Plate Nº 21 · recorded October 10, 2026
Space & AstronomyReported finding
First Solar System Planetesimals Were 83–92% Chondrules, Study Finds
A Yale-led study in Nature Astronomy finds the earliest Solar System planetesimals were 83–92% chondrules and just 8–17% matrix, pushing selective assembly of planetary ingredients back to the first million years.
By Marcus Bennett4 min read719 words
In brief
- Earliest outer Solar System planetesimals contained 83–92% chondrules and 8–17% matrix, according to a study published September 18, 2026 in Nature Astronomy.
- This is the first geochemical evidence of selective assembly within the Solar System's first million years; previous evidence dated to 2–4 million years after formation.
- Lead author Damanveer Grewal is an assistant professor of Earth and planetary sciences at Yale; co-authors are Zhongtian Zhang (Princeton) and Joanna Drążkowska (Max Planck Institute for Solar System Research).
- Researchers used two independent chemical tracers — sulfur content and iron oxidation state — in iron meteorites whose parent bodies had fully melted, erasing physical structures but preserving chemistry.
The earliest solid bodies in the outer Solar System contained 83% to 92% chondrules — millimeter-sized beads of rock forged at high temperatures — and only 8% to 17% water-rich dust, according to a study published September 18 in Nature Astronomy.
The finding represents the first geochemical evidence that this sorting process was already at work during the Solar System's first million years. Until now, scientists had documented similar selective assembly only in objects that formed 2 to 4 million years after the Solar System began.
"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 first author. "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."
What are chondrules and why do they matter?
Chondrules are small rocky spheres found inside chondrites, among the most primitive meteorites preserved in geological collections. Scientists consider them direct physical samples of the Solar System's earliest stages.
"You can hold them in your hand and know that they began as part of a process that started billions of years ago," Grewal said. "It's a timescale that's hard to wrap your head around."
Researchers had long noted that carbonaceous chondrites — primitive stony meteorites containing organic compounds and water in their minerals — show an age-related pattern. Older specimens contain more chondrules and less fine-grained dust. Younger ones carry more icy, volatile-rich material. That pattern hinted that the regions where planetesimals, the first solid planetary building blocks, were forming already favored heat-formed chondrules while excluding much of the icy dust.
How did the team test the first million years?
Confirming what happened during the Solar System's first million years has been hard. No preserved undifferentiated bodies from that period remain, leaving scientists without a direct way to measure the original balance between chondrules and matrix (the cold dust surrounding them).
Grewal and colleagues turned to iron meteorites from the outer Solar System. The parent bodies that produced these meteorites held so much aluminum-26, a radioactive isotope, that they heated up and melted completely, erasing the physical structures that could have revealed what they originally contained. Their chemistry, however, survived.
The researchers used two independent tracers linked to matrix. The first was sulfur content, which is highly concentrated in matrix. The second was the oxidation state of iron, which reflects how much water ice and oxidized dust the original body had incorporated.
What did the meteorites reveal?
Both pointed in the same direction. The team reconstructed the original compositions of the parent bodies and calculated that matrix accounted for just 8% to 17% of their starting material. That is a smaller share than has been measured in any known chondrite.
"Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor," Grewal said. "That convergence is what makes the result robust."
Why are very old chondrules rare today?
The results may also explain a long-standing puzzle. Very old chondrules are surprisingly uncommon in modern meteorite collections, even though they appear to have been abundant early on.
Grewal and his co-authors propose that many of those ancient chondrules were folded into the first generation of planetesimals, which then melted and destroyed the physical evidence. The oldest chondrules are therefore harder to find among surviving meteorites.
"These ubiquitous little beads of rock are the basic building blocks from which the planets themselves were eventually assembled," Grewal said. "And now we know they were already being sorted and incorporated into the first generation of solid bodies from the very start."
What does this change about planet formation?
The findings indicate that the process of separating and selecting planetary ingredients began almost as soon as solid bodies started to form. The young Solar System did not mix chondrules and icy dust evenly. Instead, some of its earliest planetesimals strongly favored heat-formed chondrules.
The study's co-authors are Zhongtian Zhang of Princeton University and Joanna Drążkowska of the Max Planck Institute for Solar System Research in Germany. Funding came from Yale University. The paper's DOI is 10.1038/s41550-026-02976-6.
via dx.doi.org (Original)
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