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Feeding the young Sun a super-Earth squares its helioseismology with its missing lithium
In MNRAS, Ege University's Mutlu Yildiz reports solar models that swallow a planet of five to 10 Earth masses matching interior structure and low surface lithium in the same run, a fit that needs an independent detection before it means much.
The Scientist · Science desk
What happened
- Mutlu Yildiz of Ege University argues in Monthly Notices of the Royal Astronomical Society that a super-Earth swallowed by the young Sun would have left a chemical signature still present in the solar interior.
- Standard solar models have long struggled to reproduce two helioseismic measurements at the same time: the sound-speed structure just below the convection zone and the depth of that zone.
- Separately, the Sun's surface shows a strong and well-documented shortfall of lithium relative to what standard evolution models produce.
- Yildiz used the MESA stellar-evolution code to run different accretion histories, then scored the resulting models against helioseismic constraints and measured surface abundances.
- The models that worked best had the young Sun ingesting a planet of roughly five to 10 Earth masses, a narrower range than the author expected the calculations to pick out.
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Why it matters
- constraint The imprint has not been observed, and Yildiz says definitive proof of engulfment may never be available, so the paper competes as the best-fitting model rather than settling anything about the Sun's youth.
- capability If a rocky body of a few Earth masses can cross a star's outer layers almost intact, engulfment becomes a chemistry question you can go looking for in other stars rather than only a dynamical inference from missing planets.
- precedent The question moves from whether a super-Earth could have migrated into the young Sun, which earlier modelling already allowed, to whether the star kept a receipt, which attaches an observational consequence to that older work.
What lifts this above a curiosity is the comparison Yildiz ran alongside it. Solar modellers have familiar levers to pull when a model misses the sound speed just below the convection zone or puts the base of that zone at the wrong depth, and the paper reports pulling them: the equation of state, the opacities, and different prescriptions for turbulent and convective mixing [6]. Engulfment is offered as the option that also lands the Sun's depleted surface lithium without being retuned for it [8].
The physical premise is not exotic. A young star sits inside a protoplanetary disk with substantial material moving in both directions, and planets are built from material chemically unlike the disk gas, which is the reason Yildiz went looking for a shared origin for the two problems [13]. Dropping a rock-rich body into the envelope changes the composition of exactly the layers helioseismology can see. The published account states the fit rather than the intermediate physics; how the composition change propagates into burned lithium is not spelled out there.
The mass window is where the paper is most exposed and most interesting. Five to 10 Earth masses is a factor of two wide [16], which is tight for a quantity inferred from stellar structure, and Yildiz says the convergence on so specific a range was not what he expected going in [9]. Tight predictions are the killable ones. What the account does not report is which constraint breaks first outside that window, and that is the number a sceptic would want.
Nothing here is a measurement of the Sun's history. The signature is predicted, not observed, and Yildiz is direct about the ceiling: definitive proof that the Sun swallowed a planet may not be available, and the best available outcome is independent identification of the predicted structural and chemical imprint through helioseismic or other observations [10]. He calls that detection the next step [11]. The paper also inherits its setup from work a decade earlier, which showed a super-Earth could form inside Mercury's orbit and migrate inward through the gas disk, but did not require that it ended up inside the star [15]. The long-standing question of why other systems have big super-Earths and ours has none is the motivation, not the evidence [14].
Two ideas are being tested at once, and they should be scored separately. One is that a rocky body of a few Earth masses can cross a star's outer layers losing very little mass, which is what makes any fingerprint survive to be found later [12]. The other is that our Sun actually ate one. The first is a modelling result with consequences for other stars. The second remains a hypothesis whose status depends on someone measuring the imprint that these models predict, and until that measurement exists the honest description is a model that fits several observations at once with a planet-shaped ingredient and a falsifiable mass range attached.
What to watch
- A revised opacity table or equation of state that closes the same helioseismic gap without a planet would remove the need for engulfment entirely.
- Whether a helioseismology group can name a single observable, with error bars, that the predicted imprint would fail.
- Whether the low-mass-loss survival result gets turned on other host stars, where engulfment chemistry can be compared between similar stars.