The earliest stages of planet formation may have been far more selective than scientists once realized. As the young Solar System began assembling solid objects such as planets, moons, and protoplanets, two major types of material were available. One was chondrules, millimeter-sized pieces of rock formed at high temperatures.
The other was matrix, a cold, fine-grained dust rich in water ice and organic material. New research led by Yale University suggests that the Solar System strongly favored the heat-formed material almost immediately. Researchers have found the first geochemical evidence showing that chondrules were already being preferentially incorporated into solid bodies during the Solar System's first million years.
Until now, scientists had only been able to document this type of sorting in objects that formed 2 to 4 million years after the Solar System began. The study was published Sept. 18 in Nature Astronomy. The Solar System Was Selective From the Start "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 in Yale's Faculty of Arts and Sciences, and first author of the study.
"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." Chondrules are small rocky spheres found inside chondrites, which are among the most primitive meteorites preserved in geological collections. These tiny structures offer scientists a direct physical connection to the earliest stages of the Solar System. "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." Scientists have long known that carbonaceous chondrites (primitive, stony meteorites that contain organic compounds and water among their silicate minerals) from the outer Solar System show a pattern related to age. Older examples tend to contain a greater proportion of chondrules and less matrix, while younger ones contain more of the cold, volatile-rich material. That pattern suggested that the regions where the first solid objects, known as planetesimals, were forming were already favoring heat-formed chondrules while excluding much of the icy dust.
Searching for Evidence From the First Million Years Confirming what happened during the Solar System's first million years has been difficult because no preserved undifferentiated bodies from that period remain. Scientists therefore lacked a direct way to determine the original balance between chondrules and matrix in those earliest objects. Grewal and his colleagues approached the problem by examining chemical clues preserved in iron meteorites from the outer Solar System.
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