Gravitational accretion is considered an axiom of cosmology by which planets are formed. There’s a slight problem – it’s wrong.
Below is a diagram showing why.
Case 1 is what Newton observed and correctly described the motion between M1 and M2, two independent bodies separated by distance d. There’s supposed to be nothing between M1 and M2, since Newton had to assume the space was vacuum, or on the Earth, air. This equation works because Newton’s laws are about bodies in motion, not about motionless bodies. Bodies that are not doing anything do not require explanation.
Case 2 is what results when M1 has finally moved to M2 and the distance separating them is thus zero. But there is no gravitational force acting on M1 and M2 because no motion can ever be observed between M1 and M2 requiring explanation. This is because they are effectively one object; and one object in isolation also has no mass since the existence of mass requires at least two objects. Newton’s law is about movement between objects separated by space. Since there is no space between M1 and M2, there cannot be motion, and hence there is nothing to explain. A trick to explain this is to fabricate the centre of gravity of the two objects and since these are indeed separated, Newton’s Law can be fiddled to cope with this situation. It is a nonsense however.
Case 3 is a cartoon of the Earth in which there is a mantle, outer-core and inner-core. Does the inner core attract the outer core and mantle ? No because all three components have actually the same centre of gravity. There is no distance or space between the 3 enveloping shells comprising the Earth. There is thus no gravitational force acting on the objects. And like Case 2 above, they are not Newtonian bodies since such bodies require the ability to move independently of each other by virtue of the presence of space between them. There is no space between the core, outer core and mantle. These three components are parts of the Newtonian body called the Earth.
Gravitational accretion is thus a nonsense.
So how do planets form?
The only known force is the plasma Z-pinch mechanism. It is the only force capable of compressing matter to high densities. Planets form along plasma strings and might be fizzled out stars, or planetary bodies in their own right.