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Quoting Wikipedia:

The nebular hypothesis says that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud.

The question is: how much of the current form of the Solar System is determined by the macroscopic state of the molecular cloud and its surroundings? Are the number of planets, their orbits, sizes and composition determined? if not - at what point in the evolution of the Solar System were these determined?

(I understand that the system is chaotic; but I'm interested only in macroscopic properties; surely something macroscopic is determined by the initial macroscopic state)

(by "Y is determined from X" I mean that Y is very likely to happen given X, even if the probability is not exactly 1.0)

Lior
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    What do you mean by the macroscopic state of the molecular cloud? – GiorgioP-DoomsdayClockIsAt-90 Sep 11 '23 at 05:13
  • @GiorgioP-DoomsdayClockIsAt-90 I know it's not well defined (in particular because it's not in equilibrium), but I'm mean things like the density, shape, composition, temperature as functions of position in coarse grained distances. – Lior Sep 11 '23 at 10:17

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How much of the current form of the Solar System is determined by the macroscopic state of the molecular cloud and its surroundings ? Are the number of planets, their orbits, sizes and composition determined ?

Most probably not. The wide ranges in the number, size, orbits and compositions of exoplanets observed around sun-like stars suggests that there is no such thing as a typical planetary system, and that random chance plays a large part in the development of planets. Just about the only general conclusion that we can take away from a study of exoplanets is that a sun-like star is more likely to have planets than not.

The most likely theory of the origin of the Moon is that it was formed after a Mars-sized body collided with the young Earth. If this collision had not happened then the solar system may have had an extra planet.

gandalf61
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  • thanks. "there is no such thing as a typical planetary system, and that random chance plays a large part in the development of planets." - but is this due to different planetary systems having very different macroscopic initial conditions, or that they had similar conditions yet different outcoms? – Lior Sep 11 '23 at 10:20
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    @Lior It is reasonable to assume that stars in our own galaxy that have a similar age, mass and spectral type to our own sun formed from similar initial conditions, at least at a macroscopic level. I don't think anyone can give you a more precise answer unless you tell us what exactly you mean by "macroscopic state". – gandalf61 Sep 11 '23 at 10:29