Lithium Signature Reveals Sun-Like Star Devoured Its Planet

Thirteen hundred light-years away, a sun-like star called TOI-5882 holds a secret written in its chemistry. Astronomers led by Brooke Kotten of the University of Michigan have found evidence that this distant star consumed one of its planets—not through direct observation of the catastrophic event itself, which would be impossible, but through the telltale signature left behind in the star's composition.

The clue is lithium. While stars naturally contain some lithium, planets are far richer in it. When a star engulfs a planet, that enriched material floods into the star's atmosphere, raising its overall lithium content in a way that stands out against the background of ordinary stellar chemistry. Kotten, a graduate student researcher, put it plainly: "You are what you eat, right? We know that there's much more lithium in planetary material than there is in stars. So if a star eats a planet, it's going to take on a bunch of lithium." The engulfment itself happens with stunning speed—weeks or days at most—which means astronomers cannot hope to catch it in the act. Instead, they must work backward from the evidence, like detectives reconstructing a crime from the scene.

To make their case, Kotten's team used spectroscopy to measure the light signature of TOI-5882 and determine its lithium concentration. They then assembled a control group of 62 comparable stars, matched for age, mass, and temperature, and ran multiple comparisons. TOI-5882 stood apart decisively. As Melinda Soares-Furtado, an assistant professor at the University of Wisconsin and senior author of the study, noted, the star's lithium enrichment was so pronounced that it ranked in at least the 97th percentile among its peers. The result was robust—no cherry-picking of data required. No matter how the researchers analyzed it, TOI-5882 was unmistakably anomalous.

Based on the amount of lithium detected, the team estimates that the engulfed planet had a mass somewhere between two Earths and Neptune. But TOI-5882 itself has not yet expanded into a red giant, the phase when a star naturally swells and consumes its inner planets. This raises a puzzle: how did the planet end up in the star's grasp? The team has proposed an intriguing possibility. Orbiting TOI-5882 is a brown dwarf—a massive object more than twenty times Jupiter's mass but not quite dense enough to ignite as a star. This companion body may have gravitationally nudged the planet into a collision course with its host star, though testing that hypothesis will require separate investigation.

The research, published in The Astrophysical Journal, involved fourteen experts from institutions across the United States and Chile. It builds on earlier work by Soares-Furtado, who had identified which stars possessed the right characteristics to show signs of engulfment. TOI-5882 proved to be one of the rare exceptions—a star old enough and chemically suitable for such detection. Interestingly, a handful of other stars in the control sample also showed elevated lithium, suggesting that other enrichment mechanisms may be at work, opening new avenues for future study.

The implications extend beyond this single star. In roughly five billion years, our own sun will enter its red giant phase and expand outward, engulfing Mercury, Venus, and possibly Earth. By developing methods to detect planetary engulfment in distant stars, astronomers gain insight into how common this process is across the universe and the various pathways it can take. For Kotten, who began this work as an undergraduate student in the Lamat Program at UC Santa Cruz, the research satisfies a childhood ambition. "When I was growing up, I dreamed about becoming a private investigator," she said. "I think that explains a lot about where I ended up. I do feel like a detective." The mystery of TOI-5882 is solved, but the larger questions—how many stars have consumed planets, and in how many ways can it happen—remain open.