Overcoming the Meter-size Barrier in Planet Formation Models

The meter-size barrier is a persistent problem in current planet formation models, where particles on the order of a meter in size fail to grow because they either fragment or drift into the star. Planetesimal formation at this size can be summarized in three characteristic timescales: growth, drift, and fragmentation. Accurate models of these timescales that resolve the meter-size barrier will improve our understanding of how planets form in a protoplanetary disk. Recent observations of protoplanetary disks indicate that they may be more massive than previously assumed. Decreasing the dust-to-gas ratio from typically assumed ISM values, and increasing the total disk mass, causes the growth timescale to be longer. According to our analytical model, the drift timescale is initially shorter than the growth timescale, allowing a particle to drift past the fragmentation limit of the meter-size barrier. Once beyond the barrier, the growth timescale becomes faster which permits the particle to grow quickly such that it is less susceptible to radial drift into the star. These preliminary results show that through increasing the total disk mass, the particle can potentially grow beyond the meter-size barrier. We adapt the two-population dust evolution numerical model from Birnstiel et al. (2012;15) to verify that, with a smaller dust-to-gas ratio and a smaller turbulence parameter, particles can survive the meter-size drift and fragmentation barriers and continue to grow.