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

Abstract

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 ($T_{\rm eq}=226$ K) planet straddling the radius valley ($R_{\rm p}\approx1.7 R_\oplus$), with the WINERED high-resolution spectrograph yielded a detection of planetary atmospheric escape through the measurement of excess absorption ($1.24\pm0.23\%$) 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 $1\sigma$. For LHS 1140 b, we derive $3\sigma$ upper limits of $0.72$--$1.21\%$. 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 ($f=22_{-12}^{+17}\%$). 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.