerodynamic-aided accretion of "pebbles," objects ranging from centimeters to meters in size, has been suggested as a potential solution to this long-standing problem. While pebble accretion has been demonstrated to be extremely effective in local simulations that look at the detailed behavior of these pebbles in the vicinity of a single planetary embryo, to date there have been no global simulations demonstrating the effectiveness of pebble accretion in a more complicated, multi-planet environment. Therefore, we have incorporated the aerodynamic-aided accretion physics into LIPAD, a Lagrangian code which can follow the collisional / accretional / dynamical evolution of a protoplanetary system, to investigate the how pebble accretion manifests itself in the larger planet formation picture. We find that under generic circumstances, pebble accretion naturally leads to a "oligarchic" type of growth in which a large population of planetesimals grow to similar sized planets. In particular, our simulations tend to form hundreds of Mars and Earth mass objects between 4 and 10 AU. While merging of some oligarchs may grow massive enough to form giant planet cores, left-over oligarchs lead to planetary systems that cannot be consistent with our own Solar System. We investigate various ideas presented in the literature (including evaporation fronts and planet traps) and find that none easily overcomes this tendency towards oligarchic growth.

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