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. Small initial clumps increase the orbital velocity of the gas, slowing radial drift locally, leading to their growth as they are joined by faster drifting isolated particles. 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. The formation of streaming instabilities requires solids that are moderately coupled to the gas and a local solid to gas ratio of one or greater. The growth of solids large enough to become moderately coupled to the gas is more likely outside the ice line and in regions with limited turbulence. An initial concentration of solids with respect to the gas is necessary to suppress turbulence sufficiently to allow the solid to gas ratio to reach greater than one at the mid-plane. A wide variety of mechanisms to selectively remove gas or to concentrate solids have been proposed. In the inner Solar System the formation of streaming instabilities requires a greater initial concentration of solids or the growth of solid beyond the size of chondrules.
Background Planetesimals and larger bodies are traditionally thought to have formed via a hierarchical accretion, the formation of large objects via the collision and mergers of small objects. This process begins with the collision of dust due to Brownian motion producing larger aggregates held together by van der Waals forces. The aggregates settle toward the mid-plane of the disk and collide due to gas turbulence forming pebbles and larger objects. Further collisions and mergers eventually yield planetesimals 1–10 km in diameter held together by self-gravity. 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. In the outer Solar System the planetary embryos grow large enough to accrete gas, forming the giant planets. In the inner Solar System the orbits of the planetary embryos become unstable, leading to giant impacts and the formation of the terrestrial planets. 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. As a particle grows the time required for its motion to react to changes in the motion of the gas in turbulent eddies increases. The relative motions of particles, and collision velocities, therefore increases as with the mass of the particles. 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. 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. Radial drift is the result of the pressure support of the gas, enabling it to orbit at a slower velocity than the solids. Solids orbiting through this gas lose angular momentum and spiral toward the central star at rates that increase as they grow. 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. At greater distances the growth of icy bodies can become drift limited at smaller sizes when their drift timescales become shorter than their growth timescales. Turbulence in the protoplanetary disk can create density fluctuations which exert torques on planetesimals exciting their relative velocities. Outside the dead zone the higher random velocities can result in the destruction of smaller planetesimals, and the delay of the onset of runaway growth until planetesimals reach radii of 100 km. Some evidence exists that planetesimal formation may have bypassed these barriers to incremental growth. In the inner asteroid belt all of the low albedo asteroids that have not been identified as part of a collisional family are larger than 35 km. A change in the slope of the size distribution of asteroids at roughly 100 km can be reproduced in models if the minimal diameter of the planetesimals was 100 km and the smaller asteroids are debris from collisions. A similar change in slope has been observed in the size distribution of the Kuiper belt objects. The low numbers of small craters on Pluto has also been cited as evidence the largest KBO's formed directly. Furthermore, if the cold classical KBO's formed in situ from a low mass disk, as suggested by the presence of loosely bound binaries, they are unlikely to have formed via the traditional mechanism. The dust activity of comets indicates a low tensile strength that would be the result of a gentle formation process with collisions at free-fall velocities.
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