Did this star eat its planets? A new study offers clues on 'chemical paradox' of a binary system

June 5, 2026 report Did this star eat its planets? A new study offers clues on 'chemical paradox' of a binary system Shreejaya Karantha Author Lisa Lock Scientific Editor Robert Egan Associate Editor Astronomers have investigated a puzzling binary star system in which two stars that may have formed together now show dramatically different chemical compositions. The new study, uploaded to the arXiv preprint server on May 29, hints at the possibility that one of the stars may have swallowed its own planets. Double-trouble Generally, in binary systems, the two stars form from the same molecular cloud and, as a result, have the same age and chemical composition. Any differences in their metallicity, astronomers say, hint at an event involving mass transfer or engulfment of planetary components or other internal processes. HD 81809 is one such peculiar system in which the stars are both sun-like G stars but are at different stages of evolution. The primary star, HD 81809A, has crossed the main-sequence phase, depleted its hydrogen fuel in the core but hasn't turned into a giant star yet—it is now a subgiant. On the other hand, the secondary star, HD 81809B, is still a main-sequence star. It has lithium enrichment and there is a difference in iron content between the two stars—the primary is metal-poor with an iron abundance of −0.57 dex, while the secondary has roughly solar metallicity around 0.00 dex. This difference in chemical abundance between the two stars is unexpected from the standard chemical evolution trends. The 0.57 dex discrepancy is too large to be explained by standard mechanisms. While some studies have proposed formation scenarios consistent with standard theories, inconsistencies remain in producing the observed luminosity and age of the stars. A previous study published in The Astrophysical Journal hypothesized that a recent accretion event could have changed the chemical properties of the surface layers of HD 81809B. This was also supported by the detection of a debris disk in the system. In the new paper, led by Nuno Moedas of the Technical University of Denmark, the team explores this scenario further to check whether an engulfment of metal-rich planets by the secondary star could reproduce the system's chemical properties. Building on the more physically plausible scenario from the previous study—in which both stars formed from the same molecular cloud—they used a computer simulation code called the Modules for Experiments in Stellar Astrophysics (MESA) to test the planet engulfment hypothesis by modeling different accretion events with various mass ranges and chemical mixtures. A chemical mismatch Computer simulations suggest that reproducing the star's unusually high metallicity would require the accretion of roughly 25–75 Earth masses of metals in an event occurring near the star's current age of roughly 10 billion years. But if the planet engulfment took place in the very early stage of the star's life, the required accreted mass is 150 Earth masses of metals, which the researchers say is "unlikely." So, the event must be recent. However, their analysis also predicts far more lithium than astronomers observe. The lithium mismatch would disappear if the accreted material was less than 6 Earth masses. "The models predict that such a metal-rich accretion would over-enrich lithium at the surface; matching the observed lithium instead requires accreting less than 6 M⊕," the team writes in the paper. "This tension highlights the need for precise knowledge of the accreted material's chemical composition." Nevertheless, compared to previously proposed scenarios, planet engulfment emerges as the most plausible explanation. The authors note that detecting rotation and magnetic activity on HD 81809B, which could hold telltale signs of a planet engulfment event, would provide additional evidence. Written for you by our author Shreejaya Karantha, edited by Lisa Lock, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Nuno Moedas et al, Chemical paradox in a binary system: Exploring metal enrichment in HD 81809B, arXiv (2026). DOI: 10.48550/arxiv.2605.31060 Maria Pia Di Mauro et al, On the Contradictory Case of the Binary System HD 81809 Hosting Two Pulsating Solar-like Stars Observed by TESS, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae40ad Journal information: Astrophysical Journal , arXiv © 2026 Science X Network