The Sun May Have Swallowed A Planet 5.6 Times As Massive As Earth, Study Finds

Scientists Say the Sun’s Missing Chemistry Points to a Long-Lost Planet

Billions of years ago, the Sun may have swallowed a planet roughly 5.6 times the mass of Earth, and that single event could explain several stubborn mysteries about how the Sun is built. That is the conclusion of a new study in the journal Monthly Notices of the Royal Astronomical Society, whose author gave the vanished world a nickname: Dev Dilek, a Turkish phrase meaning “Great Wish.”

Mutlu Yildiz, an astronomer at Ege University in Turkey, was not chasing a dramatic headline. For decades, scientists studying the Sun’s interior have run into a stubborn problem: their best computer models keep disagreeing, in small but persistent ways, with what instruments detect inside the star. Those disagreements show up in the Sun’s sound, its structure, and its chemistry alike, and no one has been able to explain why.

Yildiz proposes a simple explanation: sometime in the Sun’s early life, it ate a planet. His simulations show that a swallowed world buried beneath the surface could account for nearly all of these mismatches at once, including the Sun’s missing lithium.

Sun’s Chemistry Has Puzzled Scientists for Decades

Scientists know the Sun better than any other star, thanks to precise measurements of its mass, size, brightness, and age, plus helioseismic data, sound waves that ripple through the Sun and act like an ultrasound of its interior. That data shows three things models struggle to match at once: how fast sound travels at different depths, the exact boundary of the Sun’s churning outer layer, and the surface mix of hydrogen, helium, and heavier elements. Fixing one piece tends to break another; get the surface helium level right and the heavier elements come out wrong. Researchers have tweaked heat flow and internal mixing for years without clearing up every mismatch simultaneously.

A Swallowed Super-Earth Fits the Missing Chemistry

Yildiz took a different approach. Using a stellar simulation program called MESA, he modeled two early feeding episodes for the young Sun: an intake of heavy-element-rich material, standing in for a swallowed planet, followed by a batch of gas unusually poor in heavy elements, representing leftover disk material after planets had already formed. By adjusting how much was added, when, and how it settled, he checked whether the resulting model Sun matched the real one better than standard models do.

One version of his model stood out from the rest. Combining planet engulfment with a specific pattern of internal mixing, it gave the best overall match to the real Sun of any model in the study, pointing to an engulfed super-Earth weighing roughly 5.6 times Earth’s mass, or about a third of Neptune’s mass. Related best-fitting models converged on a broader range of 5 to 10 Earth masses.

In the best-fitting models, the swallowed material shows up as a heavy-element-rich layer just beneath the Sun’s churning outer zone, reshaping its internal structure in a way that lines up with real measurements. That layer comes from a model built to match the Sun’s chemistry, not a simulation that tracked a planet actually sinking through the star; whether a real planet could physically reach that depth is addressed separately, below.

Control models built without any planet-eating, but given extra flexibility through adjustable mixing settings, only partly closed the gap with observations. Yildiz ran a statistical checkup called the Bayesian Information Criterion, which penalizes a model for having too many adjustable dials before crediting it with genuine improvement. Even accounting for that penalty, the engulfment models came out ahead.

Lithium adds a second data point. Stars destroy this lightweight element over time, and meteorite evidence suggests the early solar system held a generous supply. Today the Sun’s surface has more than a hundred times less. Yildiz found that if the swallowed material was itself low in lithium, and the planet weighed between 4.6 and 5.8 Earth masses, models reproduce the Sun’s current lithium level almost exactly, overlapping neatly with the other mass estimates.

The Planet Could Have Survived the Plunge

A skeptic might reasonably ask whether a planet could survive diving into a star rather than being torn apart on approach. Yildiz tested that directly.

One check asked whether the Sun’s tidal pull would rip the planet apart before it ever reached the star’s surface, a test astronomers call Roche lobe stability. Puffy, gas-rich planets would lose material before getting that close. A compact, rocky planet already stripped of its gas envelope, however, sat right at that stability limit rather than well outside it, meaning it would not necessarily be shredded on contact.

Yildiz also modeled how such a planet would respond to crushing pressure inside the Sun’s outer layers, and calculated how much friction and heat would erode it as it plunged deeper, a process called ablation. The planet could cross the churning outer layer losing only a tiny amount of mass, arriving largely intact at the depth where the models say it eventually dissolved.

One Engulfed Planet Ties Together Several Solar Mysteries

None of this proves the Sun definitely ate a planet named Dev Dilek four and a half billion years ago. What it does show is that an engulfment scenario can potentially tie together several stubborn mismatches between solar models and real measurements, from sound-speed anomalies to the mystery of the Sun’s missing lithium. Thousands of planets orbiting other stars sit close enough that getting swallowed seems like a real possibility for many of them eventually. Seen that way, the idea that the Sun once did the same to a small world of its own is no longer such a wild theory.

---