7bn-year-old stardust found inside a meteorite is the oldest solid material on Earth

“We are stardust, billion year old carbon. We are golden, caught in the devil’s bargain. And we’ve got to get ourselves back to the garden.”

TL;DR

An international team of scientists has discovered stardust grains within the Murchison meteorite that are over 7 billion years old, making them the oldest solid materials found on Earth. These grains, composed of presolar silicon carbide, are older than the Sun and Earth, and were dated by analyzing their exposure to cosmic rays. The discovery suggests a peak in star formation around 7 billion years ago, a “baby boom” of new stars that left behind stardust. The finding adds a critical piece to our understanding of the galaxy’s history and star formation rates.

________________________

An international team of scientists discovered a 7 billion-year-old grain of stardust within a meteorite, which is older than our solar system. According to the researchers, this is the oldest solid material found on Earth. It is even older than our planet and the sun, which are 4.5 and 4.6 billion years old, respectively.

And this stardust has an Australian connection. It was retrieved from the Murchison meteorite, which hit the ground in the Victorian small town of Murchison in 1969.

“This meteorite is really a treasure trove for science,” said Philipp Heck, cosmochemist at the Field Museum in Chicago and lead author of the paper published today in the Proceedings of the National Academy of Sciences.

How do you date Stardust?

First and foremost, remove the stardust from the meteorite. The stardust is composed of grains of presolar silicon carbide, a material that formed before our solar system was born. “The presolar grains are a tiny fraction of meteorites like Murchison, only parts per million,” Dr. Heck claimed.

“We utilize a chemical mixture containing several chemical reagents, primarily acids, to dissolve everything else before extracting the minerals. “We image them, we analyse their isotopic composition to find out what type of star they came from, and then we can start our study … to find out how old they are.”

The carbon-12 to carbon-13 isotope ratios in these grains matched exactly what astronomers had found in clouds of dust and gas surrounding ageing stars such as the Egg Nebula and the Ring Nebula.

However, Dr Heck explained that typical dating procedures used by geochemists on Earth do not work for dating stardust. Instead, the researchers analyzed how long the grains were exposed to cosmic rays traveling around the universe.

When a cosmic ray—a stream of high-energy particles, primarily protons and alpha particles—penetrates a presolar grain, one of its carbon atoms is occasionally torn into fragments. Scientists can determine the age of the stardust by counting all of the fragments created by cosmic rays and knowing how frequently they occur.

A scanning electron micrograph of a dated presolar silicon carbide grain. The grain is approximately eight micrometres wide. “Each grain probably came from a different star,” Dr. Heck explained.

“Some grains are 5.5 billion years old, making them a billion years older than the Sun. There are a few grains that are two billion years older, and one grain that is three billion years older [than the Sun].

“I’m pretty sure than Murchison contains older minerals, we just haven’t found them yet.” The cosmic ray method doesn’t give us an absolute age of the stardust, said planetary scientist and meteorite hunter Phil Bland of Curtin University, who wasn’t involved in the research.

But it’s our best approximation so far.

An astronomical baby boom

The ages of these grains of stardust are significant not just for their own sake, but also for what they reveal about our galaxy’s evolutionary history. While some astronomical models assume stars originate at a constant pace, Dr Heck and his colleagues’ research demonstrates that this is not the case.

“In order to explain our age distribution, where we have many more younger grains that we would otherwise expect, we have to explain this by this dust forming from more stars than normal,” he claimed.

“We came to the conclusion that about 7 billion years ago there must have been an episode of enhanced star formation, probably about 50 per cent more stars formed than normal.” In other words, a celebrity-making baby boom. Professor Bland described the discovery as intriguing.

“That peak in star formation about 7 billion years ago … is really interesting because there’s a target that other people can chase,” Prof. Bland said. Now he says we need more data from a different meteorite.

“What I would do now is pick another sample and go through the same process and say ‘Okay, do we find that same distribution of stuff or is there something weird about this one?’.” For Dr. Heck, this latest study is one of the most interesting he’s ever worked on, but he’s confident it won’t be the last.

“Murchison is so important, and so rich and full of surprises,” he told me. “I really want to emphasise that without the foresight of the people of Murchison we wouldn’t have access to this wonderful sample … so this has really made a big, big difference.”