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Origin of visit-to-visit spectral discrepancies in JWST observations of K2-18 b

Ascertain whether the visit-to-visit discrepancies observed in JWST transmission spectra of K2-18 b arise from astrophysical variability, instrumental systematics, or both, by analyzing detector-level offsets and repeated NIRSpec/NIRISS transits to identify the dominant source of the variability.

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Background

The authors combined multiple JWST visits (NIRSpec G235H/G395H and NIRISS/SOSS) but noted inconsistencies between visits, which can affect retrieval results and the interpretation of molecular detections.

They proposed a ‘shifted average’ approach to mitigate detector-level offsets and highlighted the need for further observations to understand whether the discrepancies are driven by stellar activity, instrument effects, or both.

References

In addition to directly combining repeated visits, we noticed visit-to-visit discrepancies in the spectra. The origin of these discrepancies remains uncertain, potentially arising from astrophysical variability, instrumental systematics, or a combination of both.

— A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST (2507.12622 - Hu et al., 16 Jul 2025) in Section 3.1 (Spectral Retrievals: ExoTR retrievals)