Can we compute/simulate why the solar system has a very small number of planets? I saw a YouTube video saying that Earth was formed in the solar system which was initially a rotating disk of dust that kept colliding and forming larger and larger blocks.
What surprises me is that:

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*Eventually planets form; i.e one could conceive dust would collide but keep scattering and remain and the solar system would remain a rotating disk of dust.


*Given that planets form, there is a constant small number of them (in our solar system, 8).
Can we explain 1? Why does random colliding lead to planets?
Can we compute 2? Given initial mass of center and density of dust surrounding it, compute the number of planets and their radiuses?
 A: Yes, we can simulate this. Explaining why a given planetary system, including ours, has the specific planets it has, in terms of number, size and position, is still in its infancy, and early problems have already led to refinements, including in starting assumptions. (For example, we keep finding at least one early giant planet would be ejected, so to explain the modern Solar System's four you probably need to start with five! Indeed, such models find many planets are ultimately ejected, which matches reality.) But simulations invariably find a few large planets do in fact form.
I won't attempt to explain "why" these simulations have the outcomes they do, but I'll summarize a recent valuable video talk by Andre Izidoro on efforts at such refinements.
Izidoro begins by noting several peculiarities of our Solar System:

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*Our planets are surprisingly far from the Sun.

*The Solar System has no rocky Super-Earths, but they're common in other planetary systems, some of which are known to have several; by contrast, Jupiter-like planets, let alone ones of very low orbital eccentricity, are very rare. (To be clear, we do get giant planets, sometimes much larger than Jupiter, but they tend to be very hot.)

*Mars is surprisingly small, being $11\%$ of Earth's mass instead of at least $50\%$ as early models expected. This is possibly tied to the question of Jupiter's position.

But the research summarized in his talk offers some insights, for not only our planetary system but also others:

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*Models of planetary formation identify several bifurcation points, i.e. conditions that lead to qualitative differences between planetary systems.

*Simple models can be empirically improved by including the role of "pressure bumps", at frost lines and where temperatures sublime silicate; in particular, this matches our asteroid belt, but implies its carbonaceous and non-carbonaceous asteroids formed at different times as the Sun gradually warmed.

*We find three rings, which can explain our rocky planets, giant planets and Kuiper belt.

*How much the middle bump leaks determines which types of planet a system has, with hot (icy) super-Earths common if inner (outer) leakage is significant.

*Larger rocky planets migrate more due to planet-gas-disk gravitational interactions.

*"The migration scenario is broadly consistent with the dynamical architecture of observed hot super-Earths".

*This logic can also apply to a large planet's "system"; for example, Galilean moons may have formed in the Jovian system like hot super-Earths.

(He also mentions the need for an extra early giant planet or two to explain what we see today.)
