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The young Solar System began favoring heat-formed rocky material over icy, organic-rich dust almost immediately after its formation, according to new research led by Yale University scientists.
The study provides the first geochemical evidence that chondrules—millimeter-sized rocky spheres formed at high temperatures—were preferentially incorporated into solid bodies during the Solar System’s first million years.
The findings, published Sept. 18 in Nature Astronomy, suggest that the earliest stages of planet formation were more selective than previously thought.
Researchers found that the earliest bodies in the outer Solar System were composed of about 83% to 92% chondrules, with relatively little matrix—the cold, fine-grained dust rich in water ice and organic material.
“Our work shows that this assembly process was remarkably selective from the very beginning,” said Damanveer Grewal, an assistant professor of Earth and planetary sciences at Yale and the study’s lead author.
Chondrules are among the oldest known solid materials in the Solar System. They are preserved in chondrites, primitive meteorites that provide clues to conditions during the earliest stages of planetary formation.
Scientists have previously observed that older carbonaceous chondrites from the outer Solar System generally contain more chondrules and less matrix, while younger samples contain greater amounts of cold, volatile-rich material.
That pattern suggested that the earliest planetesimals—the building blocks of planets—preferentially incorporated heat-formed material while excluding much of the icy dust.
However, confirming this process during the first million years has been difficult because no preserved undifferentiated bodies from that period are known.
To investigate the earliest stage, Grewal and his colleagues examined chemical signatures preserved in iron meteorites from the outer Solar System. These meteorites provided indirect evidence of the composition of the first solid bodies.
The findings indicate that selective assembly of planetary building blocks was already underway very early in Solar System history, offering new clues about how planets and other large bodies began to form.