If proto-Earth had all its parts and chemistry replaced to become the Earth we know today, can the two still be considered the same planet? That’s the planetary version of Theseus’s Paradox, an old philosophical puzzle about identity and perception. The popular consensus until recently was that Earth’s chemistry changed completely after a giant meteorite impact, leaving nothing behind from its proto-Earth days.

A new finding suggests that conception may be wrong. In a Nature Geoscience paper published earlier this week, researchers report detecting a chemical signature that appears to have miraculously resisted change for billions of years. Specifically, the team—an international collaboration between the U.S., China, and Switzerland—found an odd imbalance of potassium isotopes in ancient rock samples. Chemical analyses revealed the anomaly couldn’t have emerged from any known geological processes on modern Earth.

Theseus’s planet?

Planetary scientists have long suspected that a Mars-sized meteorite slammed into Earth some 4.5 billion years ago. The impact triggered a literal, astronomical makeover, transforming what was once a rocky, lava-filled environment into the Earth we know today.

The general understanding was that, over time, whatever materials or processes formed proto-Earth either transformed or were replaced by ones more familiar to researchers today. It was, of course, a reasonable explanation: that the “resetting” of Earth’s chemistry miraculously created the conditions that eventually led to life.

A potassium anomaly

Naturally, scientists are still hoping to learn more about our planet’s earliest days. For the new paper, the authors zoomed in on potassium. On Earth, the common element normally exists in a specific combination of potassium-39 and potassium-41, with a tiny portion of potassium-40.

Previous work by the study’s lead authors, however, showed that extraterrestrial objects—such as meteorites—have distinct potassium profiles, typically with a slightly higher proportion of potassium-40.

Building on this knowledge, the team dug deep into the oldest available rocks on Earth, such as powdered rocks from Greenland and Canada and lava deposits in Hawaii. At the lab, they ran the samples through various techniques in analytical chemistry.

Surprisingly, the potassium profile they ended up with was unlike anything researchers had ever seen—neither on Earth nor in cosmic objects. In fact, the “deficit” of potassium-41 was so bizarre that spotting it was “like spotting a single grain of brown sand in a bucket rather than a scoop full of yellow sand,” the researchers told MIT News.

An ongoing mystery

Was there really no feasible, natural way for this chemistry to have emerged? Multiple simulations and follow-up investigations of all known meteorites and geological processes seemed to point to the same answer: no. According to the paper, the most viable explanation for this material’s existence is that it was left over from proto-Earth.

“This is maybe the first direct evidence that we’ve preserved the proto-Earth materials,” Nicole Nie, study co-lead author and a planetary scientist at MIT, explained to MIT News. “We see a piece of the very ancient Earth, even before the giant impact. This is amazing because we would expect this very early signature to be slowly erased through Earth’s evolution.”

That said, we may as well end up finding something, like an odd meteorite, with the same potassium anomaly, in which case the signature wouldn’t necessarily be the surviving remnants of proto-Earth.

Either way, the findings demonstrate that there’s still a lot for us to learn about our own Earth—lessons that, perhaps, may guide us away from any missteps we’re making while studying things beyond Earth.