New research published in Monthly Notices of the Royal Astronomical Society suggests that the Sun may have engulfed a super-Earth about five to ten times more massive than Earth early in its formation, potentially explaining long-standing solar mysteries including an unusually depleted surface lithium level and unexpected sound-speed patterns.
Stubborn Chemical Puzzles in the Solar Atmosphere
Astrophysicists have spent centuries modeling stellar evolution. Yet our own star continues to harbor stubborn chemical and physical puzzles.
Chief among them is a severe deficiency of lithium in the solar atmosphere. The proto-solar nebula from which our system formed billions of years ago is believed to have had two orders of magnitude more lithium than is currently seen in the Sun’s outer shell. At the same time, helioseismic measurements tracking pressure waves rising through the solar interior reveal that the speed of sound near the bottom of the convection zone does not match standard models.
MESA Stellar-Evolution Modeling and Professor Mutlu Yildiz
To investigate whether these two separate anomalies might share a single origin in the early history of the Solar System, Professor Mutlu Yildiz of Ege University in Turkey deployed the MESA stellar-evolution code. Researchers tested various accretion histories and alternative explanations involving opacity, the equation of state, and turbulent mixing.
"Our new study suggests that a planet several times more massive than Earth may have fallen into the young Sun and left a lasting chemical imprint deep inside it," Professor Mutlu Yildiz said.
The modeling points toward a scenario in which the young Sun ingested a rocky super-Earth roughly five to ten times the mass of our planet. The planetary characteristics that best reproduce the known features of the Sun suggest an object about 5.6 times the size of Earth: a rocky, but lithium-poor, super-Earth.
Solving Lithium Deficit and Convection Zone Anomalies
Standard solar models based on the standard physics of stellar evolution have had difficulty reproducing some helioseismic observations simultaneously. This is particularly true for the sound-speed structure just below the convection zone and the depth of the solar convection zone. The newly published calculations indicate that planetary engulfment can alter the internal structure of a host star while leaving observable chemical fingerprints.
"By modelling the Sun’s evolution and comparing the results with precise observations of its interior, we find that the ingestion of a super-Earth could help explain long-standing differences between standard solar models and observations, including subtle changes in the Sun’s internal structure and its depleted lithium abundance," Professor Yildiz told the Royal Astronomical Society.
Protoplanetary Discs and Migration Pathways
The study notes that young stars are surrounded by protoplanetary discs. Substantial amounts of material can move between the disc and the star here. Because planets are made of material that is chemically different from the gas in the disc, an early planetary engulfment event would leave a chemical signature inside the young Sun.
Furthermore, researchers found that such a world could survive its passage through the Sun’s outer layers while losing very little mass. This suggests that planets may leave detectable fingerprints inside their host stars long after they have disappeared.
While our solar system currently lacks super-Earths, astronomers have long wondered why many other star systems appear to have large super-Earths. The research cites previous research from a decade ago by Martin & Livio (2016), which suggested that one or more super-Earths could have formed inside the orbit of Mercury and migrated inward through the gas disc, potentially falling into the young Sun.
Future Observational Steps and Evidence
While the paper does not definitively prove that our star swallowed a planet billions of years ago, researchers emphasize that the model successfully addresses multiple independent mysteries at once. Professor Yildiz noted that while it may not be possible to definitively prove the Sun swallowed a planet, if the predicted structural and chemical signature could be independently identified through helioseismic or other observations, it would provide strong evidence for such an event happening billions of years ago.
"Our paper asks whether the Sun itself could still carry observable evidence that such an engulfment actually happened, and we believe it could," Professor Yildiz concluded. "The next step is to see if these fingerprints can be independently detected."
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