A previous post introduces the Parent Vortex and the initial givens for the formation of the Solar System. Each of the 10 densely populated regions of atoms around the 10 big clumps of iron atoms is a protoplanet. A protoplanet is like a big “particle” within the gaseous flow of the Parent Vortex. Each of these big particles is not necessarily spherical at this point. Gravity and rotational motion combine to create a system that separates these big “particles” from the mainstream flow. This behavior is called cyclone separation (or cyclonic separation).
I really like the explanation in Wikipedia. The article in Wikipedia provides the physics of what happens in cyclonic separation. It sets up the criteria for the case where forces balance so that a particle has no incentive to spiral inwards to the center, nor fly away from the center. This happens when the radial velocity has caused enough drag force to counter the centrifugal and buoyancy forces.
From the equations of motion, we find that “if the density of the fluid is greater than the density of the particle, the motion is (-), toward the center of rotation and if the particle is denser than the fluid, the motion is (+), away from the center.” For the 10 densely populated regions, the particle is denser than the fluid, so it moves radially away from the center of the Vortex, until it is no longer present within the Vortex flow. The protoplanet “particle,” however, retains its angular momentum and goes into orbit around the Parent Vortex’s center of mass.
Cyclone technology for vacuum cleaners works using the same physics. So validation for the physics of cyclone separation is provided every day through the fact that people find cyclone-technology vacuum cleaners efficacious for the removal of dirt, dust and allergens.
Our solar system had 10 protoplanets in its nascent vortex. It is possible that an arbitrary solar system could not have any protoplanets. Or there could be more than 10.
No protoplanets means that in the initial nebula (i.e. the portion of a nebula which birthed the system), there were no big clumps of metal atoms. This would happen if metal atoms were well-mixed within the parent cloud, or the system had a very small amount of metal atoms, i.e. the system was metal-poor. The ability for a parent vortex to have no particles explains why some stars do not have orbiting planets. Near as we can tell, Barnard’s Star is one of these; it does not have a planetary system. There is a claim by a reader (Ronald) here, Barnard’s Star: No Sign of Planets, that Barnard’s Star is a type of star that is known to be metal-poor.