As a solar system evolves, the disk from which the planets will form is initially bright and thick with dust. During the first 10 million years or so, gaps appear in the disk as planets form and clear out paths. Over time, the debris disk thins from gravitational interactions with planets. Steady pressure from the starlight and stellar winds push out the dust as well. Eventually, only a thin debris disk remains at the edge of the system.
Video Credits: NASA / JPL-Caltech / R. Hurt (SSC/Caltech)