When the Solar System began taking shape about 4.6 billion years ago, it was essentially a huge disk of gas and dust surrounding the young Sun. Over time, some of that material came together to form the planets, moons, asteroids, and other solid bodies we see today.
But what exactly were these first solid bodies made from?
According to a new study led by researchers at Yale University, the answer may reveal an important choice made very early in Solar System history. The first planet-building bodies appear to have strongly favored chondrules—tiny, heat-formed pieces of rock—over fine, icy dust known as matrix.
The research, published in Nature Astronomy, provides the earliest geochemical evidence yet that this sorting was already happening within the first million years of Solar System formation.
Two Main Ingredients for Building Planets
The young Solar System contained a mixture of different materials.
One important ingredient was chondrules. These are millimeter-sized, rounded grains of rock that were heated and melted before cooling rapidly. They are commonly found inside primitive meteorites called chondrites.
The second major ingredient was matrix. Unlike chondrules, matrix consists of extremely fine-grained material. It could contain water ice, organic molecules, and other volatile-rich components.
As these materials moved around the young Solar System, they could eventually collide and stick together, gradually forming larger objects called planetesimals. These planetesimals later became some of the building blocks of planets and moons.
The new research suggests that this process was not simply a random mixture of everything available.
Instead, the earliest planetesimals appear to have been strongly enriched in chondrules.
An Early Preference for Chondrules
Previous research had already provided clues that the Solar System was sorting these materials.
Scientists had studied carbonaceous chondrites, which are primitive meteorites containing minerals, organic compounds, and evidence of water. They found that some of the older objects contained relatively more chondrules and less matrix.
That suggested something interesting: when the earliest solid bodies were forming, fine-grained, icy material may have been separated from the larger chondrules.
However, there was a major problem.
Scientists did not have surviving, unchanged planetesimals from the first million years of Solar System history. Many of those earliest bodies experienced intense heating and melting, destroying the original physical arrangement of their ingredients.
So researchers needed another way to reconstruct what those ancient objects were made of.
The Clue Hidden Inside Iron Meteorites
Damanveer Grewal, an assistant professor of Earth and planetary sciences at Yale and the study's first author, and his colleagues turned to iron meteorites.
At first, iron meteorites might seem like an unusual place to look for evidence about ancient dust and chondrules.
But these meteorites came from parent bodies that formed extremely early in Solar System history. Those bodies contained significant amounts of radioactive aluminum-26, which produced heat as it decayed.
The heat was powerful enough to melt the parent bodies completely.
That melting destroyed much of the original physical evidence. In other words, scientists could no longer simply look at the rock and see how much chondrule or matrix material had originally been present.
However, chemistry can preserve clues even when physical structures disappear.
The researchers identified two independent chemical tracers that could reveal how much matrix the original bodies had contained.
Sulfur and Iron Reveal the Past
The first tracer was sulfur.
Sulfur tends to be concentrated in matrix material. Therefore, the amount of sulfur preserved in the meteorites can provide information about how much matrix was present in their original parent bodies.
The second tracer was the oxidation state of iron.
This chemical property can indicate how much water-rich and oxidized material had been incorporated into an ancient body.
These two clues provided independent ways of estimating the original matrix content.
And remarkably, both methods pointed toward the same conclusion.
The earliest planetesimals represented by these iron meteorites contained only about 8% to 17% matrix.
That is an extremely low amount compared with many later-forming chondritic objects.
The Earliest Planetesimals Were Mostly Chondrules
Based on the chemical evidence, the researchers estimate that some of the earliest bodies in the outer Solar System were composed of approximately 83% to 92% chondrules.
That means these ancient objects were overwhelmingly dominated by heat-processed rocky material, while only a small fraction consisted of the fine, volatile-rich matrix.
This is significant because it pushes evidence for material sorting much further back in time.
Earlier studies had documented this type of sorting in objects that formed roughly 2 to 4 million years after the Solar System began forming.
The new research suggests the process was already occurring during the first million years.
In other words, the Solar System may have been selectively organizing its raw materials almost from the beginning.
How Did This Sorting Happen?
The study points toward a process called aerodynamic sorting.
The young Solar System contained a rotating disk of gas and solid particles. Different particles behaved differently as they moved through this gas.
Their size, shape, density, and interaction with the surrounding gas could affect where they traveled and how easily they became incorporated into growing bodies.
Chondrules and fine matrix particles therefore did not necessarily move through the young Solar System in exactly the same way.
This difference could have allowed growing planetesimals to collect significantly more chondrules than matrix.
The result was a kind of natural sorting system operating inside the early Solar System.
Why Are Ancient Chondrules So Difficult to Find?
The findings also help explain a long-standing mystery.
Chondrules are extremely common in many primitive meteorites, but the oldest chondrules themselves are relatively difficult to identify in the meteorite record.
The new study offers an explanation.
Some of the oldest chondrules may have been incorporated into the first generation of planetesimals. Many of those bodies later became hot enough to melt completely.
When that happened, the original chondrules were destroyed or transformed.
Their physical evidence disappeared, even though their chemical signatures could survive in other forms.
Iron meteorites therefore act as an unusual kind of historical record. They can preserve chemical information about materials that no longer exist in their original form.
A New Look at Planet Formation
The discovery provides an important piece of the story of how planets began.
Rather than simply accumulating whatever material happened to be nearby, the earliest planet-building process appears to have been influenced by the physical behavior of different particles in the young Solar System.
Tiny millimeter-sized chondrules may have been particularly important.
These small rocky beads formed through intense heating events billions of years ago. Later, they became part of larger bodies, which eventually contributed to the construction of planets and other objects.
As Grewal and his colleagues' findings suggest, this selective assembly was already underway at an extremely early stage.
The research also demonstrates how scientists can reconstruct events that happened billions of years ago—even when the original objects have disappeared.
By studying the chemistry locked inside meteorites, researchers can piece together the history of the Solar System and understand how its first solid bodies were assembled.
The next time you see a meteorite containing tiny round grains, those seemingly insignificant particles can be viewed in a very different way.
They may represent some of the ancient building blocks from which the planets themselves were eventually assembled.
Reference: Grewal, D.S., Zhang, Z. & Drążkowska, J. Planetesimal compositions governed by aerodynamic sorting from the onset of Solar System formation. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02976-6
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