Fifty years ago, a fireball streaked across the skies of Victoria, Australia, scattered into three fragments, and crash-landed, spraying fragments over 13 square kilometers (5 square miles). Now, scientists have discovered stardust trapped inside the meteorite, time-stamping the interstellar grains to 5-7 billion years ago.
The grains of stardust were trapped inside meteorites long ago—even before the sun formed—where they remained unchanged for billions of years, until one such meteorite fell 50 years ago in Australia. These “time capsules” offer clues about what was going on in our patch of the universe before the sun formed; for example, the grains suggest a surprising boom in star formation.
“This is one of the most exciting studies I’ve worked on,” said Philipp Heck, associate professor at the University of Chicago, curator at the Field Museum and lead author of a paper published Jan. 13 in the Proceedings of the National Academy of Sciences. “These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy.”
Though they may seem fixed to humans, stars have life cycles. They’re born when bits of dust and gas floating through space find each other and heat up; they burn for millions to billions of years, and then they die. When they die, they pitch the particles that formed in their winds out into space, and those bits of stardust eventually form new stars, along with new planets and moons—as well as meteorites.
But such “presolar” grains are hard to come by. They’re rare—found only in about 5% of meteorites that have fallen to Earth, and they’re tiny—a hundred of the biggest ones would fit on the period at the end of this sentence. But the Field Museum has the largest portion of the Murchison meteorite, a treasure trove of presolar grains that fell in Australia in 1969 and that the people of Murchison, Victoria made available to science. Presolar grains for this study were isolated from the Murchison meteorite for this study about 30 years ago at the University of Chicago.
“It starts with crushing fragments of the meteorite down into a powder,” said Jennika Greer, a graduate student at the Field Museum and the University of Chicago and co-author of the study. “Once all the pieces are segregated, it’s a kind of paste, and it has a pungent characteristic—it smells like rotten peanut butter.”
This “rotten-peanut-butter-meteorite paste” was then dissolved with acid, until only the presolar grains remained.
Once the presolar grains were isolated, the researchers used a rigorous process based on cosmic rays to figure out from what types of stars they came and how old they were.
“We used exposure age data, which basically measures their exposure to cosmic rays, which are high-energy particles that fly through our galaxy and penetrate solid matter,” said Heck, whose research focuses on pioneering new ways to understand astrophysical questions by studying meteorites. “Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form.” By measuring how many of these new cosmic ray-produced elements are present in a presolar grain, we can tell how long it was exposed to cosmic rays, which tells us how old it is.
The researchers learned that some of the presolar grains in their sample were the oldest ever discovered—based on how many cosmic rays they’d soaked up, most of the grains had to be 4.6 to 4.9 billion years old, and some grains were even older than 5.5 billion years. (For context, our sun is 4.6 billion years old, and the Earth is 4.5 billion.)
But the age of the presolar grains wasn’t the end of the discovery. Since presolar grains are formed when a star dies, they can tell us about the history of stars. And 7 billion years ago, there was apparently a bumper crop of new stars forming—a sort of astral baby boom.
“We have more young grains that we expected,” Heck said. “Our hypothesis is that the majority of those grains, which are 4.9 to 4.6 billion years old, formed in an episode of enhanced star formation. There was a time before the start of the solar system when more stars formed than normal.”
This finding is fresh evidence in a debate between scientists about whether or not new stars form at a steady rate, or if there are highs and lows in the number of new stars over time. “Some people think that the star formation rate of the galaxy is constant,” Heck said. “But thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy seven billion years ago with samples from meteorites. This is one of the key findings of our study.”
As almost a side note to the main research questions, the researchers also learned that presolar grains often float through space stuck together in large clusters, “like granola,” Heck said. “No one thought this was possible at that scale.”
Heck and his colleagues look forward to all of these discoveries furthering our knowledge of our galaxy. “With this study, we have directly determined the lifetimes of stardust. We hope this will be picked up and studied so that people can use this as input for models of the whole galactic life cycle,” he said.
“It’s the next best thing to being able to take a sample directly from a star,” Greer said. “Once learning about this, how do you want to study anything else? It’s awesome; it’s the most interesting thing in the world.”