Around 4.5 billion years ago, an interstellar molecular cloud collapsed. At its centre, the Sun was formed; around that, a disc of gas and dust appeared, out of which the earth and the other planets would form. This thoroughly mixed interstellar material included exotic grains of dust: “Stardust that had formed around other suns,” explains Maria Schönbächler, a professor at the Institute of Geochemistry and Petrology at ETH Zurich. These dust grains only made up a small percentage of the entire dust mass and were distributed unevenly throughout the disc. “The stardust was like salt and pepper,” the geochemist says. As the planets formed, each one ended up with its own mix.

Thanks to extremely precise measurement techniques, researchers are nowadays able to detect the stardust that was present at the birth of our solar system. They examine specific chemical elements and measure the abundance of different isotopes – the different atomic flavours of a given element, which all share the same number of protons in their nuclei but vary in the number of neutrons. “The variable proportions of these isotopes act like a fingerprint,” Schönbächler says: “Stardust has really extreme, unique fingerprints – and because it was spread unevenly through the protoplanetary disc, each planet and each asteroid got its own fingerprint when it was formed.”

Studying palladium in meteorites