  An unsolved issue in the standard core accretion model for gaseous planet
formation is how kilometre-sized planetesimals form from, initially,
micron-sized dust grains. Solid growth beyond metre sizes can be difficult both
because the sticking efficiency becomes very small, and because these particles
should rapidly migrate into the central star. We consider here how metre-sized
particles evolve in self-gravitating accretion discs using simulations in which
the gravitational influence of the solid particles is also included.
Metre-sized particles become strongly concentrated in the spiral structures
present in the disc and, if the solid to gas density ratio is sufficiently
high, can fragment due to their own self-gravity to form planetesimals
directly. This result suggests that planetesimal formation may occur very early
in the star formation process while discs are still massive enough to be
self-gravitating. The dependence of this process on the surface density of the
solids is also consistent with the observation that extrasolar planets are
preferentially found around high metallicity stars.
