  Turbulence acting on mixes of gas and particles generally evenly diffuses the
latter through the former. However, in the presence of background gas
temperature gradients a phenomenon known as turbulent thermal diffusion appears
as a particle drift velocity (rather than a diffusive term). This process moves
particles from hot regions to cold ones. We rederive turbulent thermal
diffusion using astrophysical language and demonstrate that it could play a
major role in protoplanetary discs by concentrating particles by factors of
tens. Such a concentration would set the stage for collective behavior such as
the streaming instability and hence planetesimal formation.
