Debris Disk - Origin

Origin

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.

Typical debris disks contain small grains 1–100 μm in size. Collisions will grind down these grains to sub-micrometre sizes, which will be removed from the system by radiation pressure from the host star. In very tenuous disks like the ones in the Solar System, the Poynting–Robertson effect can cause particles to spiral inward instead. Both processes limit the lifetime of the disk to 10 Myr or less. Thus, for a disk to remain intact, a process is needed to continually replenish the disk. This can occur, for example, by means of collisions between larger bodies, followed by a cascade that grinds down the objects to the observed small grains.

For collisions to occur in a debris disk, the bodies must be gravitationally perturbed sufficiently to create relatively large collisional velocities. A planetary system around the star can cause such perturbations, as can a binary star companion or the close approach of another star. The presence of a debris disk may indicate a high likelihood of terrestrial planets orbiting the star.