The curious case of the Sun’s missing lithium

In our latest analysis, now published as a Research Note of the AAS, we focus on a star we know well: the Sun. When it comes to lithium, though, the Sun doesn't behave like its stellar siblings.

This work builds directly on our earlier study of lithium in FGK-type stars (see also our blog post summarising that work). There, we demonstrated that lithium depletion is primarily determined by three parameters: effective temperature, metallicity, and stellar age (with mass excluded due to high collinearity). Our model captured broad trends across stellar populations, but when applied to the Sun, it flagged our nearest star as an outlier.

Using a statistical survival model designed to account for upper limits in lithium measurements, we compared the Sun's lithium abundance to over 1100 Sun-like stars from the Gaia-ESO Survey. Based on its temperature, metallicity, and age, we’d expect the Sun to have a photospheric lithium abundance of about 1.71 dex. But its actual value is just 0.96 dex: a discrepancy far too large to be explained by measurement uncertainty alone!

1. The physics of lithium destruction

The key to understanding lithium depletion lies in stellar convection. In Sun-like stars, the outer convective zone circulates material from the photosphere down into hotter interior regions. Lithium is destroyed at temperatures above ~2.5 million K, so the depth and efficiency of this mixing directly affect how much lithium survives. That makes lithium an unusually sensitive probe of stellar interiors and their evolution over time (see the illustration showing the solar inner structure below).

Fig. 1: Sun's interior and lithium destruction. Cross-section diagram of the Sun's interior, illustrating the core, radiative zone, and convective zone with directional flow arrows showing energy and heat transfer processes. The base of the convective zone is shown for reference (Basu & Antia, 1997). The variation of the convective layer impacts lithium destruction; the larger the convective layer, the more effective the lithium destruction is, as it leads to higher temperature regions. Image licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

2. What makes the Sun different?

Several hypotheses could explain why the Sun's convection destroys more lithium than predicted:

1. Deeper convection zone: Enhanced early rotation or magnetic activity could have temporarily extended the Sun's convective envelope, exposing more lithium to destruction.

2. Lack of planetary engulfment: Some Sun-like stars may replenish their surface lithium by engulfing planetary material. The Sun shows no evidence of such events, which could partly explain its lower abundance.

3. Elemental composition effects: The Sun has an unusual distribution of refractory elements compared to similar stars. These differences may influence opacity and internal structure in subtle ways that affect convective mixing and lithium depletion.

This result isn’t just a curiosity. The Sun is a fundamental reference point in stellar astrophysics. If it doesn’t fit our models, that’s a clue that something important is missing in our understanding of stellar interiors. We're continuing to explore this anomaly with new data and refined models, and we invite others to join the investigation.

For more details, see our newly accepted RNAAS paper: https://iopscience.iop.org/article/10.3847/2515-5172/adf29c.