Planets form from the coagulation of tiny solid particles (dust) in a gaseous protoplanetary disk, requiring growth over 40 orders of magnitude in particle mass. A crucial stage in planet formation involves making kilometer-sized planetesimals from millimeter to centimeter sized pebbles. This image illustrates this process: aerodynamical interactions between the gas and the pebbles collect the latter into very dense clumps (bright regions), almost as if by design. In turn, these clumps become planetesimals: the building blocks of planets. The image is taken from a hydrodynamical simulation of a protoplanetary disk, in which the dynamics and feedback from millions of small solid particles is included self-consistently. The unstable clumping of particles was predicted in research conducted at Princeton by A. Youdin and J. Goodman.