In planetary science a streaming instability is a hypothetical mechanism for the formation of planetesimals in which the drag felt by solid particles orbiting in a gas disk leads to their spontaneous concentration into clumps which can gravitationally collapse.
Massive filaments form that reach densities sufficient for the gravitational collapse into planetesimals the size of large asteroids, bypassing a number of barriers to the traditional formation mechanisms.
This process begins with the collision of dust due to Brownian motion producing larger aggregates held together by van der Waals forces.
The growth of the largest planetesimals then accelerates, as gravitational focusing increases their effective cross-section, resulting in runaway accretion forming the larger asteroids.
Later, gravitational scattering by the larger objects excites relative motions, causing a transition to slower oligarchic accretion that ends with the formation of planetary embryos.
[3] A number of obstacles to this process have been identified: barriers to growth via collisions, the radial drift of larger solids, and the turbulent stirring of planetesimals.
For silicates the increased collision velocities cause dust aggregates to compact into solid particles that bounce rather than stick, ending growth at the size of chondrules, roughly 1 mm in diameter.
[4][5] Icy solids may not be affected by the bouncing barrier but their growth can be halted at larger sizes due to fragmentation as collision velocities increase.
At 1 AU this produces a meter-sized barrier, with the rapid loss of large objects in as little as ~1000 orbits, ending with their vaporization as they approach too close to the star.
[19][20] Streaming instabilities, first described by Andrew Youdin and Jeremy Goodman,[21] are driven by differences in the motions of the gas and solid particles in the protoplanetary disk.
The difference in velocities results in a headwind that causes the solid particles to spiral toward the central star as they lose momentum to aerodynamic drag.
[23] The clusters shrink as energy is dissipated by gas drag and inelastic collisions, leading to the formation of planetesimals the size of large asteroids.
[23] Impact speeds are limited during the collapse of the smaller clusters that form 1–10 km asteroids, reducing the fragmentation of particles, leading to the formation of porous pebble pile planetesimals with low densities.
[26] Collapsing swarms with excess angular momentum can fragment, forming binary or in some cases trinary objects resembling those in the Kuiper belt.
[31] In the outer Solar System the largest objects can continue to grow via pebble accretion, possibly forming the cores of giant planets.
The drag felt by the solids moving toward the region also creates a back reaction on the gas that reinforces the elevated pressure leading to a runaway process.
[9] Moderately coupled solids, sometimes referred to as pebbles, range from roughly cm- to m-sized at asteroid belt distances and from mm- to dm-sized beyond 10 AU.
The moderately coupled solids that participate in streaming instabilities are those dynamically affected by changes in the motions of gas on scales similar to those of the Coriolis effect, allowing them to be captured by regions of high pressure in a rotating disk.
[38] Stars with higher metallicities are more likely to reach the minimum solid to gas ratio making them favorable locations for planetesimal and planet formation.
[43] If the magnetic field of the disc is aligned with its angular momentum the Hall effect increases viscosity which can result in a faster depletion of the inner gas disk.
[44][45] A pile up of solids in the inner disk can occur due to slower rates of radial drift as Stokes numbers decline with increasing gas densities.
[67][68] Solids may also be concentrated locally if disk winds lower the surface density of the inner disc, slowing or reversing their inward drift,[69] or due to thermal diffusion.
[70] Streaming instabilities are more likely to form in regions of the disk where: the growth of solids is favored, the pressure gradient is small, and turbulence is low.
[7] In an early proposal dust settled at the mid-plane until sufficient densities were reached for the disk to gravitationally fragment and collapse into planetesimals.
[82] The difference in orbital velocities of the dust and gas, however, produces turbulence which inhibits settling preventing sufficient densities from being reached.
If the average dust to gas ratio is increased by an order of magnitude at a pressure bump or by the slower drift of small particles derived from fragmenting larger bodies,[83][84] this turbulence may be suppressed allowing the formation of planetesimals.
As this process cascades to smaller eddies a fraction of these clumps may reach densities sufficient to be gravitationally bound and slowly collapse into planetesimals.
[93] A similar fractal growth of porous silicates may also be possible if they are made up of nanometer-sized grains formed from the evaporation and recondensation of dust.
In this model collisional dampening and gas drag dynamically cool the disk and the bend in the size distribution is caused by a transition between growth regimes.
As their eccentricities were damped due to gas drag and tidal interaction with the disk the largest and smallest objects would be lost as their semi-major axes shrank leaving behind the intermediate sized planetesimals.