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How did planetesimals form from dust and gas in the solar nebula?

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asteroid Ida and its satellite, Dactyl — Small body Asteroid Ida and its satellite, Dactyl, photographed by the Galileo spacecraft on August 28, 1993, from a distance of about 10,870 km (6,750 miles). Ida is about 56 km (35 miles) long and shows the irregular shape and impact craters characteristic of many asteroids. The Galileo image revealed that Ida is accompanied by a tiny companion about 1.5 km (1 mile) wide, the first proof that some asteroids have natural satellites.

Planetesimals formed within the solar nebula through a process of rapid settling and collision. As the nebula cooled, solid materials—initially grains of metal-containing silicates—condensed from the gaseous phase and settled toward the disk’s midplane.1

This settling occurred relatively quickly, with particles ranging from 1 micrometre to 1 centimetre in size reaching the midplane in timescales of 10 years to 100,000 years. Once these particles concentrated at the midplane, the local density increased, facilitating growth through collisions. As these objects grew, their increasing gravitational fields further accelerated the accretion process. Calculations indicate that this mechanism allowed for the formation of objects approximately 10 kilometres (6 miles) in size within just 1,000 years; these bodies are classified as planetesimals, the fundamental building blocks of planets.1

Beyond the snow line, the condensation of water ice provided additional solid material, allowing objects in the outer solar system to acquire significantly more mass than those forming in the warmer inner regions.1