Protoplanetary Disks and Accretion

The gaseous, dusty disks that orbit young stars, supplying the raw material for planets and channelling mass onto the central star.

Definition

A protoplanetary disk is a rotating disk of gas and dust surrounding a newly formed star, within which planets form and through which mass accretes onto the star.

Scope

This topic covers the formation, structure, thermal and chemical stratification, and dissipation of protoplanetary disks, together with the physics of how disk material accretes onto the star and how angular momentum is transported outward. It includes the alpha-viscosity framework, magnetorotational and other turbulence, the role of the snow line and dust traps, and observational diagnostics from infrared and millimetre imaging such as rings, gaps, and spiral arms.

Core questions

• How is angular momentum transported so that gas can accrete onto the star?
• What sets a disk's temperature and density structure, and where does the snow line fall?
• How and when do disks dissipate, setting the clock for giant-planet formation?
• What do the rings and gaps seen in disk images reveal about embedded planets and dust dynamics?

Key theories

Alpha-disk model

Disk accretion is parameterized by an effective turbulent viscosity proportional to the gas pressure and a dimensionless parameter alpha, allowing tractable predictions of disk structure and evolution without resolving the underlying turbulence.

Viscous angular-momentum transport

Outward transport of angular momentum, plausibly driven by magnetorotational instability or disk winds, lets the bulk of the disk gas spiral inward and accrete while a small outer portion carries angular momentum away.

Mechanisms

Material in a disk loses angular momentum through turbulent stresses or magnetized winds and spirals inward onto the star; dust settles to the midplane and drifts radially, concentrating at pressure maxima where it can grow into planetesimals. Stellar irradiation and viscous heating set a radial temperature gradient that fixes the condensation fronts of water and other volatiles.

Clinical relevance

Disk structure determines the inventory and location of solids and volatiles available to planets, directly shaping the compositions and orbits of the planets that ultimately form.

History

Disk-accretion theory was placed on a quantitative footing by the 1973 alpha-disk prescription of Shakura and Sunyaev, originally developed for accreting compact objects but adopted broadly for protoplanetary disks. High-resolution millimetre imaging in the 2010s revealed that disks are commonly structured into rings and gaps, reframing the observational study of planet formation in situ.

Debates

What drives disk accretion?

Whether accretion is dominated by magnetorotational turbulence, magnetized disk winds, or other mechanisms remains unsettled, especially in the cold, weakly ionized outer disk.

Key figures

• Nikolai Shakura
• Rashid Sunyaev
• Philip Armitage
• Sean Andrews

Seminal works

• shakurasunyaev1973
• andrews2020

Frequently asked questions

How long do protoplanetary disks last?

Observations of young clusters indicate that the gas-rich phase of most disks dissipates within a few million years, which limits how quickly gas-giant planets must form.

What causes the rings seen in disk images?

Rings and gaps may be carved by forming planets, or arise from dust trapping at pressure bumps and at condensation fronts; both interpretations are actively studied.