A group of Australian researchers working in Antarctica found stardust in freshly melted snow, discovering large amounts of a rare isotope known as iron-60 that is not natively found on Earth.

The study, published in the journal Physical Review Letters, ruled out the chance that iron-60 found in the snow was made by human action and argued it was delivered to Earth by some type of interstellar falling rock.

Earth’s most abundant element is iron, but iron-60 has four more neutrons than the well-known element. Experts argued that iron-60 can be found in the Earth’s crust, but the source can’t be the same as the new finding because it was in snow that has accumulated in recent decades.

The team collected 500 kg (1,100 lb) of Antarctic snow from around the Kohnen Station, shipped it to Munich, melted it down, and analyzed it. The solid components were separated from meltwater and processed using a few different chemical methods.

“Our analyses allowed us to rule out cosmic radiation, nuclear weapons tests or reactor accidents as sources of the iron-60,” says Dominik Koll, an author of the study. “As there are no natural sources for this radioactive isotope on Earth, we knew that the iron-60 must have come from a supernova.”

The location gave further clues to origin of this isotope. The snow it was found in was at most 20 years old, and the researchers reasoned that they couldn’t have come from too far away in the cosmos or they would have dissipated.

“If the gas cloud hypothesis is correct, then material from ice cores older than 40,000 years would not contain interstellar iron-60,” says Koll. “This would enable us to verify the transition of the solar system into the gas cloud – that would be a groundbreaking discovery for researchers working on the environment of the solar system.”

Researchers said the source of iron-60 must be a supernova, “not so near as to kill us, but not too far to be diluted in space,” Koll argued. Particles were likely picked up as Earth travels through the Local Interstellar Cloud, a 30-light-year wide region of space that our solar system is currently passing through, he said.

Nevertheless, more research is necessary to understand where and when the iron-60 got to Earth — it has a half-life of 2.6 million years — which Koll said will require more data and ice cores that go deeper into the planet, reaching older dust.