1. Gravity:
- As the dust and gas in the protoplanetary disk settled and cooled, tiny particles began to clump together due to weak gravitational attraction.
- These clumps, called "dust bunnies," grew larger as they attracted more particles, eventually reaching sizes of millimeters or centimeters.
2. Aerodynamic Drag:
- The gas in the protoplanetary disk exerted drag on the dust particles, causing them to slow down and settle towards the midplane of the disk.
- This concentration of dust in the midplane facilitated collisions and further accretion.
3. Turbulent Flow:
- The gas in the protoplanetary disk wasn't perfectly smooth but exhibited turbulence, creating eddies and swirls.
- These turbulent flows could concentrate dust particles together, increasing the likelihood of collisions and growth.
4. Electrostatic Forces:
- Dust particles can carry electrostatic charges, which can attract or repel each other.
- These electrostatic forces could play a role in the initial clumping of dust particles.
5. Ice:
- The presence of ice in the outer regions of the protoplanetary disk was crucial. Ice, being more sticky than rock, allowed dust particles to stick together more effectively, contributing to the formation of larger planetesimals.
6. Pebble Accretion:
- As planetesimals grew, they began to accrete pebbles, which were larger than dust particles.
- Pebble accretion was a much more efficient process than dust accretion, allowing for rapid growth of planetesimals.
7. Runaway Growth:
- Once planetesimals reached a certain size, their gravity became strong enough to attract material from a wider area.
- This "runaway growth" allowed planetesimals to quickly accrete large amounts of mass, eventually forming the cores of planets.
8. Fragmentation:
- As planetesimals collided, some could fragment into smaller pieces. These fragments could either accrete onto other planetesimals or be ejected from the protoplanetary disk.
The formation of planetesimals is a dynamic and complex process involving a combination of these factors. These processes worked together to create the building blocks of planets that we see today.
