  Planetesimal formation is a crucial yet poorly understood process in planet
formation. It is widely believed that planetesimal formation is the outcome of
dust clumping by the streaming instability (SI). However, recent analytical and
numerical studies have shown that the SI can be damped or suppressed by
external turbulence, and at least the outer regions of protoplanetary disks are
likely weakly turbulent due to magneto-rotational instability (MRI). We conduct
high-resolution local shearing-box simulations of hybrid particle-gas
magnetohydrodynamics (MHD), incorporating ambipolar diffusion as the dominant
non-ideal MHD effect, applicable to outer disk regions. We first show that dust
backreaction enhances dust settling towards the midplane by reducing turbulence
correlation time. Under modest level of MRI turbulence, we find that dust
clumping is in fact easier than the conventional SI case, in the sense that the
threshold of solid abundance for clumping is lower. The key to dust clumping
includes dust backreaction and the presence of local pressure maxima, which in
our work is formed by the MRI zonal flows overcoming background pressure
gradient. Overall, our results support planetesimal formation in the
MRI-turbulent outer protoplanetary disks, especially in ring-like
substructures.

Earth and Planetary Astrophysics