Study suggests Earth’s core carbon came from outer solar system

The team found a uniquely heavy carbon-13 signature in the magma it linked to the core-mantle boundary. Carbon-13 is a heavier form of carbon measured in the analysis, so an unusually large share indicates a distinctive isotopic composition, meaning a particular mixture of carbon versions with different atomic masses.

“We interpret this as carbon leaking from the core,” Grewal said. The wording matters: researchers did not recover core material. They inferred core carbon from lava whose connection to the boundary region they described as a belief rather than a direct sample.

Where did that carbon originate?

Grewal also led the origin analysis. The core carbon was too heavy to have come from the rocky, carbon-poor material of the inner solar system that built most of Earth. Its composition instead matched carbonaceous chondrites, a class of carbon-rich meteorites from the outer solar system.

Those meteorites contain organic compounds and water, according to the study’s description. The isotope match points specifically to carbonaceous chondrites, and the researchers’ broader conclusion suggests that most of Earth’s carbon originated in the outer solar system. The result offers an answer to a longstanding debate over the origins of carbon on Earth, although the conclusion remains an inference from Hawaiian lavas rather than a direct examination of core material.

When could the meteorites have delivered it?

The researchers identified only one feasible window for carbonaceous chondrites from the outer solar system to become permanent core material: the final stage of Earth’s main assembly.

• Before that stage, subsequent collisions could have sent much of the incoming carbon back into space.
• During the final stage, incoming carbonaceous chondrite material could remain and become part of the core.
• After that stage, Earth’s core would already have been closed to further delivery.

This timing limits the origin story. It does not require carbon to have existed in Earth’s material from the beginning; the proposed source would have arrived late, after most of the planet had assembled.

What does the finding change?

Beyond the origin question, the researchers said the result may have implications for the role of core carbon during episodes of intense volcanic activity. That application remains a possible implication rather than a demonstrated outcome.

The central conclusion still carries qualifications. The researchers describe a heavy carbon-13 signal in magma they believed represented the core-mantle boundary, then interpret that signal as carbon escaping from the core. The comparison with carbonaceous chondrites and the late-delivery window together make the outer solar system a plausible source, not a closed question.

The paper, “Isotopically heavy carbon in the core,” appeared in the Proceedings of the National Academy of Sciences in 2026. Its citation lists Yves Moussallam as lead author and gives the DOI as 10.1073/pnas.2610912123.

The study supplies the first observational estimate of core carbon’s isotopic composition while leaving its distant origin provisional.