What if a missing planet left a trace inside the Sun? Could the Sun swallow a super-Earth when the solar system was young? Our system has no super-Earth in the close-in orbits common around many other stars. Some such compact worlds orbit very close to their stars; Curiosmos has explained what makes hot Jupiters so unusual. A new study asks whether one might have formed here, then spiralled into the young Sun—and whether the star could still carry a record of that lost world.
The idea is not based on a planet being seen fall into the Sun. It comes from a computer model of the Sun’s interior. In a paper published in Monthly Notices of the Royal Astronomical Society, astronomer Mutlu Yıldız and colleagues tested whether an early episode of planet material falling into the Sun could improve the fit between solar models and measurements of the real star.
Their best-fitting scenario points to a planet about 5.6 times Earth’s mass. That is a result of the model, not a measured object or a confirmed event. The interesting question is whether the Sun’s internal structure preserves a fingerprint that future observations can test.
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Why solar models leave a puzzle
The Sun’s surface is visible, but what is happening beneath it has to be inferred. One way is helioseismology: researchers study the star’s natural oscillations and use the way sound waves travel through it to map its interior. The resulting sound-speed profile can be compared with calculations of how a star like the Sun should evolve.
That comparison has not always lined up. Standard solar models have struggled to match the sound speed inferred just below the Sun’s convective envelope, as well as the measured depth of that envelope. Changing the Sun’s assumed chemical composition, opacity, or the way material mixes can improve parts of the fit, but those adjustments bring their own constraints.
The new study explores another possibility: the young Sun may have accreted material in more than one stage. The team used the MESA stellar-evolution code to model an early addition of metal-rich planetary material, followed by metal-poor gas left over from planet formation. “Metals” here means elements heavier than hydrogen and helium, the ingredients rocky planets are rich in.
A chemical layer beneath the Sun’s convection zone
In the model that best matched the helioseismic constraints, the dissolved heavy elements gathered in a narrow region beneath the convective zone, around 0.67 to 0.70 of the Sun’s radius. The added material changes the opacity and the way the interior is stratified, which in turn changes how sound travels through that layer.
The preferred case corresponds to about 5.6 Earth masses; related best-fitting models fall roughly between 5 and 10 Earth masses. The result is not a direct measurement of the Sun’s past. It is the amount of planetary material that gives the researchers’ model its strongest match under the assumptions they tested.
There is a longer history behind the idea. In 2016, Rebecca Martin and Mario Livio proposed that super-Earths could have formed in the early solar system and migrated inward, potentially falling into the Sun. The new paper takes that possibility a step further: it asks whether such an event could leave an observable signature in the Sun’s present-day interior, rather than considering migration alone.
Does the model explain the Sun’s lithium?
The Sun contains far less lithium in its visible atmosphere than the original solar nebula is thought to have contained. That long-standing difference gives the authors another test, but it does not make the planet-ingestion explanation automatic.
In their calculations, the observed lithium depletion can be reproduced only for a particular combination of conditions: the accreted material must be lithium-poor, the engulfed object must fall in a specified mass range, and mixing beneath the convection zone must remain limited enough to preserve the heavy-element layer. The lithium result therefore depends on what the material was made of and how it mixed—not simply on a planet entering the star.
Could a rocky planet reach that deep?
The paper also estimates whether a compact rocky planet could pass through the Sun’s outer convective envelope without losing all its mass first. Its calculations of classical drag and ablation suggest that a dense rocky body might survive long enough to carry material deeper into the star.
That is a physical-feasibility calculation, not a complete movie of the event. The authors do not model the full three-dimensional process of orbital decay, disruption, shocks, and mixing. The depth and shape of the proposed chemical fingerprint still depend on how planetary material actually behaves inside a young star.
What would count as evidence?
The key prediction is specific: a localized heavy-element enhancement beneath the Sun’s convective zone should leave a corresponding signal in the star’s internal structure and oscillations. Researchers would need to test for that pattern independently, while also accounting for competing explanations involving opacity, solar composition, and mixing.
The model’s advantage is that it makes the missing planet a testable possibility. Its limit is just as important: a good fit is not proof that the event happened. The Sun may once have swallowed a super-Earth, but the case will turn on whether helioseismic and chemical measurements reveal the predicted layer—and whether other solar models can explain the same clues without it.
If that buried signature is found, our familiar eight-planet system may have begun with a ninth world that never survived long enough to become part of the family we see today.
Earlier proposal: Martin and Livio, “On the Formation of Super-Earths with Implications for the Solar System” (2016).
