Timescales of TRAPPIST-1 stellar variability affecting transmission spectroscopy

Ascertain the characteristic timescales of the different stellar variability phenomena on the TRAPPIST-1 M8V host star that impact JWST/NIRSpec PRISM transmission spectroscopy of TRAPPIST-1 e, so that the effectiveness of Gaussian-process-based stellar contamination mitigation can be validated and improved using transits separated by varying intervals.

Background

The transmission spectra of TRAPPIST-1 e observed with JWST are strongly impacted by stellar contamination due to time-varying surface inhomogeneities on the ultracool M-dwarf host. The authors employ a Gaussian process (GP) framework to mitigate this contamination but acknowledge uncertainty regarding how well it removes stellar signals versus planetary signals.

To robustly distinguish stellar variability from planetary atmospheric features and to validate the GP approach, knowledge of the timescales over which different stellar phenomena (e.g., spots, faculae, granulation, flares) evolve is needed. The authors propose comparing transits separated by different lengths of time to identify variability timescales, but note that these timescales are presently unknown.

References

To break this degeneracy, we can perform a second test comparing transits separated by different lengths in time to identify stellar variability on different timescales (as we do not yet know the timescale(s) to expect for every kind of stellar variability).

— JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e  (2509.05407 - Glidden et al., 5 Sep 2025) in Section 5.4 (Caveats of Stellar Contamination Correction and the Path Forward)

However, it remains unknown whether fully convective M dwarfs can exhibit such variability on short timescales, and therefore whether the W24 signal could be attributed to stellar variability.

— No Persistent Helium Absorption in LHS 1140 b: Four JWST/NIRISS SOSS Transits and Multi-epoch Stellar He I Variability  (2608.19120 - Gressier et al., 19 Aug 2026) in Section 4.4, “Helium variability in LHS 1140”