A Yale-led study published in Nature Astronomy reveals the Solar System’s earliest planetesimals were composed of 83% to 92% chondrules—heat-formed rock beads—with minimal icy, volatile-rich matrix. The research, based on iron meteorite analysis, challenges prior assumptions about planetary formation.
Scientists have long debated how the Solar System’s earliest solid bodies formed, but a new study led by Dr. Damanveer Grewal of Yale University offers clarity. By analyzing iron meteorites from the outer Solar System, researchers found that the first planetesimals were overwhelmingly chondrule-rich, with only 8% to 17% matrix—a stark contrast to later-formed objects. Our work shows that this assembly process was remarkably selective from the very beginning,
Grewal said, emphasizing the early preference for heat-forged material over icy dust.
Chondrules: The Building Blocks of Planets
Chondrules, millimeter-sized spheres of rock found in meteorites, are remnants of the Solar System’s earliest days. These ubiquitous little beads of rock
were the primary ingredients in the first planetesimals, according to the study. You can hold them in your hand and know that they began as part of a process that started billions of years ago,
Grewal explained. The research suggests that chondrules dominated the outer Solar System’s earliest bodies, with matrix—rich in water ice and organics—playing a minimal role.
The study’s findings align with patterns observed in carbonaceous chondrites, which show older samples contain more chondrules and less matrix. However, direct evidence from the Solar System’s first million years was previously lacking. Grewal and his team overcame this gap by examining iron meteorites, whose chemistry preserved clues about their parent bodies. Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor,
Grewal noted, highlighting the reliability of their conclusions.
Unraveling the Early Solar System’s Chemistry
The researchers identified two key chemical tracers: sulfur concentration and iron oxidation state. Sulfur, highly enriched in matrix, and iron’s oxidation level, which reflects water content, allowed the team to reconstruct the composition of ancient planetesimals. Their analysis revealed that matrix accounted for just 8% to 17% of these early bodies, far less than in later-formed objects. The earliest bodies in the outer solar system were built from 83% to 92% chondrules, with very little of the icy, volatile-rich dust that dominates later-forming objects,
the study states.
This discovery also addresses a longstanding puzzle: why ancient chondrules are rare in modern meteorites. Grewal suggests many were incorporated into the first planetesimals, which later melted and destroyed physical evidence. “Many of those ancient chondrules were probably incorporated into the first generation of planetesimals. Those bodies subsequently melted, destroying the physical evidence and making the oldest chondrules much harder to find in surviving meteorites,” he said.
Implications for Planetary Formation and Exoplanets
The study’s insights extend beyond our Solar System.
The research underscores the Sun’s early selectivity in assembling planetary building blocks. By prioritizing chondrules over icy dust, the Solar System’s formation process was remarkably selective from the very beginning,
as Grewal emphasized. This challenges assumptions that planetary systems form through uniform mixing of materials, suggesting instead a more nuanced, selective process.
For now, the research offers a clearer picture of how our cosmic neighborhood began—starting with fire, not ice.