Earliest Planet-Building Rocks Were Up to 92 Percent Heat-Formed Beads

The first solid bodies in the outer Solar System were built almost entirely from tiny beads of heat-formed rock, not from icy dust, according to a study in Nature Astronomy reported by ScienceAlert. Researchers say it is the first geochemical evidence of what the earliest planetesimals were made of.

The finding suggests that planet-building was selective from the start, with some ingredients favored over others rather than a random mix of whatever material was nearby.

Fire and ice in the young Solar System

ScienceAlert described two main ingredients around the early Sun. The “fire” was chondrules, small fragments of rock formed by intense heat. The “ice” was matrix, a cold, fine-grained dust loaded with water ice and organic molecules. Planetesimals, the larger objects that could grow into planets and moons, formed from a mix of the two.

How the mix changed over time

• First million years: The new study found that bodies born in this period were dominated by chondrules.
• From about 2 million years: Earlier work had shown chondrules were winning out over matrix in forming planetesimals.
• From about 4 million years: The icy matrix began to win out.

How the team read the meteorites

An international team analyzed iron meteorites that fell to Earth. These carry chemical fingerprints of how chondrules and matrix were mixed. Radioactive dating, together with the fact that their parent bodies melted completely, places the meteorites at the very start of the Solar System, ScienceAlert reported.

The researchers looked for two tracers. One was the estimated amount of sulfur, which is far more abundant in matrix than in chondrules. The other was the oxidation of the iron, which shows how much water ice and oxidized dust a body took in. Both pointed the same way. Damanveer Grewal, an Earth and planetary scientist at Yale University, said the earliest bodies in the outer Solar System were built from 83 percent to 92 percent chondrules, with very little icy dust.

Grewal said the two tracers independently tell the same story, which is what makes the result robust. ScienceAlert noted that each approach involves some educated guesswork about the larger planetesimals the iron fragments once belonged to, but that the agreement between them is significant.

Why chondrules may have dominated

The researchers attribute the early dominance to aerodynamic sorting. Chondrules are stronger and more compact, which increased the chances that they would clump together. Over the following few million years something changed that balance, though what is not yet entirely clear.

The result also helps explain why so few early-formed chondrules appear in the meteorite record. The rocks they belonged to later melted from internal heat, which erased the evidence.

What this means for understanding planets

For readers curious about where planets come from, the study offers a more detailed picture of the first steps. The materials that eventually assembled planets were being sorted and built into the first generation of solid bodies from the very start, Grewal said.

Because the work establishes what happened in our own Solar System, ScienceAlert reported, it could give experts a new perspective on how planets and moons formed elsewhere in the Universe. The study describes the early Solar System only, so it does not show that other planetary systems followed the same pattern.

Around Prague, based on reporting by ScienceAlert