roughly a centimetre coagulate via sticking [e.g. @Dominiketal2007], while for the largest sizes, gravity of the planetary bodies plays a dominant role [e.g. @Ida2010].

Between these two limits, there is a gap that neither mechanism seems able to leapfrog. Beyond a size of 1 cm - 1 m, collisions occur at velocities of order $1-10\:\mathrm{m\:s^{-1}}$. These lead either to fragmentation or to bouncing [@Zsometal2010], rather than to systematic aggregation. Worse yet, particles of this size will drift sunward at the dramatic rate of 1 AU per century [@Weidenschilling1977]. This is because solids tend to orbit the Sun at the full Keplerian speed, while the gas moves slightly slower because of the support from an outward directed pressure force. Thus, the solids experience a headwind that removes angular momentum via gas drag. This effect is the cause of the “metre-size barrier”.

Self-gravity of the dust could in principle promote accretion past this barrier provided some process enhances the dust density. A classic scenario for this is one in which the vertical component of the Sun’s gravity causes sedimentation to the disc midplane and increases the dust density [@GoldreichWard1973]. Although @Weidenschilling1980 had suggested that Kelvin-Helmholtz instabilities could prevent the Roche density from 