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 (Teq=226 K) planet straddling the radius valley (Rp≈1.7R⊕
Nearby in the stack
), with the WINERED high-resolution spectrograph yielded a detection of planetary atmospheric escape through the measurement of excess absorption (
1.24±0.23%
) 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
1σ
. For LHS 1140 b, we derive
3σ
upper limits of
0.72
--
1.21%
. Individual visits disfavor the reported WINERED absorption at
2.6
--
3.7σ
, while their joint constraint disfavors a persistent signal at
4.5σ
. 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%
). 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.
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