A debris disk is a circumstellar disk of dust and debris in orbit around a star. Debris disks have been found around both mature and young stars, as well as at least one debris disk in orbit around an evolved neutron star.
Younger debris disks can constitute a phase in the formation of a planetary system following the protoplanetary disk phase, when terrestrial planets may finish growing. During the formation of a Sun-like star, the object passes through the T-Tauri phase during which it is surrounded by a disk-shaped nebula.
Out of this material are formed planetesimals, which can undergo an accretion process to form planets. The nebula continues to orbit the pre-main-sequence star for a period of 1-20 million years until it is cleared out by radiation pressure and other processes.
Additional dust may then be generated about the star by collisions between the planetismals, which forms a disk out of the resulting debris. At some point during their lifetime, at least 45% of these stars are surrounded by a debris disk, which then can be detected by the thermal emission of the dust using an infrared telescope.
Repeated collisions can cause a disk to persist for much of the lifetime of a star. They can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets.
Debris disks are often described as massive analogs to the debris in the Solar System. Most known debris disks have radii of 10-100 astronomical units (AU); they resemble the Kuiper belt in the Solar System, but with much more dust.
The presence of a debris disk may indicate a high likelihood of terrestrial planets orbiting the star.