Astronomers have identified six red dwarf stars containing unusually high amounts of lithium, an element that should have vanished from stars of their age. The discovery points to a possible explanation: these stars may have recently swallowed rocky planets, leaving behind a rare chemical trace.
The finding emerged from a study of red dwarfs in stellar clusters, where stars form from the same cloud of gas and dust and typically share similar ages and chemical compositions. Finding evidence that a star has consumed a planet is rarely straightforward. The process generally destroys the victim and much of the direct proof. Astronomers must instead search for subtle clues hidden in a star’s chemistry, motion, or physical properties long after the event has taken place.
Lithium Stood Out Among Hundreds Of Stars
The research team, led by Robin Jeffriesof Keele University, analyzed observations from the Gaia-ESO Spectroscopic Survey (GES), a project that gathered detailed chemical information on thousands of stars across the Milky Way. As reported in Monthly Notices of the Royal Astronomical Society, the scientists focused on red dwarfs located in stellar clusters. These stars rapidly destroy lithium during their early evolution, meaning very little of the element should remain later in life.
After refining their sample to 318 red dwarfs capable of displaying a lithium signature, the researchers found six stars with lithium abundances far above expectations.
“We found that a few of the red dwarf stars we studied contained lithium, a chemical element that should not be there,” Jeffries said in a statement published by the Royal Astronomical Society. “Therefore even a small amount of lithium stands out clearly in these stars – a bit like throwing paint onto a blank canvas.”
The unusual chemical signature immediately drew attention. In mature red dwarfs, fresh lithium is generally interpreted as evidence that the element arrived after the star’s formation.
Red Dwarfs Provide An Unusual Laboratory
The search for signs that stars have consumed planets is known as necroplanetology. Much of this work has traditionally focused on stars nearing the end of their lives, where traces of engulfed planetary material can be easier to detect.
Red dwarfs offer a different perspective. They are smaller, cooler, and dimmer than stars like the Sun, and they burn their fuel at a much slower rate. While the Sun’s lifespan is estimated at about 10 billion years, red dwarfs can survive for tens of billions or even trillions of years.
The study highlights another characteristic that makes them particularly useful: their ability to destroy lithium quickly. Any lithium detected in their atmospheres therefore represents a relatively recent addition rather than a remnant from the star’s birth.
Researchers view this property as a powerful diagnostic tool. If lithium appears in an otherwise normal red dwarf, it can serve as a marker of a recent external event, including the possible ingestion of planetary material.
Alternative Theories Failed the Test
Before concluding that planets might be involved, the team examinedseveral alternative scenarios. One scenario suggested that the six stars might actually be younger objects that had entered established clusters later. Younger stars naturally retain more lithium. Data on stellar colors and motions, however, showed that all six stars were genuine members of their respective clusters.
The scientists also examined whether unusual stellar behavior could explain the observations. Rapid rotation and strong magnetic activity can affect how stars transport material internally and, in some cases, influence lithium depletion. The evidence pointed elsewhere. Rather than being unusually active, the six lithium-rich stars were among the slowest rotators in their clusters.
The final step involved testing whether a recent influx of lithium-rich material could reproduce the observed chemical signatures. The team’s models indicated that it could. The calculations showed that consuming roughly three to ten Earth masses of rocky material would be sufficient to generate the lithium levels measured in the stars. The team determined this interpretation aligns most closely with the observations.
