This clip shows the pebble density as it evolves in the computer simulation. The black regions have no pebbles, blue regions have a moderate density, and bright regions have high density of pebbles. The square represents a small part of the disk of gas and dust that surrounds the star before the planets form, referred to as the protoplanetary disk, seen from above.
The drifting pebbles first concentrate in an elongated filament. The filament then breaks into seven gravitationally bound mini-planets. This is only possible because the abundance of heavy elements in the simulated disk is slightly higher than the solar value. Disks that are less dirty, referred to as lower metallicity or fewer heavy elements, would not be able to form planets in these simulations.