This still is from a simulation that shows the pebble density as it evolves around a star. 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.
Credit: Anders Johansen/Sterrewacht Leiden or Leiden Observatory