- In 1969, a 4.6-billion-year-old meteorite struck Murchison, Australia.
- The meteorite contained fragments of stardust called presolar grains.
- This stardust is between between 5 billion and 7 billion years old – older than the sun and our solar system.
- Most of the grains in the Murchison meteorite came from various stars that formed around the same time. This suggests stars are born in bursts, rather than at a constant rate.
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Meteorite fragments found in Australia appear to have brought rare, interstellar passengers to Earth: pieces of stardust older than the sun.
The space rock broke up in the atmosphere above Australia on September 28, 1969, and scientists later collected pieces scattered across the town of Murchison in the state of Victoria.
These fragments, called presolar grains, are incredibly tiny – about 100 would fit on the period at the end of this sentence. But they offer astronomers insight into how stars formed in the early stages of our galaxy.
For a new study published in the journal Proceedings of the National Academy of Sciences, scientists analysed presolar grains from one of these meteorite fragments.
They discovered that the grains are the oldest solid material ever found on Earth. They’re between 5 billion and 7 billion years old.
“They’re solid samples of stars – real stardust,” Philipp Heck, the lead author of the study, said in a press release.
The grains were incorporated into the ancient meteorite (which was later named Murchison after the spot it touched down) 4.6 billion years ago, during the time when the solar system was forming, Heck told Business Insider.
Stars form when clouds of dust collapse and heat up. They burn for billions of years then eventually collapse and cool, scattering some of their ingredients into space. Over time, the freed stardust particles condense to form new stars and planets, or – in the case of these particular grains – meteorites.
Each individual grain of stardust has a chemical composition unique to the parent star it came from. So measuring the carbon and silicon isotopes each grain contains allowed Heck’s team to determine what type of star it came from.
The group looked at 40 grains made of a mineral called silicon carbide, which “is almost as hard as diamond and can survive for billions of years unchanged,” Heck said.
The researchers were able to determine the grains’ ages based on their chemical composition, since that reveals how long a grain was exposed to cosmic rays over the course of its lifetime. These rays are high-energy particles that zip through space at the speed of light and pierce solid matter.
“Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form,” Heck said in the press release.
His team discovered that about 60% of the grains are between 4.6 and 4.9 billion years old, while some other grains are even older: 5.5 billion years or more. Our sun, by comparison, is 4.6 billion years old, and Earth is 4.5 billion years old.
That makes this stardust is the oldest material from the solar system ever found on Earth. It beats the oldest rocks on Earth, which were previously considered the most ancient material: zircon crystals discovered in Australia in 2014. Those are only 4.4 billion years old.
Previous studies have noted that similar presolar grains have ages ranging from 5 billion to 4.6 billion years old, but the upper age limit of this Murchison stardust takes the the cake.
That makes these presolar grains useful pieces of evidence in the study of our galactic history.
Diamonds in the rough
Meteorites containing stardust are incredibly rare. Heck said about 70,000 meteorites are currently known to science, and of those, “at most 5% of these contain presolar grains.”
“Only a few meteorites are as large as Murchison and as rich in grains as Murchison,” he said.
The microscopic size of these grains – the biggest is only 30 micrometres across – make them challenging to study, however.
“It starts with crushing fragments of the meteorite down into a powder,” Jennika Greer, a co-author of the study, said in a press release. Then the crushed meteorite gets dissolved with acid until only the presolar grains remain.
“It’s like burning down the haystack to find the needle,” Heck said in the release.
Tens of thousands of presolar grains were extracted from the Murchison meteorite, but only 40 were large enough to be dated, he added.
Presolar grains tell us how and when stars formed in the early universe
Since the majority of the 40 grains Heck’s team studied were around the same age, his team hypothesised that most came from stars that formed in one big burst.
Astronomers have long debated whether new stars form at steady rate or in accelerated bursts. The new study is evidence of the latter.
“Thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy 7 billion years ago,” Heck said in the release.
In other words, he added, “there was a time before the start of the solar system when more stars formed than normal.”
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