Dr. Nandkumar M. Kamat
Our five-billion-year-old solar system has many mysteries. Earth was formed roughly half a billion years after the sun became active. We do not know exactly what happened during this period. The sun may still preserve evidence of violent episodes from the infancy of the solar system.
A study by astronomer Mutlu Yıldız of Ege University, Turkey, raises the possibility that the young sun engulfed a rocky planet several times the mass of the Earth. Yıldız’s paper, ‘Planetary engulfment as a solution to solar-model discrepancies and its implications for planetary systems’, published in the Monthly Notices of the Royal Astronomical Society in 2026, investigates whether material from a vanished planet could have altered the sun sufficiently for traces of that event to remain measurable today. His preferred model points to a super-Earth of approximately 5.6 Earth masses, while related models favour a range of approximately five to 10 Earth masses.
This question arises because even the sun, our best-studied star, has not yielded all its secrets. Solar models are tested against helioseismology, which is the study of oscillations travelling through the sun. These waves allow scientists to infer the speed of sound through much of the solar interior and provide precise measurements of the depth of the convection zone and the abundance of helium at the surface of the sun. Standard solar models reproduce much of this well; however, significant discrepancies remain. Yıldız concentrated particularly on the differences in the sound speed profile and the region around the base of the convection zone.
Solar metallicity, the astronomical term for the abundance of elements heavier than hydrogen and helium, introduces another difficulty because changing the assumed composition to improve one part of a model can worsen another part. Therefore, a satisfactory model must simultaneously meet several independent observations. Lithium provides an additional test for the sun’s history. Its present photospheric abundance is much lower than that expected from the material from which the solar system was formed. Lithium is readily destroyed when carried into sufficiently hot stellar layers; therefore, its abundance contains information about mixing within the sun. In Yıldız’s calculations, the standard model gives a present lithium abundance considerably higher than the observed value of ~ A(Li)=1.1. Models involving lithium-poor accreted material bring the predicted abundance much closer to the observations. Therefore, any explanation based on planetary engulfment must account for the internal sound-speed profile and the chemical composition measured at the solar surface.
Planetary engulfment is physically conceivable because young planetary systems are highly dynamic. Planets can migrate as they interact with their surrounding discs and other planets. Some move outward, while others can be driven towards their parent stars. Yıldız considers whether such an event occurred during the early evolution of our own system and calls the hypothetical engulfed planet “Dev Dilek.”
This study treats it as a super-Earth or hot Neptune-like body. Stars do not need to completely erase the evidence of material falling into them. If the accreted material differs chemically from the stellar material and is not subsequently mixed throughout the star, an altered layer can survive for a long period. Helioseismology may provide a means of detecting these structural consequences. To test this possibility, Yıldız used MESA, a widely used framework for modelling stellar structure and evolution. His models include the early accretion of metal-rich material representing planetary engulfment, followed by metal-poor disc accretion associated with planet formation. A rocky planet contains a much larger proportion of heavy elements than the sun, which consists overwhelmingly of hydrogen and helium. Therefore, adding several Earth masses of rocky material contributes little to the sun’s total mass but can substantially alter the chemistry of a restricted solar layer. In the successful models, the material from the engulfed planet settled below the convection zone at approximately 0.96 to 0.973 solar masses in the model’s mass coordinate. This concentration of heavy elements changes the opacity and internal stratification and consequently alters the calculated sound-speed profile.
The most successful model, designated DD1020, combined planetary engulfment with turbulent mixing and provided the best overall agreement with the helioseismic and photospheric constraints examined in the study. This indicates an engulfed planet of approximately 5.6 Earth masses, which is approximately one-third of Neptune’s mass. Closely related models generally point towards five to ten Earth masses. Particularly successful models constructed with an initial solar mass of 0.97 solar masses produce very small differences between the calculated and helioseismically inferred sound speeds and favour a narrower planetary mass of approximately 4.6–5.8 Earth masses. This range arises because several observations must be satisfied together: the sound-speed profile, depth of the convection zone, surface helium abundance, surface heavy-element abundance, and present lithium abundance. Could these improved results arise simply because a more complicated model has more parameters available
for adjustment?
Yıldız constructed control models containing turbulent mixing and variable mixing length prescriptions but without planetary engulfment. They produced partial improvements but did not achieve the same overall agreement as the engulfment models. The study also applied the Bayesian Information Criterion, which considers model complexity, and concluded that the improvement cannot be attributed solely to additional adjustable parameters. Within the assumptions of the study, the added heavy elements from planetary material produce a localised change in the solar interior that improves the agreement with several independent measurements.
However, a major physical question remains. Could a rocky planet survive long enough after entering the sun to carry several Earth masses of material through the convective envelope? Yıldız examined Roche-lobe stability, planetary compression under high pressure, aerodynamic drag, and ablation. For a reference calculation involving a ten-Earth-mass rocky planet, the passage through the convective envelope takes only a few hours. As the pressure increases, the planet is compressed, and its cross-sectional area decreases, reducing the drag and classical aerodynamic ablation. Even under assumptions chosen to favour erosion, the calculated mass loss before reaching the base of the convection zone remains negligible compared to the planet’s initial mass. Fragmentation, shocks, thermal expansion, hydrodynamic instabilities, and dissolution in deeper layers could eventually destroy it, and complete hydrodynamic treatment remains for future work.
The lithium constraint makes this hypothesis more challenging. Mixing beneath the convection zone must be sufficient to account for the observed depletion of lithium yet remain shallow enough not to erase the heavy-element enhancement deposited by the engulfed planet. If the metal-rich region were redistributed into deeper layers, the improvement in the helioseismic sound-speed profile would disappear. Therefore, the hypothesis must satisfy two competing requirements: sufficient mixing to explain lithium depletion but not enough to destroy the chemical signature of engulfment.
The study finds a limited range of conditions in which both requirements can be satisfied, along with the other solar constraints. None of this establishes that a super-Earth once existed and was swallowed by the sun. It is a modeling hypothesis that achieves good consistency with several observations and passes the initial tests of whether such an event is physically possible. A more comprehensive treatment would still have to follow the planet’s orbital evolution, deformation, fragmentation, and eventual dissolution inside the young sun.
Independent evidence is required before Dev Dilek can be considered anything more than a hypothetical lost world. The larger significance lies in the possibility that the present solar system may not contain a complete inventory of the substantial planets that once formed around it. Stellar interiors can retain chemical and structural signatures in incompletely mixed layers, and the sun can be examined with exceptional precision using helioseismology. If future studies independently identify the predicted fingerprints, evidence for a vanished planet may have survived for billions of years inside the sun itself.
