The young Solar System resembled a giant construction site: around the newborn Sun swirled a disk of gas and dust, and from this dust gradually assembled stones, boulders, and then planetary embryos — planetesimals. Today, their fragments fall to Earth as meteorites. By composition, meteorites fall into two big families: they once formed on opposite sides of an invisible barrier in the disk — likely near the orbit of the future Jupiter.

To determine when the birth of these bodies began, the authors built a refined model of their thermal history. It included chemical reactions and phase transitions during heating, as well as natural variations in composition. It turned out that the parent bodies of iron meteorites from both groups — the “inner” and the “outer”—began forming almost simultaneously, roughly 0.95 million years after the appearance of the first solid particles (the so-called CAIs — calcium-aluminum-rich inclusions). In contrast, the parent bodies of stony meteorites, chondrites, were born later and at different times for different groups.

By comparing the ages with the proportions of isotopic “signatures” in meteorites, the scientists reconstructed the motion of material through the disk. They found that the material from both groups began crossing the barrier exactly when planetesimal formation started. It seems that the same event both triggered the birth of planetary embryos and opened a “gate” for mixing. What that phenomenon was remains unclear, but now its traces can be sought in models of disk evolution. Thus, meteorites that fell to Earth billions of years later help us reconstruct the first steps of our planetary system.