7-billion-year-old stardust in meteorite that crashed 50 years ago is the oldest material ever found on Earth
The study shows proof contrary to what scientists believe that the rate of star formation is constant. It is direct evidence that there was a time before the start of the solar system when more stars formed than normal.
In a meteorite that fell 50 years ago in Australia, scientists have now discovered stardust that formed five to seven billion years ago, the oldest solid material ever found on Earth. For context, our Sun is 4.6 billion years old, and the Earth is 4.5 billion.
The materials examined by the research team from the Field Museum of Natural History, Chicago, and the University of Chicago, among others, are called presolar grains — minerals formed before the Sun was born. These are solid samples of stars, real stardust, explains the team. These bits of stardust became trapped in meteorites where they remained unchanged for billions of years, "making them time capsules of the time before the Solar System."
According to the researchers, interstellar dust is an important component of our galaxy. It influences star formation as well as the thermal and chemical evolution of the galaxy.
"This is one of the most exciting studies I've worked on. These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy," says lead author of the study Philipp Heck, a curator at the Field Museum, and associate professor at the University of Chicago.
Although dust only presents approximately 1% of the mass in the interstellar medium, it carries a large fraction of the elements heavier than helium, including the elements that form terrestrial planets and are essential for life. Thus, interstellar dust is a key ingredient of stars and habitable planetary systems, making increased knowledge about its composition and lifecycle desirable, says the researchers in their findings.
“It's so exciting to look at the history of our galaxy. Stardust is the oldest material to reach Earth, and from it, we can learn about our parent stars, the origin of the carbon in our bodies, the origin of the oxygen we breathe. With stardust, we can trace that material back to the time before the Sun. It's the next best thing to being able to take a sample directly from a star," says study co-author Jennika Greer, a graduate student at the Field Museum and the University of Chicago.
The researchers explain that stars have life cycles: they are born when bits of dust and gas floating through space find each other and collapse in on 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 and meteorites.
However, presolar grains are tiny and hard to come by. They are found only in about five percent of meteorites that have fallen to Earth.
Field Museum has the largest portion of the Murchison meteorite, a treasure trove of presolar grains that fell in Australia in 1969 and was made available for science. Presolar grains for this study were isolated from the Murchison meteorite about 30 years ago at the University of Chicago.
The team analyzed 40 grains of silicon carbide, incorporated into the Murchison CM2 meteorite 4.6 billion years ago, for "neon isotopes produced by interactions with cosmic rays." The rates of formation of such cosmogenic isotopes can be used to date the particles.
“We determined interstellar cosmic ray exposure ages of 40 large presolar silicon carbide grains extracted from the Murchison CM2 meteorite,” says the study published in the Proceedings of the National Academy of Sciences (PNAS).
Once the presolar grains were isolated, the researchers figured out from what types of stars they came and how old they were.
The researchers learned that some of the presolar grains in their sample were the oldest ever discovered. Based on how many “cosmic rays they had soaked up,” most of the grains had to be 4.6-4.9 billion years old, and some grains were even older than 5.5 billion years.
According to the experts, “cosmogenic neon dating” used in the study is a viable method for obtaining galactic information prior to the formation of the Sun.
But the age of the presolar grains was not the end of the discovery. Since presolar grains are formed when a star dies, they can tell us about the history of stars. And seven billion years ago, there was apparently a bumper crop of new stars forming-a sort of "astral baby boom," say the researchers.
There is 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 experts think that the star formation rate of the galaxy is constant. However, thanks to these grains, scientists now have direct evidence for a period of enhanced star formation in the galaxy seven billion years ago with samples from meteorites. According to Heck, this is one of the key findings of the study.
“We have more young grains that we expected. 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," says Heck.
The researchers also learned that presolar grains often float through space stuck together in large clusters, "like granola," says Heck. According to Heck, “no one thought this was possible at that scale."
The analysis shows that 60% of the particles returned ages of less than 300 million years before the formation of the Solar System, compatible with estimates of interstellar lifetimes of 100-200 million years.
“However, particles with ages more than 1 billion years before the formation of the Solar System suggest that these grains were shielded in dense clumps that helped the particles survive supernova shockwaves in the interstellar medium. Some of the grains' histories may have included aggregates, including mantles of ices or organic compounds,” says the study.
The research team believes that the study findings will further our knowledge of the 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," says Heck.