Did Earth form from icy pebbles instead of cosmic dust?

Scientists Challenge Leading Theory of Earth's Origins

Astronomers propose a new scenario for how "stardust" became part of our planet.

December 16, 2025, 12:22 GMT

Astronomers have proposed a new explanation for how stardust became part of Earth's composition.

Researchers from the University of Copenhagen argue that much of the material forged in supernova explosions reached the early solar system not as dust particles, as previously believed, but trapped in ice. This finding also calls into question the classic model of Earth's formation through collisions of large protoplanets.

A preprint of the study is available on arXiv.

The idea that humans are literally made of "star stuff" has long been part of scientific culture. Many elements and isotopes essential for life are known to form only in the cores of massive stars and during supernova explosions. For years, scientists assumed these products dispersed across the galaxy as tiny dust grains, later embedding themselves in emerging planetary systems.

But new research points to an alternative delivery mechanism.

The key to revising the old model was a rare isotope of zirconium—Zr-96. Produced exclusively in supernova explosions, it serves as a convenient "marker" of stellar origin. The team analyzed samples from different types of meteorites, treating them with weak acetic acid. This method allowed them to separate water- and ice-associated components from solid mineral grains.

The results were surprising: the concentration of Zr-96 in the dissolved "aqueous" fraction was thousands of times higher than in the solid rock. This suggests that a significant portion of supernova material traveled through interstellar space embedded in icy particles.

In essence, atoms born in stellar cataclysms were incorporated not into dust but into ice, which later found its way into protoplanetary disks.

This mechanism neatly explains the distribution of stellar isotopes in the solar system. The closer a planet formed to the Sun, the more intense the heating and the more ice evaporated. The inner planets—Mercury, Venus, and Earth—thus lost a substantial share of their "supernova" isotopes, while outer worlds like Uranus and Neptune retained far greater amounts.

Astronomers have long observed this linear relationship in the data but, until now, lacked a compelling explanation.

These findings also have deeper implications. Earth is depleted in the isotope zirconium-96 compared to the asteroids long thought to have contributed to its formation. If the planet had grown from collisions between massive bodies, it would have inherited more of this isotope. Yet its scarcity aligns well with the galactic accretion model, in which Earth formed from streams of tiny, icy "pebbles."

As these pebbles crossed the so-called snow line, their ice vaporized, carrying away stellar isotopes with it.

The researchers emphasized that their conclusions require further verification, but if confirmed, they could dramatically reshape our understanding of the early solar system. If Earth truly grew from icy pebbles rather than collisions of large protoplanets, it shifts the focus in the debate over the origins of the raw materials that gave rise to life.