Transitional disks are a type of protoplanetary disk - a rotating disk of gas and dust surrounding a young star - that display a gap or hole in their inner regions, typically around 1-10 astronomical units from the central star. These gaps are believed to be cleared out by young, forming planets in the disk. Transitional disks are considered a key stage in the evolution of planetary systems, as they represent a critical phase in which planets are actively forming and shaping the structure of the disk. Researchers study transitional disks to understand the process of planet formation and the dynamics of protoplanetary systems.