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Assuming the accretion disk model for the formation of the Solar System, the most important problem, according to wikipedia, is "how the material, which is accreted by the protostar, loses its angular momentum."

If angular momentum is conserved, then as our Sun lost angular momentum in formation, excess gas and dust in the disk must have gained angular momentum. How much matter (if any) reached escape velocity and was ejected from the Solar System? When compared to the total mass of the Solar System, was this a significant portion?

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It is actually the other way around:
First a massive accretion disc can form, through which material looses angular momentum and accretes onto the star radially, hence being angular momentum poor.

However, during the initial free-fall phase, before the disc forms, infalling material can be 'rejected' at the star, either via high pressure gradients or magnetic interactions. This causes a polar jet to form around the protostar, which emits material back into the interstellar medium. Measured jet mass-loss rates are up to 1/2 the measured accretion rates.

Furthermore, the remaining fraction of material that did not form planets is ejected at the end of the disc lifetime via photoevaporation by the young stars' X and UV photons. What fraction this is in comparison to the mass in the planets is unknown, as the overall planet forming efficiency is unknown.

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