Sometimes I wonder: how do planets or even smaller asteroids form from an interstellar dust? Certainly, the matter aggregation process induced by gravity has a strong logic: under proper conditions, smaller particles stick together forming grains which later also clump together, consequently forming increasingly larger bodies. But the time needed to accomplish this is still perplexing to most of us.
What time scale exactly are we talking about? According to scientific publication appearing on arXiv.org, you need at least ~105 (100 000) years to grow a millimeter-sized pebbles, or chondrules – as they are called by astronomers. Solids of this class may be of critical importance in the formation of planetisimals and current models of planetary formation lack a comprehensive theory clarifying how such chondrules are formed.
The authors of this study from Lund University, Sweden, used a numerical modeling to simulate a protoplanetary disk in which solar system-like planets are being formed. Their initial assumption was that disk contains a significant component of gas and solid particles that are initially micrometer-scale in size and form approximately 1% of the total disk mass. “Over time, particle sizes can grow by coagulation, and the mass ratio between gas and solids may change if particles migrate through the disk, or if the gas becomes depleted”, explain the scientists.
Concentrations, speeds and dimensions of dust particles, distances between them, and resulting clumping conditions were analyzed by the team. Numerical modeling revealed that dense particle clumps may form at a certain distance from the host star, because friction resulting from surrounding dust and gas slows such solids down and they migrate inwards to the star due to gravitational attraction. Therefore, larger bodies like asteroids may form only at a certain distance from the star where proper disk mass and turbulent viscosity is present.
Another estimate – the probability of sticking collisions between mm-sized particles – is mainly dominated by their speeds and distances between them. Even though a single collision over a course of ~105 years seems somewhat a rare event to expect, the total number of dust particles involved in this process is large. Meanwhile, the lifetime of the planetary disk which is the source of matter for the planetisimals to be built is significantly longer (~107 years) for a large multitude of such collisions to happen.
“A high concentration of particles is not required for this result. As the particles grow, the streaming instability becomes more effective, leading to the formation of planetisimals”, the authors say. Then asteroids may form directly from millimeter-size chondrules, particularly if weak turbulence is present in the surrounding space which facilitates the formation of solid aggregates few millimeters in size, the team concludes.
Written by Alius Noreika