A meteorite that fell near the Australian town of Murchison in 1969 was found to contain grains of stardust up to around 7 billion years old — the oldest solid material ever identified on Earth, formed long before the Sun or planets existed.

On 28 September 1969, a meteorite broke up in the sky over Murchison, a small town in Victoria, Australia. Fragments fell across the surrounding farmland, and local people collected them and made them available to science. The total recovered came to roughly 100 kilograms.

The Murchison meteorite has been one of the most studied meteorites in the world ever since, partly because so much of it was recovered and partly because of what it contains. It is a carbonaceous chondrite, a primitive type of meteorite rich in carbon compounds, and it carries, among other things, amino acids and other organic molecules. But the finding that concerns this piece is a different one. Sealed inside Murchison are tiny grains of material that are older than the Sun.

What a presolar grain is

The grains are called presolar grains, and the name is exact. They are bits of solid matter that formed before the solar system did.

Stars produce dust. Toward the end of a star’s life, as it sheds material into space, solid grains condense in the outflowing gas, in this case grains of silicon carbide, a hard compound of silicon and carbon. That dust drifts through the galaxy. Some of it, eventually, gets swept into the cloud of gas and dust that collapses to form a new star and its planets.

Most such grains do not survive that process. The heat and chemistry of forming a new solar system destroys them, and they are reprocessed into new material. A presolar grain is one of the rare survivors: a grain that formed around some earlier star, drifted through the galaxy, was caught up in the formation of our solar system, and was never melted down. It came through intact, carrying its original composition, and ended up locked inside an asteroid. A piece of that asteroid is what fell on Murchison.

These grains are not common. They are found in only a small fraction of meteorites, and even in those they make up only a few parts per million of the material. They are also extremely small. The grains in the Murchison study were measured in micrometres.

How the ages were measured

In January 2020, a team led by Philipp Heck of the Field Museum and the University of Chicago published a study in the journal PNAS that dated a set of these grains. The grains themselves had been extracted from Murchison fragments at the University of Chicago about three decades earlier, and kept until methods improved enough to date them well.

The dating method does not measure the grains the way a rock on Earth is dated. It uses cosmic ray exposure. While a presolar grain drifts through space, it is constantly struck by high-energy cosmic ray particles. Some of those collisions produce new atoms inside the grain. Those products accumulate at a roughly known rate, so measuring how much has built up gives an estimate of how long the grain was exposed, drifting in space, before it was sealed inside the parent asteroid. The longer the exposure, the longer the grain had likely spent drifting through interstellar space.

It is worth being clear that this is an exposure age, and it carries uncertainty. It is a well-grounded estimate, not a precise reading, and the study presented its ages with error bars rather than as exact figures.

What the numbers actually were

The headline figure from the study, the one that travelled, was about 7 billion years. That number is real, and it is correctly described as the oldest solid material identified on Earth. But it is worth being precise about it.

Seven billion years was the upper end. The study dated about 40 grains, and they did not all come back that old. According to the research, the majority, roughly 60 per cent, were younger than about 4.9 billion years, predating the solar system by 300 million years or less. Only a small fraction, around 8 per cent, came back older than about 5.5 billion years. The oldest few grains were the ones that reached toward 7 billion years.

So the accurate version is this. Most of the presolar grains in the study were older than the 4.6-billion-year-old Sun, but not dramatically older. A few were much older. And the very oldest, with the uncertainty in the measurement included, sat at around 7 billion years. For comparison, the universe itself is about 13.8 billion years old, so the oldest Murchison grains formed when the universe was roughly half its current age.

What the grains tell us

The ages were not the only result. The distribution of ages was itself informative.

The Heck team found that a large share of the grains clustered in age, as though many of them had formed in the same era. They proposed that this points to a burst of star formation in the Milky Way several billion years ago, a period when an unusual number of stars were born, lived their cycles, and produced dust. A cluster of grains of similar age is consistent with a cluster of stars having formed, aged, and shed material together. That interpretation is an inference from the age distribution, not a direct observation, but it shows the kind of thing presolar grains can be used for. They are not only old. They are a physical sample of galactic history.

What Murchison offers, in the end, is a matter of holding something genuinely older than the planet it landed on. The rock formed in the early solar system. The grains inside it formed before there was a solar system at all, around stars that died before the Sun existed. A meteorite fall near an Australian dairy town in 1969 turned out to have delivered, intact, some of the oldest solid matter anyone has ever been able to put under a microscope.