Do Planets Steal a Star's Lithium? A New Look at 450 Stars
Lithium (Li) is a delicate element that can be destroyed inside stars at relatively low temperatures. Because of this, scientists use the amount of lithium found in a star’s outer layers as a clue to its internal structure and history. There has been a long-standing question in astronomy: does the presence of planets around a star lead to more lithium depletion? Some earlier studies suggested that the formation of planets could affect how a star rotates and mixes material inside, causing it to lose more lithium. Other studies, however, argue that lithium levels mostly depend on a star’s natural properties, such as age and mass, and are unaffected by planetary systems. This paper, led by Jinxiao Qin and collaborators, aims to resolve this debate using a large and carefully selected sample of stars.
Building a Matched Sample of Stars
To compare stars fairly, the team used data from the Keck Observatory’s high-resolution spectrograph. They selected 279 planet-host stars (HS) from the California-Kepler Survey, which followed up on exoplanet discoveries from NASA’s Kepler mission. For the comparison group, they chose 171 isolated stars (IS) from earlier studies that had undergone long-term radial velocity monitoring—ensuring no planets had been detected. All stars had similar characteristics: they were FGK-type main-sequence stars (similar to the Sun), with effective temperatures between 4600 and 6700K and a wide but overlapping range of chemical compositions. Importantly, all observations were done using the same instrument, minimizing differences due to equipment or measurement techniques.
Measuring Lithium the Right Way
To find out how much lithium each star contains, the researchers used a method called spectral synthesis, focusing on lithium’s signature wavelength at around 6708 angstroms. They calculated the abundance of lithium, called A(Li), using both local thermodynamic equilibrium (LTE) and non-LTE approaches. Non-LTE calculations provide more accurate results in many cases, especially for stars with higher lithium levels. In fact, they found that non-LTE corrections became increasingly important when lithium abundance exceeded about 2.5 dex. They also estimated uncertainties by adjusting key stellar parameters (like temperature and metallicity) and studying how these changes affected lithium results.
What the Results Show
The big takeaway is that stars with planets and those without show very similar lithium abundances. Even in the narrow temperature range of 5600 to 5900K—where earlier studies had reported differences—the planet-hosting stars did not appear to be more lithium-depleted. The researchers also separated their stars into “evolved” and “unevolved” categories based on surface gravity. For both groups, the overall lithium patterns stayed the same regardless of whether planets were present. In unevolved stars, they did notice a bit more scatter in lithium values, but this was better explained by differences in stellar mass. Heavier stars tend to retain more lithium because their outer layers are not mixed as deeply.
Accounting for Activity Levels
One possible issue was whether the method of selecting isolated stars (based on lack of radial velocity variations) might bias the IS group toward low magnetic activity, which could in turn affect lithium readings. However, both groups showed similar levels of chromospheric activity, as measured by an index based on the Ca II H\&K spectral lines. Although a few stars in the 5600–5900K range did show higher activity—and tended to have higher lithium—these were not enough to skew the overall results. The authors argue that such stars are probably younger and haven’t had time to lose their lithium yet.
Comparisons with Previous Work
To put their findings in context, the team compared their results with two other large studies: one by Israelian et al. (2009), which had suggested lower lithium in planet-hosting stars, and another by Llorente de Andrés et al. (2024), which found no such difference. The new results agree more closely with the latter, showing that lithium depletion does not depend strongly on whether or not a star has planets. This larger and more carefully matched sample helps explain why earlier studies might have found different results—they may have relied on smaller, less uniform samples.
Conclusion: Planets Don't Drain Lithium
The authors conclude that there is no strong evidence that hosting planets causes stars to deplete more lithium. Instead, lithium abundance seems to depend mainly on a star’s internal properties and evolutionary stage. While a few stars show unusual lithium levels—especially if they are more active—these are likely due to age or mass rather than planetary effects. Looking ahead, the team suggests that even larger and more uniform samples will be needed to confirm these results across all types of stars. But for now, the mystery of lithium depletion appears to have less to do with planets than once thought.
Source: Qin
