Planetary migration is the term used to describe the phenomenon in which planets within a planetary system change their orbits over time. This can be caused by interactions with other planets, gravitational forces from nearby stars, or even the presence of planetesimals or debris within the system. Planetary migration is a key concept in understanding the formation and evolution of planetary systems, as it can affect the final positions of planets, their compositions, and their overall stability. Studying planetary migration can help astronomers better understand how planets form and evolve within their solar systems.