Presolar grains — solid pieces of stardust — are unraveling a hidden history of the solar system.
What if you could hold a piece of a star in your hands?
The hypothetical evokes everything from the cartoonish collectable stars of a Nintendo game, to the ethereal divinities that handle celestial objects in mythology.
But for scientists who study presolar grains, which are tiny specks of solidified stardust, it’s a normal part of everyday life. In recent years, laboratories have received a wealth of new grains from pristine asteroid samples delivered to Earth by a series of space missions. These grains, along with the stardust preserved in meteorites that fall on our planet, are the last corporeal remnants of our stellar progenitors, bearing the fruits of their nucleosynthetic labor.
Presolar grains are the “only physical samples we have of stars,” Sheri Singerling, a planetary scientist at Goethe University Frankfurt who studies these grains, tells Supercluster.“ They bring ground truth to the other fields of astrophysics that either simulate these environments (modeling or laboratory work) or look at them from afar (observations with telescopes).”
“Time and time again, these grains have contained information that theory and observations did not predict,” she noted. “They are little treasure-troves of new insights just waiting to be studied.”
What are Presolar Grains?
Presolar grains are solid minerals, on the scale of nanometers to micrometers, that crystallized in the gassy outflows and ejecta of stars that existed before the Sun was born 4.6 billion years ago. Only senescent stars make presolar grains; main-sequence stars in the prime of their lives, like the Sun, do not shed enough gas into space to produce solid stardust.
Presolar grains can be made by a variety of different dying stars, from those like the Sun to colossal behemoths many times larger. After their parent stars have collapsed, these grains drift through the interstellar medium inside clouds of star-forged atomic gasses, famously described by Carl Sagan as “starstuff,” from which our solar system, and our own bodies, are built.
“All these grains from all these different stars basically fly around and get mixed up in the interstellar medium, they end up being incorporated into molecular clouds,” said Larry Nittler, a cosmochemist and self-described 'Interstellar Dust Buster' at Arizona State University.
“The cloud is a mixture of gas and a whole lot of dust that's formed from lots and lots and lots of stars, over a period of maybe 100 million to 500 million years,” he added, noting that the uncertainty in the time frame is because “we don't really know how long dust grains can survive in the interstellar medium.”
Because the Sun and planets condensed from these clouds billions of years ago, presolar grains were baked into the elemental mix of the solar system. Scientists have cataloged thousands of grain varieties — containing silicon carbides, diamonds, graphite, and more — in the Presolar Grain Database.
By studying their compositions, scientists can match the origin of these grains to various types of stars.
“We can determine the type of star that the grains of stardust come from by measuring the composition of different isotopes in the lab, and then comparing these isotopic compositions to spectroscopic observations of stars and to astrophysical models of stellar nucleosynthesis,” Ann Nguyen, a planetary scientist at NASA Johnson Space Center who specializes in presolar grains, tells Supercluster.
“We know our solar system was seeded by material from oxygen-rich and from carbon-rich red giants, asymptotic giant branch stars, supernovae, and novae,” she added. “There are likely other types of stars as well.”
In one stunning example, a team discovered that presolar grains from the Murchison meteorite, which fell in Australia in 1969, belonged to a group of massive stars that perished a few hundred million years before the birth of the Sun, according to a 2020 paper. These ancient crystals are the oldest material ever handled by humans.
Where are Presolar Grains Found?
At the dawn of the space age, presolar grains were unheard-of, even as a hypothetical. But hints of their existence started emerging in the 1960s during laboratory experiments on ancient meteorites. The space rocks contained weird isoptic radios of the noble gasses neon and xenon that could not be explained by contemporaneous models of solar system formation.
These findings prompted scientists, including John Reynolds and Donald Clayton, to propose the existence of presolar grains. The prediction was eventually confirmed experimentally by the chemist Edward Anders in the 1980s using an ion probe that could resolve the isotopic composition of the grains in unprecedented detail.
“They discovered that it’s not just the noble gasses, but that all the elements were just isotopically wacky,” explained Nittler, who said he felt lucky to be on the “ground floor” of the field as he began his graduate research in the 1990s. “It was the doggedness of Ed Anders to try and do this, combined with technical maturity. This whole field has continuously grown along with advances in technology. They couldn’t have been found sooner because we didn't have the technology to analyze them.”
At that time, scientists were reliant on meteorites as their source of presolar grains.
Though some stardust had been baked into Earth and other planets at the dawn of the solar system, the grains have survived in much higher concentrations in more primitive and unaltered bodies, like asteroids.
“Most material in our solar system has been homogenized,” explained Singerling. “This is just a fancy way of saying it all kind of blended together and balanced out to have roughly similar chemistries. Things like heating or interactions with fluids tend to ‘erase’ the chemical signatures that tell us a grain is presolar. We look for presolar grains in certain types of meteorites, what we call primitive meteorites, because these meteorites have not changed much since the solar system formed.”
In the 21st century, however, scientists who study presolar grains have benefitted from another technological leap — sample-return missions. Japan’s Aerospace Exploration Agency (JAXA) has launched a pair of spacecraft, named Hayabusa 1 and 2, that collected a few grams of samples from the surface of the asteroids Itokawa and Ryugu, and returned them to Earth in, respectively, 2010 and 2020. NASA’s OSIRIS-REx, following those successes, hauled more than four ounces of samples back from asteroid Bennu, which arrived on Earth in September 2023. These missions have expanded both the number and varieties of presolar grains available for research.
“Sample-return has been such a huge boon for planetary science,” said Singerling. “That being said, we are still in the early days when it comes to studying these samples.”
What Can We Learn from Presolar Grains?
Presolar grains sit at the nexus of multiple different fields like astrophysics, planetary science, mineralogy, and cosmochemistry, to name a few. They open direct windows into the bellies of bygone stars, the environment of interstellar space, the origin of the solar system, and the evolution of specific bodies within it.
With that in mind, it’s no surprise that these grains can shed light on a host of open questions that touch on phenomena ranging from atomic to galactic scales. Variations in the composition of grains can also reveal details about where their parent asteroids formed, how much they’ve moved over time, and how processes like collisions or exposure to water have shaped them.
“Studying stardust in different samples can inform us on the degree of heating and hydration that a parent asteroid experienced,” Nyugen said. “We are seeing some differences in the populations of presolar grains, both in terms of stellar sources and abundances, found in meteorites, Ryugu, and Bennu.”
“For instance we found some small pockets of material in Ryugu that are less altered and have extremely high abundances of a specific type of presolar grain,” she continued. “Along with other traits, these ‘clasts’ likely came from a region of the solar system that isn’t represented by other materials that we have for study. These clasts could have been incorporated onto Ryugu when it was forming. We are seeing some interesting differences in the Bennu sample, but we’ve just scratched the surface. We’re still processing the data and conducting more analyses of different stone fragments from Bennu.”
In addition to serving as probes of asteroid evolution, presolar grains can illuminate conditions surrounding the birth of the solar system. The Sun likely had older siblings that formed from the same molecular cloud; the deaths of stars may have triggered the birth of the Sun and planets, or at the very least coincided with it. It’s possible that grains from these massive stellar siblings might be identified in future studies.
“The first generation of massive stars in a molecular cloud, which may be forming right before the Sun, can reach their end of their life, explode, and put new dust, and newly synthesized stuff in the supernova, back into the cloud,” Nittler said. “One of the big questions we have had for years, and are still trying to figure out, is: do we have evidence for that in presolar grains?”
“The Occam’s razor is that all the grains are from just a huge number of stars throughout history,” he continued. “But it would be cool to see if we can have evidence for, say, one or two specific supernovae that went on right before the birth of the solar system. Is there some concentration of supernova grains of one type or another? There's some isoptic clues for it, but it's still pretty ambiguous.”
For Singerling, another compelling mystery is how presolar grains even survive in interstellar space long enough to be integrated into new star systems, like our own.
“It might seem like the environment between stars, what we call the interstellar medium (ISM), is empty and, as a consequence, calm,” she said. “However, it is far from that. There are many destructive processes taking place in the ISM, such as irradiation from high energy particles, turbulent motions from supernovae shock waves, and collisions between particles to name a few. All of these processes should destroy presolar grains or at least change their structures quite dramatically. That's what the theoretical calculations imply.”
“And yet we find them, and some of them are quite large and intact,” she added. “There are theories on why this might be, but it is still an open question. It is one I hope can be resolved a bit more in the coming years. As for how, it comes down to a shear numbers game. We need more people studying more grains and collaborating with astrophysicists. The instruments we use to study these grains are expensive, and finding funding for the research itself can be challenging.”
These are just some of the kaleidoscopic insights locked inside presolar grains that animate the interdisciplinary community of experts who study them. In this field, which marries the spectral nature of stars with hard geochemical analysis, it’s possible to not only peek into the stars of the past, but to actually reach out and touch them.
“For the last 30 years, I feel so privileged to have been part of this,” said Nittler. “It blows my mind. Every time I find a new grain, I get excited. Even though I've been doing it for so long, it’s like: ‘This was once a piece of a star!’”