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American Association for the Advancement of Science

Studying the solar system's first solids

Calcium-aluminum-rich inclusions (CAIs) formed by condensation of volatilized dust in the hot, innermost region of the proto-planetary disk, whereas chondrules formed farther from the sun by accretion and melting of dust particles. Contrary to accepted models, this study shows that chondrules started forming contemporaneously with CAIs about 4,567.30 ± 0.16 Mya and continued their formation for another three million years. [Image credits: Background image - NASA/JPL-Caltech/T. Pyle (SSC); Chondrule - Mia Olsen]

The first solids to form in our solar system more than four and a half billion years ago were calcium-aluminum-rich inclusions, or CAIs, and round grains called chondrules. They can both be found in meteorites today.

Until now, however, most researchers believed that CAIs formed in our solar system first, and then chondrules began forming one or two million years after them—even though the two grains seem to form at the same time in distant, developing star systems.

Now, researchers have discovered that chondrules and CAIs actually began forming at the same time—around 4.567 billion years ago—in our solar system too. James Connelly from the University of Copenhagen in Denmark, along with colleagues from around the world, took measurements with uranium and lead isotopes in order to confirm this.

The difference, they say, was that chondrules continued forming for three million years while CAI formation was a relatively brief event. But, the two grains did begin forming at the same time in our solar system. So, the processes that form chondrules and CAIs in our solar system may not be so different from the processes that form them in other star systems after all.