No Persistent Helium Absorption in LHS 1140 b: Four JWST/NIRISS SOSS Transits and Multi-epoch Stellar He I Variability

The search for atmospheres on temperate terrestrial planets is important for understanding how these objects form and evolve, and their potential habitability. Recent transit observations of LHS 1140 b, a temperate ($T_{\mathrm{e}\mathrm{q}}=226$ K) planet straddling the radius valley ($R_{\mathrm{p}}\approx 1.7R_{\oplus }$), with the WINERED high-resolution spectrograph yielded a detection of planetary atmospheric escape through the measurement of excess absorption ($1.24\pm 0.23\mathrm{\%}$) in the metastable helium triplet, although a later second visit resulted in a non-detection. We analyze four transits of LHS 1140 b and two of LHS 1140 c observed with JWST/NIRISS SOSS. We find no evidence of helium absorption in either planet, with all amplitudes consistent with zero within $1\sigma$. For LHS 1140 b, we derive $3\sigma$ upper limits of $0.72$–$1.21\mathrm{\%}$. Individual visits disfavor the reported WINERED absorption at $2.6$–$3.7\sigma$, while their joint constraint disfavors a persistent signal at $4.5\sigma$. We find that the stellar He I line of LHS 1140 varies substantially between NIRPS and WINERED epochs, with both its strength and fractional variability consistent with the behavior of other M dwarfs. We also identify a moderate correlation between the 2024 He I depth and seeing, suggesting a possible seeing-dependent instrumental contribution. If the high-resolution detection is indeed planetary, the rate of such atmospheric loss events must be relatively low ($f=22_{-12}^{+17}\mathrm{\%}$). Alternatively, stellar He I variability may contribute to the reported excess absorption. Additional high-resolution observations, both in and out of transit, are required to distinguish between these scenarios.