A critical phase in the standard model for planet formation is the runaway growth phase .
During runaway growth bodies in the 0.1–100 km size range ( planetesimals ) quickly produce a number of much larger seeds .
The runaway growth phase is essential for planet formation as the emergent planetary embryos can accrete the leftover planetesimals at large gravitational focusing factors .
However , torques resulting from turbulence-induced density fluctuations may violate the criterion for the onset of runaway growth , which is that the magnitude of the planetesimals ’ random ( eccentric ) motions are less than their escape velocity .
This condition represents a more stringent constraint than the condition that planetesimals survive their mutual collisions .
To investigate the effects of MRI turbulence on the viability of the runaway growth scenario , we apply our semi-analytical recipes of Paper I , which we augment by a coagulation/fragmentation model for the dust component .
We find that the surface area-equivalent abundance of 0.1 \mu \mathrm { m } particles is reduced by factors 10 ^ { 2 } – 10 ^ { 3 } , which tends to render the dust irrelevant to the turbulence .
We express the turbulent activity in the midplane regions in terms of a size s _ { \mathrm { run } } above which planetesimals will experience runaway growth .
We find that s _ { \mathrm { run } } is mainly determined by the strength of the vertical net field that threads the disks and the disk radius .
At disk radii beyond 5 AU , s _ { \mathrm { run } } becomes larger than \sim 100 km and the collision times among these bodies longer than the duration of the nebula phase .
Our findings imply that the classical , planetesimal-dominated , model for planet formation is not viable in the outer regions of a turbulent disk .