Vol. 579
In section 10. Planets and planetary systems

How to form planetesimals from mm-sized chondrules and chondrule aggregates

In young circumstellar disks, dust grains rapidly migrate onto the central star owing to gas drag. To avoid getting lost, they must form large enough planetesimals. One of the leading mechanisms to explain this is the streaming instability that explains how grains that are affected by gas drag tend to clump together like cyclists in the “Tour de France”. Under the right conditions, this can form large planetesimals (~100km) directly from small grains. Carrera et al. examine when this clumping occurs. They show that it may happen when the dust-to-gas ratio is greater than 1.5% in mass and for a particle “Stokes number” between 0.0025 and 4 that corresponds to a particle between millimeter and meter sizes for a standard “MMSN” disk around 2 AU. This implies that millimeter-size chondrules in the 2-4 AU region would clump into planetesimals in a relatively late stage when the protosolar disk became less massive than a fraction of the MMSN value. This thus offers a new perspective on the formation of asteroids in our solar system that could aid in interpreting meteorite records.