Physics
The Process of Planetesimal Accretion in Protoplanetary Disks
Quick fact
In a protoplanetary disk, dust grains can stick together upon collision, growing from micrometer sizes to kilometer-sized planetesimals in as little as a few million years.
Why this is interesting
Every planet we know began as invisible dust. How do microscopic grains turn into worlds?
Read the full explanation
Understanding The Process of Planetesimal Accretion in Protoplanetary Disks
Imagine a giant swirling disk of gas and dust surrounding a young star. The dust grains are not uniformly spread; they are tiny specks (micrometers in size) floating in the gas. As they jostle and collide, gentle forces—like static electricity—cause them to stick together, forming small clumps. These clumps grow larger as they collide with other clumps, building centimetre-sized pebbles. The process sounds simple, but there are challenges: as particles grow, they interact with the gas and can spiral inward toward the star due to drag, a problem known as the meter-size barrier. To build planetesimals—bodies at least a kilometer across—gravity must take over. Once a clump becomes massive enough, its own gravity can pull in more material, accelerating growth. This step-by-step process of sticking and accreting is what turns specks of dust into the seeds of planets.
A deeper explanation
The mechanism of planetesimal accretion hinges on two competing forces: growth through collisions and loss through drag. In the early disk, tiny dust grains (micrometers) collide at low velocities. Van der Waals forces and electrostatic attractions make them stick, forming fractal aggregates. These grow into centimetre-sized pebbles. Here, the meter-size barrier emerges: pebbles experience strong aerodynamic drag from the gas, causing them to spiral inward and be lost to the star before they can grow further. The key to overcoming this barrier is the streaming instability—a process where local concentrations of pebbles create a feedback loop: the drag on pebbles alters the gas motion, concentrating pebbles into dense clumps. When the density becomes high enough (the Hill density), the clump collapses gravitationally, forming a planetesimal directly, bypassing the problematic intermediate sizes. Once a planetesimal is kilometer-sized, its gravity is strong enough to continuously attract nearby pebbles and small bodies, leading to runaway accretion. This explains why planetesimals are the critical seeds of planets.