An international team of astrophysicists from Ege University (Turkey) has proposed a hypothesis that could resolve three long-standing questions in solar physics simultaneously. Calculations indicate that if the Sun swallowed a super-Earth—a rocky planet 5 to 10 times the mass of Earth—during the early stages of its evolution, a chemical “fingerprint” of that event would still be detectable within the solar interior.
For decades, standard solar models based on the physics of stellar evolution have failed to simultaneously reproduce several observed parameters. First, the speed of sound just below the convection zone diverges from helioseismic measurements—currently the most precise tool for probing a star’s internal structure. Second, the modeled depth of the convection zone itself does not match reality. Third, the Sun’s surface exhibits a severe lithium deficit: levels are 140 times lower than theoretical predictions suggest. The researchers hypothesized that all three mysteries might stem from a single event that occurred 4.6 billion years ago.
Image credit: NASA, ESA, CSA, Ralf Crawford (STScI)
The researchers modeled two successive accretion episodes. The first involved the ingestion of heavy-element-rich planetary material (the super-Earth), while the second involved the star accreting metal-poor gas from the protoplanetary disk remaining after planet formation. This sequence was not manually adjusted to fit the desired outcome; rather, it reflects a realistic process wherein planet formation depletes the gas of heavy elements, causing the star to subsequently accrete material that is already element-poor.
The key result of the study is model DD1020 (featuring a planet designated “Dev Dilek”), which successfully satisfied all observational constraints simultaneously. The engulfed planet—with a mass of approximately 5.6 Earth masses (or 5 to 10 Earth masses according to broader models)—left behind a localized enrichment of heavy elements directly beneath the convective zone, at a depth corresponding to 0.96–0.973 of the solar radius. This altered the material’s composition and internal stratification—precisely the changes needed to resolve discrepancies regarding the speed of sound and the depth of the convective zone.
At the same time, the model accounted for the observed lithium deficit. The team demonstrated that if the engulfed material was lithium-poor and the additional turbulent mixing beneath the convective zone remained shallow, the resulting surface lithium concentration would match actual observations. Notably, the required mixing depth is physically consistent with independent estimates for the tachocline (a transition region in the upper third of the Sun).
The authors went beyond chemical modeling to verify the scenario’s physical feasibility. Calculations showed that a dense, rocky planet could pass through the young Sun’s convective envelope with virtually no mass loss; classical aerodynamic breakup and ablation would strip away a negligible fraction of the material. This process is aided by the planet’s compression under mounting external pressure: its radius shrinks by up to 29%, reducing its effective cross-section and further suppressing erosion. Additional stability analysis confirmed that such a planet would not be torn apart by tidal forces before entering the stellar envelope.
A statistical test demonstrated that the model’s results could not be attributed simply to an increase in the number of adjustable parameters. Optimized control models—incorporating turbulent mixing and a variable mixing parameter but excluding planetary engulfment—yielded only partial results and fell significantly short of the performance of the “engulfment” model. The next step will be to seek independent confirmation through helioseismic or spectroscopic observations. If the “chemical fingerprint” of a super-Earth within the Sun’s interior is confirmed, it would mean that the star still preserves information about events from the era of the Solar System’s formation—one need only be able to detect it.
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