A study published in Nature Astronomy reveals that the earliest planetesimals in the solar system contained up to 92% heat-forged chondrules, with only 8–17% volatile-rich matrix, as aerodynamic sorting separated rocky grains from ice-rich dust during the first million years of the solar system.
The early solar system’s first solid bodies, known as planetesimals, were primarily composed of heat-forged chondrules rather than ice-rich dust, according to a study published in Nature Astronomy. Researchers analyzed iron meteorites and found that these ancient objects contained between 83% and 92% chondrules—millimeter-scale rocky particles formed under extreme heat—while only 8% to 17% of their mass consisted of volatile-rich matrix, a fine dust containing water ice and organic molecules. This discovery challenges previous assumptions about the uniformity of early solar system materials and highlights the role of aerodynamic sorting in shaping planetary building blocks.
Aerodynamic Sorting Shaped Early Planetesimals
During the solar system’s first million years, a gas-rich disc surrounded the young Sun, and particles within it behaved differently based on size and composition. Larger, heat-processed chondrules were preferentially separated from finer, ice-laden matrix through aerodynamic forces, leaving the earliest planetesimals with a disproportionately high concentration of rocky material. This process, described in the Nature Astronomy study, explains why ancient chondrules are rare in meteorite collections—many were incorporated into bodies that later melted, erasing their original structure.
Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor,
said Damanveer Grewal, the study’s first author. The findings align with analyses of iron meteorites, which preserve chemical clues about their parent bodies. By examining sulfur levels and iron oxidation states, the team confirmed that the earliest planetesimals contained minimal volatile-rich dust, a stark contrast to later-formed objects.
Iron Meteorites Reveal Lost Cosmic History
Direct study of the earliest planetesimals is impossible, as their original structures were destroyed by internal heating from radioactive aluminum-26. Instead, researchers turned to iron meteorites—metallic remnants of melted parent bodies. These meteorites, which reached Earth, retain chemical signatures of their origins. By reconstructing the sulfur content and oxidation states of these meteorites, the team inferred that their parent bodies had chondrule fractions of 83% to 92%, with matrix comprising only 8% to 17%.
This approach resolved a longstanding puzzle: why ancient chondrules are scarce in meteorite records. Many were trapped in early planetesimals that later melted, obliterating their physical evidence. The earliest original rocks were melted away,
the study notes, emphasizing that today’s meteorites are not direct snapshots of the solar system’s infancy but rather fragments of later-stage bodies.
Ryugu Samples Add Context to the Puzzle
While the Nature Astronomy study focused on iron meteorites, analyses of Ryugu asteroid samples—collected by JAXA’s Hayabusa2 probe—provide complementary insights. Researchers including researchers from the Museum found that Ryugu’s parent asteroid formed in the solar system’s outer regions, where ice melted and reacted with minerals to create hydrated materials. This process, which occurred within two million years of the solar system’s formation, aligns with the idea of selective planetesimal assembly.
We found that Ryugu's parent asteroid formed in the cold, outer regions of the solar system where there was both carbon dioxide and water ice,
said Ashley King, a Museum researcher who co-authored the analysis of Ryugu. The asteroid’s rubble-pile structure and composition suggest it formed from fragments of a larger body, which was later shattered by a collision. This supports the broader narrative of a solar system where materials were dynamically sorted and reassembled over time.
Why This Matters for Solar System Origins
The findings reshape understanding of how planets and moons formed. By showing that the earliest planetesimals were matrix-poor, the study suggests that the solar system’s outer regions were not a uniform mix of ice and rock but a dynamic environment where physical processes filtered materials. This selective assembly may have influenced the composition of terrestrial planets, including Earth, which received water and organic compounds from later asteroid impacts.
Understanding the composition of asteroids can help explain how the early solar system developed,
said Ashley. The research also underscores the importance of sample-return missions like Hayabusa2, which provide direct evidence of processes that cannot be fully reconstructed from meteorites alone.
Future studies will focus on refining the timeline of aerodynamic sorting and its impact on planet formation. Researchers are also analyzing additional meteorite samples to confirm the 8–17% matrix threshold and explore variations across different solar system regions. The James Webb Space Telescope and upcoming asteroid missions may provide further insights into the distribution of chondrules and matrix in protoplanetary disks.
For now, the study confirms that the solar system’s earliest building blocks were not random but shaped by physical forces that favored heat-forged rock over icy dust. This discovery not only clarifies the origins of planetesimals but also highlights the intricate processes that governed the solar system’s evolution.