Old Times Not Forgotten

Age: Radioisotope 207Pb/ 206Pb dating of refractory inclusions (CAIs) found within chondritic meteorites, the oldest Solar System solids known, yields an age of 4.568 Gyr. Dating with other isotope systems yields similar ages. Chondrules, as well as most differentiated meteorites that originated within small bodies, solidified only a few million years later (§ 8.7). Rocks formed on the Moon and Earth are younger: lunar rocks are typically between 3 and 4.4 Gyr old, and terrestrial rocks are ≲ 4 Gyr old, although terrestrial mineral grains as old as 4.4 Gyr have been found.

de Pater, Imke; Lissauer, Jack J.. Planetary Sciences (p. 512). Cambridge University Press. Kindle Edition.

The ages of the refractory inclusions and the chrodrites tell us when they first solidified, almost certainly when the solar system was first condensing. From studies of regions of stellar formation, and deductions from basic physics, we know that stellar systems form in relatively dense molecular clouds containing a lot of dust. As the most of the mass concentrates in the star (I'm not considering multi-star systems here) most of the angular momentum ends up in the outer reaches which are flattened into a disk by gravity and viscous dissipation. The micron sized dust particles in these disks stick together, accumulating over a few million years into planetesimals each formed from roughly a trillion, trillion, trillion (10^36) dust grains.


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