Ancient stardust became the seeds of planetary solids

New research suggests leftover grains from earlier stars acted as nucleation points, solving a long-standing puzzle about how the solar system’s first solids formed so quickly. The Allende meteorite fell over Mexico in 1969 and remains the largest known primitive meteorite. Its fragments preserve chemical records from the solar system’s earliest days and provided the samples in which Caltech researchers detected ancient stardust that helped seed the first solids. (Wikimedia commons via CNS) (CN) — Microscopic grains of ancient stardust, leftovers from stars that lived and died long before our sun existed, likely acted as the seeds for the earliest solid materials in the solar system, new research from Caltech shows. In the same way snowflakes form around dust particles in Earth’s atmosphere, these presolar grains appear to have provided the starting surfaces around which the solar system’s first solids crystallized more than 4.5 billion years ago. The findings, published July 29 in Science Advances , come from detailed analysis of fragments of the Allende meteorite, a primitive space rock that fell over Mexico in 1969. Meteorites are like fossils from the early solar system, preserving a record of what our astronomical neighborhood looked like at the time of its formation 4.5 billion years ago. Allende, the largest known primitive meteorite, has long been a treasure trove for scientists studying that distant era. Its carbon-rich matrix has previously yielded tiny mineral grains with chemical signatures completely unlike anything formed in our solar system, evidence they originated in earlier generations of stars. Until now, those exotic presolar grains had only been found in cooler parts of meteorites. The new study, led by former Caltech graduate student Ren Marquez and geochemist Francois Tissot, detected their chemical fingerprints inside calcium-aluminum-rich inclusions, or CAIs. These refractory objects are widely regarded as the first solids to condense out of the hot, gaseous disk that surrounded the newborn sun. “In general, the Earth and meteorite samples are quite similar, with only very tiny chemical differences,” Marquez explained in a news release. “Detailed studies of these anomalous grains in primitive meteorites revealed signatures that are so wildly different that the only way to explain them is that they came from a different generation of stars that preceded our sun. This was the first dramatic evidence that showed that the solar system may not be as homogeneous as we thought." The researchers conclude that some of the ancient stardust somehow survived the intense heat of the early solar system and served as nucleation points. Without such seeds, minerals would have taken far longer to form as the disk cooled. The presolar grains, though scarce, appear to have provided the crucial substrate that allowed the rest of the CAI material to grow around them. “Nucleation is a very difficult process if there is no surface upon which to grow,” Tissot said. Presolar grains acting as seeds “solves an otherwise unaddressed problem in cosmochemistry.” The work builds on earlier hints that CAIs contained presolar material and strengthens the picture of a solar system that was never a perfectly uniform “soup” of gas and dust. Instead, it incorporated and was shaped by debris from previous stars. Beyond rewriting cosmic history, the high-precision techniques developed for the study may also find practical uses, including more sensitive analysis of tiny biomedical samples. As Tissot noted, fundamental discoveries often lead to unexpected applications: “We didn’t discover electricity by studying the candle.” Our weekly newsletter Closing Arguments offers the latest about ongoing trials, major litigation and rulings in courthouses around the U.S. and the world, while the monthly Under the Lights dishes the legal dirt from Hollywood, sports, Big Tech and the arts.