Ancient stardust grains were seeds for the earliest solids in the solar system
How did the first solids that made up planets, moons and asteroids form from the "hot soup" that characterized our solar system's earliest years? New research from Caltech analyzing pieces of a meteorite has found that, just as snowflakes crystallize around grains of dust, ancient stardust grains left over from long-gone suns may have been the seeds of our solar system's earliest solids.
The research was conducted in the laboratory of Francois Tissot, professor of geochemistry and Heritage Medical Research Institute Investigator, and appears in the journal Science Advances on July 29.
Allende's record of the early solar system
In early 1969, just months before Apollo astronauts would return the first rock samples from the moon, the Allende meteorite blazed through Earth's atmosphere and broke apart, scattering more than two tons (1.8 tonnes) of fragments over Mexico.
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 is, to date, the largest primitive meteorite found on Earth, and its many fragments contain a wealth of information that has helped scientists—such as Caltech researchers Gerald Wasserburg and Dimitri Papanastassiou (BS '65, PhD '70)—understand the formation and earliest history of the solar system.
Stardust where it wasn't expected
In the 1980s, scientists at the University of Chicago studying similarly primitive meteorites discovered tiny nanodiamond grains that had chemical compositions totally different from anything in our solar system. These minerals, scientists concluded, were the remnants of an ancient star that existed and died before our own sun was born.
"In general, the Earth and meteorite samples are quite similar, with only very tiny chemical differences," says Ren Marquez (PhD '24), a former graduate student in Tissot's laboratory and first author on the study. "This led researchers to initially theorize that the early solar system was a giant homogeneous soup of gases, with everything having formed from the same starting materials. But 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 the solar system may not be as homogeneous as we thought."
Since this discovery in the 1980s, scientists have uncovered a variety of other types of presolar grains. All these grains were found in one place: the carbon-rich matrix within primitive meteorites, regions that formed under cooler conditions. No presolar grains had ever been found in components formed in the solar system's hotter regions—until now.
Seeds for the first solids
After several years of developing new techniques with unprecedented precision, Marquez analyzed tiny fractions of the Allende meteorite that formed early in the solar system's history, when it was still quite hot. These components, called calcium-aluminum-rich inclusions or CAIs, were the very first solids to condense from the hot environment of the early solar system. In a previous paper, Marquez and collaborators discovered signatures hinting that CAIs also contained presolar stardust.
The new study confirmed these earlier findings. Significantly, the work also shows that these presolar grains potentially shaped the makeup of the solar system material found alongside them. The researchers theorize that these fragments from earlier stars somehow survived intact through our own system's hot early years and acted as nucleation points for the solar system's first-formed solids. The grains, while not abundant, seem to have been a crucial structural substrate around which the rest of the CAIs coalesced.
Many kinds of substances are known to precipitate through an initial seed, from proteins to magma to snowflakes.
"Nucleation is a very difficult process if there is no surface upon which to grow," Tissot explains. "Without the presolar dust grains disrupting an otherwise homogeneous mix of gases, minerals should take a long time to condense as the solar system cooled. Presolar grains acting as seeds for this early condensation solve an otherwise unaddressed problem in cosmochemistry."
From meteorites to medical tools
In future work, the researchers aim to determine the exact chemical compositions of the presolar grains.
Tissot emphasizes that fundamental science, such as this study, can also lead to unexpected benefits to society. The new techniques developed by Marquez are not only useful for studying fragments of ancient stardust but can also be applied to biomedical research for studying tiny samples of blood and tissue. Indeed, Tissot's lab is currently engaged in a project that is applying similar techniques to improve detection of osteoporosis.
"There can be a tension between funding fundamental research and applied research with an immediate, obvious benefit, but we rarely know where the next most important technology comes from," Tissot says. "As my grandfather used to say, 'We didn't discover electricity by studying the candle.'"
Publication details
Ren T. C. Marquez et al, Stardust as nucleation seeds for the earliest solids in the Solar System, Science Advances (2026). DOI: 10.1126/sciadv.ady2311
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