Protoplanetary Disks: Observation, Measurement & Evidence

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.

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 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.

Infrared excess reveals warm dust, while ALMA resolves millimeter emission into rings and gaps at scales comparable to planetary systems. Spectral lines trace gas composition and velocity, allowing tests of near-Keplerian rotation. Some disk structures may be carved by planets, but snow lines, magnetic effects, or dust dynamics can mimic planet-like gaps.