The Sun May Have Eaten a Planet Five to Ten Times Earth's Mass When It Was Young. A New Model Says the Scars Are Still Inside It.

The sun has two long-standing problems that solar physicists have never managed to fix at once. Its surface has far less lithium than it should, and the standard model of how a star like it works cannot quite reproduce what helioseismology, the study of sound waves rippling through the solar interior, actually measures just below the convection zone. A new paper in Monthly Notices of the Royal Astronomical Society argues both problems have the same cause: early in its life, the sun swallowed a planet several times the mass of Earth, and the chemical residue is still there.

"Our new study suggests that a planet several times more massive than Earth may have fallen into the young sun and left a lasting chemical imprint deep inside it," said Mutlu Yildiz, a professor at Ege University in Turkey and the paper's author. "By modeling the sun's evolution and comparing the results with precise observations of its interior, we find that the ingestion of a super-Earth could help explain long-standing differences between standard solar models and observations."

Yildiz used the MESA stellar-evolution code to build versions of the sun with different accretion histories, then compared each against the helioseismic constraints and the sun's observed surface abundances. He also tested the usual alternative explanations, tweaks to the equation of state, to opacity, and to how turbulent and convective mixing are handled. The engulfment models did something the others did not: they matched the sound-speed structure below the convection zone, the depth of that zone and the lithium depletion simultaneously. And they converged on a narrow answer. The planet that works is a super-Earth of roughly five to ten Earth masses.

"We thought planetary engulfment might affect the solar structure but did not expect the calculations to converge on such a specific super-Earth mass range," Yildiz said. "That was one of the most interesting outcomes of the study." The modeling also found that a planet of that size would survive its plunge through the sun's outer layers while losing very little mass, meaning the material would be delivered intact into the interior rather than stripped off at the surface.

The idea itself is not new. Astronomers have long puzzled over why so many other star systems host super-Earths, the most common type of planet found so far, while ours has none at all. A decade ago, researchers proposed that one or more super-Earths could have formed inside Mercury's orbit and migrated inward through the gas disk until they fell into the young sun. That work showed a pathway. It did not show that it happened. "Our paper asks whether the sun itself could still carry observable evidence that such an engulfment actually happened, and we believe it could," Yildiz said.

Proving it would require independently detecting the predicted structural and chemical signature through helioseismic or other observations, something the paper lays out as the next step. If it holds, the sun's chemistry would be carrying a fingerprint of a planet that vanished more than four billion years ago, and the same technique could be turned on other stars to find the worlds they ate.