The grain, no wider than a bacterium, was extracted from a rock that fell on a Victorian dairy farm in 1969. Its age, published in January 2020 by a team led by Field Museum cosmochemist Philipp Heck, pushes the known limit of solid matter on Earth back by more than two billion years and offers the first physical evidence that star formation in our galactic neighbourhood occurred in bursts rather than at a steady rate.
The Fall That Saved the Science
At 10:58 on the morning of 28 September 1969, residents of Murchison, a farming town roughly two hours north of Melbourne, heard explosions in a clear sky. A fireball split into fragments, left a smoke trail that lingered for at least two minutes, and scattered stones across a strewn field measuring at least 11 kilometres by 3 kilometres. A large fragment punched through the roof of a hay shed. The smell that followed was sharp and alcoholic, faintly like methylated spirits.
Two brothers, Peter and Kim Gillick, aged 10 and 11, recovered roughly a third of the total mass by walking the paddocks. The next day a farmer named Arnold Brisbane carried samples to his local newspaper, which contacted the University of Melbourne. That single phone call sent the stones to a laboratory while they were still fresh.
The timing was almost absurdly lucky. Clean laboratories had just been assembled across the United States in anticipation of the Apollo lunar samples. A contaminant-free environment was waiting for an organic-rich meteorite nobody had expected. That accident of scheduling is why Murchison, a rare CM2 carbonaceous chondrite that never melted and never fully recrystallised, became one of the most studied stones in scientific history.
Reading a Cosmic Clock in Neon
The meteorite as a whole is about 4.6 billion years old — roughly the age of the solar system. The record-setting age belongs to something far smaller: individual presolar grains of silicon carbide, most under a micrometre across, buried inside the meteorite’s matrix. Under a scanning electron microscope they look like flecks of soot. Several thousand could fit on the head of a pin.
Their isotope ratios do not match anything the Sun has ever produced. They match the atmospheres of dying stars — specifically asymptotic giant branch stars in their late, bloated stage before shedding their outer layers. When the parent star died, the dust drifted into interstellar space and stayed there.
Cosmic rays provided the clock. High-energy particles pour through the galaxy, and every so often one slams into a grain and chips atoms out of the crystal lattice, producing tiny amounts of new isotopes — most usefully neon-21. The longer a grain floats exposed in interstellar space, the more neon-21 accumulates. Once the grain is swept into a molecular cloud and locked inside a forming solar system, the exposure stops.
Heck’s team, working with collaborators at ETH Zurich, the Australian National University, Washington University in St. Louis, and Lawrence Livermore National Laboratory, measured the neon in 40 large presolar silicon carbide grains. Their paper in PNAS reported exposure ages running from about 3.9 million years to roughly three billion years before the start of the solar system. ETH Zurich’s noble-gas laboratory put the bulk of the sample at 4.6 to 4.9 billion years old, with the oldest material reaching five to seven billion.
Acid, Patience, and Stardust
The procedure is unforgiving. Researchers crush milligrams of Murchison, then bathe the powder in a sequence of acids strong enough to dissolve the silicates, leaving only the toughest material behind. What settles at the bottom of the beaker is, almost literally, stardust — the residue of stars that died before the Sun existed.
The distribution of ages was the real surprise. If stars in this part of the Milky Way had been forming at a steady rate across galactic history, presolar grain ages should be spread evenly. They were not. The pattern pointed to an episode of enhanced star formation around seven billion years ago, in the neighbourhood of what would eventually become the Sun. Stars born in that surge swelled, shed dust into the interstellar medium, and left it drifting until something pushed it into the molecular cloud that collapsed roughly 4.6 billion years ago.
Astronomers had argued for decades about whether star formation in the galaxy is steady or bursty. The Murchison grains handed the burst side of that argument a physical sample to point at — a stone that fell on a dairy paddock in Victoria, carrying dust that recorded the surge directly rather than by inference from starlight.
How Old Is Seven Billion Years?
The comparisons only half help. The universe is approximately 13.8 billion years old, so a seven-billion-year-old grain has existed for roughly half the age of the cosmos. It was already ancient when the cloud that became the Sun had not yet started to collapse. It drifted alone in interstellar space for longer than complex multicellular life has existed on Earth.
For scale, the oldest mineral grains on Earth — the Jack Hills zircons in Western Australia — come in at around 4.4 billion years. The Sun is about 4.6 billion. The Earth, about 4.54 billion. The Murchison stardust predates all of them by a margin that dwarfs the entire span of terrestrial geology.
What Happens Next
The claim rests on one paper and one meteorite. It is not yet a consensus about the entire galaxy’s star-formation history. What it is, is a data point that biases the argument — and one that other teams will test by running the same neon-isotope analysis on presolar grains from other carbonaceous chondrites. If the burst pattern holds across multiple meteorites, the case for episodic star formation in the solar neighbourhood strengthens considerably.
The Murchison meteorite itself, now over five decades in curated collections, still has material to give. New analytical techniques — sharper mass spectrometers, faster ion probes — will keep extracting data from grains that were invisible to the instruments of 1969. Each presolar grain is a sealed record of a single dead star, and Murchison carries thousands of them. The oldest solid ever held on Earth is unlikely to stay the only one of its kind for long.
— Priya Nair, science desk, AXO News