During the solar system’s first million years, its earliest outer worlds formed from material that was up to 92 percent heat-forged rock, according to a new analysis published in Nature Astronomy.
The first solid worlds assembled in the outer solar system did not simply collect a representative scoop of everything around them. A new analysis of carbonaceous iron meteorites indicates that their parent planetesimals contained only 8 to 17 percent fine, volatile-rich matrix. The rest, as much as 92 percent by mass, was dominated by millimetre-scale rocky particles called chondrules that had been intensely heated before they cooled. The result pushes evidence for selective planet building into roughly the solar system’s first million years.
Reconstructing the Lost Planetesimals From Iron Cores
These early bodies were planetesimals, the asteroid-scale predecessors from which planets and moons grew, rather than completed outer planets. No intact example from this earliest generation survives. Their lost composition has instead been reconstructed from the metallic cores that reached Earth as iron meteorites. The finding also does not mean that the outer disk lacked ice. The surrounding material was rich in cold dust, water ice and organics. The surprise is that so little of that fine-grained component entered the first known bodies.
Primitive carbonaceous meteorites preserve two visibly different ingredients. Chondrules are compact, usually rounded silicate particles made when dust or earlier solids experienced short bursts of high temperature. Some were melted into droplets; others were thermally sintered. Matrix is the material between them, consisting of much finer dust that largely avoided comparable heating and retained more volatile elements, carbon-rich compounds and water-bearing material.
Radioactive Heat and the Destruction of Original Textures
Planetesimals assembled during the solar system’s opening epoch inherited abundant aluminium-26. This radioactive isotope had a half-life of about 717,000 years, so its decay supplied a powerful but rapidly fading source of internal heat. Bodies that formed early received enough aluminium-26 to melt extensively. Metal separated from silicate and descended into their interiors, forming cores. The process erased the starting mixture’s visible texture: chondrules dissolved into magma, ice reacted or escaped, and fine matrix ceased to exist as a recognisable component.
Later collisions broke some of those differentiated worlds apart. Pieces of their metal cores became the carbonaceous iron meteorites available today. Researchers used two independent chemical proxies to reconstruct what went into them. Damanveer Grewal of Yale University, Zhongtian Zhang of Princeton University and Joanna Drążkowska of the Max Planck Institute for Solar System Research utilized bulk sulfur, which in carbonaceous material is much more concentrated in fine matrix than in chondrules. The resulting inventories were too low for matrix-rich starting material.
A Journey from the Solar Nebula to Planetary Discs
This early epoch connects directly to the broader birth of the solar system, which began life as a vast, swirling cloud of gas and dust known as the solar nebula. About 4.6 billion years ago, this gigantic cloud was transformed into our Sun. Gravitational collapse caused dust and gas to be continually tugged to the centre of the cloud, making its core very hot and dense.
“It became a snowball effect. As more matter got pulled in, the centre got denser, increasing the gravity and pulling even more dust inwards.”
Tim Gregory, Researcher
About 99.9% of the material fell into the middle of the cloud and became the Sun, triggering nuclear fusion. The remaining 0.1% orbited around the Sun, forming a flat disc called the protoplanetary disc where the planets formed.
The Role of Asteroids and Chondrites in Modern Science
The asteroids left over from this era serve as pristine time capsules. Many of these rocks orbit the Sun in an area between Mars and Jupiter known as the asteroid belt, where the biggest object, Ceres, has a diameter of nearly 600 miles. Bodies that did not melt early on are a type of meteorite known as chondrites, which contain the first solids that formed in the solar system.
“They’re very valuable to us as scientists, because they contain material that Earth and the other planets were originally made from, frozen in time. The study of these rocks can tell us a lot about what conditions were like in the disc, when planets were still forming.”
Tim Gregory, Researcher
Unresolved Mysteries of the Solar Atmosphere
While early planet formation shaped the solid bodies of the solar system, active solar mechanisms continue to baffle researchers studying our nearest star. The corona, the tenuous outermost layer of the solar atmosphere, spikes upwards of 2 million degrees Fahrenheit, while just 1,000 miles below, the underlying surface simmers at a balmy 10,000 F. How the Sun manages this feat remains the coronal heating problem, one of the greatest unanswered questions in astrophysics.
The story of this puzzle begins with a green spectral line observed during an 1869 total solar eclipse. Scientists initially thought they discovered a new element called coronium, though a Swedish physicist later identified it as iron ionized 13 times. Understanding these solar extremes, alongside the chondrule-rich composition of the earliest outer worlds, demonstrates how chemical and physical data continue to provide information about the solar system.