Protoplanetary Disks: Physical Mechanisms

A disk contains gas, micron-sized dust, larger aggregates, and evolving planetesimals, with strong radial and vertical gradients in temperature, density, pressure, and composition. The inner disk is hotter, while farther from the star ices can condense beyond temperature-dependent snow lines. The disk gradually loses gas and dust through accretion onto the star, planet formation, winds, and photoevaporation.

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.

Dust grains collide and stick, settle toward the midplane, drift radially through gas drag, and can concentrate in pressure maxima. Gas accretes toward the star while angular momentum moves outward. Growth from grains to planetesimals must overcome barriers such as fragmentation and rapid radial drift, motivating mechanisms such as streaming instability.