A meteorite that fell to earth half a century ago in Australia contains particles of stardust that formed between five and seven billion years ago making it the oldest known solid material on Earth. [Heck, P.R. et al., Proc Natl Acad Sci (2020); DOI: 10.1073/pnas.1904573117]

"This is one of the most exciting studies I've worked on," explains Philipp Heck of the University of Chicago, USA. "These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy." The pre-solar grains found in this meteorite are particles that may have been expelled from a dying star many billions of years ago that were perhaps always destined to ultimately coalesce with other particles in space. In this case they found themselves on our planet and have effectively been unchanged for billions of years, representing a picture of minerals that existed in our part of the Milky Way galaxy before the solar system had formed. Such pre-solar grains are rare, found only in one in twenty meteorites.

Graduate student Jennika Greer explains the process of investigating these grains from the so-called Murchison meteorite. "It starts with crushing fragments of the meteorite down into a powder," she explains. "Once all the pieces are segregated, it's a kind of paste and it has a pungent characteristic it smells like rotten peanut butter." The paste is then dissolved with acid to release the pre-solar grains from the surrounding mineral matrix.

"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," explains Heck. "Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form."

The pre-solar grains are formed when a star dies, so they can tell us a lot about stellar history. Around seven billion years ago, there was apparently a bumper crop of new stars forming. "We have more young grains that we expected," adds Heck. "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."

The discovery could settle an astronomical controversy. "Some people think that the star formation rate of the galaxy is constant," explains Heck. "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."