Our solar system is currently passing through the Local Interstellar Cloud, a region of highly diluted gas and dust between the stars. On its path, Earth continuously accumulates iron-60, a rare radioactive isotope of iron produced in stellar explosions. This has now been confirmed by an international research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) through the analysis of Antarctic ice tens of thousands of years old. From the steady but time-varying influx, the researchers conclude that the radioactive isotope has been stored within the cloud since a long-past stellar explosion. The results have been published in the journal Physical Review Letters.

Iron-60 is formed in the interiors of massive stars and is ejected into space when they explode. Geological archives show that our solar system was hit twice by iron-60 from supernovae millions of years ago. In more recent times, however, there have been no nearby stellar explosions—and thus no direct supply of iron-60. When scientists discovered iron-60 in Antarctic surface snow less than twenty years old a few years ago, the question of its origin arose.

"Our idea was that the Local Interstellar Cloud contains iron-60 and can store it over long time periods. As the solar system moves through the cloud, Earth could collect this material. However, we couldn't prove this at the time," explains Dr. Dominik Koll from the Institute of Ion Beam Physics and Materials Research at HZDR.

In recent years, the team led by Koll and Prof. Anton Wallner analyzed additional samples, including deep-sea sediments up to 30,000 years old. Iron-60 was also found there, but competing theories remained. The new Antarctic ice samples date back 40,000 to 80,000 years. Their analysis now makes it clear: the Local Interstellar Cloud is the likely source.

"This means that the clouds surrounding the solar system are linked to a stellar explosion. And for the first time, this gives us the opportunity to investigate the origin of these clouds," says Koll.

Our solar system entered the Local Interstellar Cloud several tens of thousands of years ago and will leave it again in a few thousand years. At present, we are located near its edge.