The problem
A pebble orbiting a young star feels drag from the surrounding gas and drifts inward. How can small solids gather into bodies held together by gravity before drift or destructive collisions remove them?[1]
Small solids in a disk around a young star can collide and fragment or drift toward the star before becoming bound planetary building blocks. Where can they gather densely enough for gravity to take over?
In one modeled route, a concentration of pebbles exerts drag on the gas, reducing the headwind that drives inward drift. Dense filaments move inward more slowly and collect other pebbles. This feedback is the streaming instability; sufficiently dense clumps can contract into planetesimals. Its effectiveness depends on the surrounding disk, rather than following from the existence of the instability alone.[1] [2]
Scope
The conditions and mechanisms that turn disk solids into planetesimals, tested against surviving populations and observed planet-forming disks.
What remains open
Simulations support this route, but observed sizes and compositions must be connected to birth populations after later evolution. In a 2025 one-dimensional disk model, icy pebbles release smaller, slower-drifting silicate cores at the water-ice line, allowing solids to pile up. Varying carbon and oxygen changes the modeled yield. Its predicted trend in planet occurrence is an observational test, not a measured universal threshold.[1] [2]
Why it matters
Those bodies become the building blocks of planets.[1]
Surviving Solar System bodies provide evidence about the solid seeds available for later planet growth. A formation model must connect their properties to the original disk while accounting for subsequent change. Chemical predictions add another test: they connect the composition of planet-forming material to the frequency of planets around other stars.[1] [2]
Definitions
3 terms
planetesimals
Solid bodies large enough to serve as seeds for subsequent planet growth. In the cited chapter, the term refers to bodies above kilometer scales, where gas-driven drift is much weaker than for pebbles.[1]
streaming instability
A feedback between drifting solids and gas: solids exert drag on the gas and reduce their headwind, helping dense pebble filaments develop. Instability growth alone does not guarantee gravitational collapse.[1]
water-ice line
The region where inward-drifting water ice evaporates. In the cited disk model, icy pebbles release smaller silicate cores that drift more slowly, helping solids accumulate.[2]
Sources
- Formation of Planetesimals in the Outer Solar System
Notes on Formation of Planetesimals in the Outer Solar System
Researcher-authored chapter, published in Centaurs (IOP, May 2025); sections on streaming instability, observational constraints and future priorities support the scoped remaining gap.
- How does the chemical composition of solids influence the formation of planetesimals?
Notes on How does the chemical composition of solids influence the formation of planetesimals?
Authors’ manuscript of Astronomy & Astrophysics 701, A47: simulations expose disk-composition dependence and an explicitly testable prediction, not a measured universal formation threshold.