Determine the atmospheric chemistry and cloud state of ε Eridani b

Determine whether ε Eridani b’s atmosphere is in chemical equilibrium or disequilibrium and whether it is cloudy or clear, including the confidence with which clouds are expected to be present, in order to establish which atmospheric model is physically plausible.

Background

The JWST/NIRCam data contain a low-signal-to-noise feature near the predicted position of ε Eridani b. Its interpretation depends strongly on the atmospheric assumptions: chemical-equilibrium clear-atmosphere models assign greater significance to the feature, whereas disequilibrium and cloudy models more naturally treat the NIRCam measurement as an upper limit. The paper therefore leaves unresolved which chemistry and cloud prescription should receive greater physical weight.

Resolving this ambiguity is important because the inferred atmospheric metallicity, effective temperature, surface gravity, thermal flux, and predicted Roman Coronagraph reflected-light brightness all depend on the adopted atmospheric model. The authors indicate that additional observations are needed to distinguish among these possibilities.

References

Indeed, we have no information on whether the planet is in equilibrium or in disequilibrium, so we cannot say which model carries more weight in terms of what is physically possible for $ b. The same holds for the question of clouds: is the atmosphere cloudy or clear? At $\sim$1.1~Gyr we would expect some clouds to be present, but with what degree of confidence?

— An Updated Model for Epsilon Eridani b and Prospects for Imaging with the Roman Coronagraph  (2609.19131 - Llop-Sayson et al., 16 Sep 2026) in Section 4.1.1, “Is the low-SNR feature the planet?”