Planetesimal and Core Accretion

The hierarchical growth of planetary building blocks, from dust grains to kilometre-scale planetesimals to the protoplanets and giant-planet cores.

Definition

Planetesimal and core accretion is the process by which dust in a protoplanetary disk grows hierarchically into planetesimals and then into protoplanets and giant-planet cores through collisions and gravitational accretion.

Scope

This topic covers how solid material grows across many orders of magnitude in size: the sticking and coagulation of dust, the still-debated jump to gravitationally bound planetesimals, and the gravitationally focused accretion that drives runaway and oligarchic growth of protoplanets. It includes the streaming instability and pebble accretion as mechanisms for overcoming growth barriers, and the assembly of the roughly ten-Earth-mass cores required to trigger gas-giant formation.

Core questions

• How do dust grains overcome the bouncing, fragmentation, and radial-drift barriers to reach planetesimal size?
• What triggers the transition from collisional growth to gravitationally dominated runaway accretion?
• How fast can a giant-planet core reach the critical mass for gas capture before the disk disperses?
• When does growth proceed by accreting planetesimals versus accreting small pebbles?

Key theories

Runaway and oligarchic growth

Once bodies are large enough for gravitational focusing, the largest planetesimals grow fastest in a runaway phase, then settle into oligarchic growth where a few dominant protoplanets accrete the surrounding swarm at comparable rates.

Critical-core gas capture

A solid core that reaches roughly ten Earth masses can no longer maintain a static gaseous envelope and undergoes runaway gas accretion, the key step in core-accretion theory of giant-planet formation.

Streaming instability

Aerodynamic coupling between solids and gas can concentrate pebbles into dense filaments that collapse directly into planetesimals, offering a route past the metre-size growth barrier.

Mechanisms

Small grains grow by sticking collisions until radial drift and fragmentation halt further coagulation; concentration mechanisms such as the streaming instability then assemble planetesimals, which grow by mutual gravitational accretion. Gravitational focusing makes the largest bodies dominate, and pebble accretion can rapidly build massive cores by capturing aerodynamically slowed solids.

Clinical relevance

The efficiency and timing of solid growth determine whether a region of a disk produces only small bodies, terrestrial planets, or gas-giant cores, and thus shapes the architecture of the whole system.

History

Safronov's planetesimal theory in the 1970s established the hierarchical-growth framework. The 1996 calculations of Pollack and collaborators quantified the critical-core gas-capture scenario for giant planets. Since the 2000s, the streaming instability and pebble accretion have been developed to address long-standing barriers in growing planetesimals and massive cores quickly enough.

Debates

How are planetesimals first assembled?

Whether the metre-size barrier is bridged mainly by the streaming instability, by other concentration mechanisms, or by direct collisional growth is still actively investigated.

Key figures

• Viktor Safronov
• James Pollack
• Jack Lissauer
• Anders Johansen

Seminal works

• safronov1972
• pollack1996
• johansen2014

Frequently asked questions

What is the metre-size barrier?

It is the difficulty that solids around a metre in size face: they collide too fast to stick yet drift inward toward the star too quickly, so growth past this size requires special concentration mechanisms.

Why does a giant planet need a ten-Earth-mass core?

At roughly that mass the core's gravity can no longer hold a stable gaseous envelope, so it begins to accrete disk gas rapidly and grows into a gas giant.