  One of the main problems in planet formation, hampering the growth of small
dust to planetesimals, is the so-called radial-drift barrier. Pebbles of cm to
dm sizes are thought to drift radially across protoplanetary discs faster than
they can grow to larger sizes, and thus to be lost to the star. To overcome
this barrier, drift has to be slowed down or stopped, or growth needs to be
sped up. In this paper, we investigate the role of porosity on both drift and
growth. We have developed a model for porosity evolution during grain growth
and applied it to numerical simulations of protoplanetary discs. We find that
growth is faster for porous grains, enabling them to transition to the Stokes
drag regime, decouple from the gas, and survive the radial-drift barrier.
Direct formation of small planetesimals from porous dust is possible over large
areas of the disc.
