Protoplanetary Disks: Structures & Vocabulary

Protoplanetary disks are rotating disks of gas and dust around young stars in which planets can form. They arise naturally when a collapsing cloud core conserves angular momentum, causing infalling material to flatten into a disk rather than fall directly into the growing star from every direction.

Dust grains collide and sometimes stick, forming progressively larger aggregates. Gas drag changes the motion of solids, and local concentrations can accelerate growth into planetesimals. Once bodies become massive enough, gravity dominates their growth and they accrete neighboring material. Meanwhile viscosity and magnetic stresses move angular momentum outward, allowing some disk gas to spiral inward.

Young stars are commonly observed with infrared excess emission produced by warm circumstellar dust. High-resolution millimeter observations reveal rings, gaps, spirals, and other structures in disks, while spectroscopy measures disk gas. These observations show that disks are widespread around young stars and evolve on million-year timescales.

Disk structure includes a dense midplane, warmer surface layers, radial pressure gradients, dust populations, gas, snow lines, rings, gaps, spirals, and sometimes inner cavities. Dust contributes little mass compared with gas but dominates much of the continuum emission used to map solid material.