A team using the James Webb Space Telescope (JWST) looked for helium in the atmosphere of the nearby planet LHS 1140 b and found none. Four transits observed with the NIRISS/SOSS instrument between 2023 and 2026 showed no absorption at the wavelength of the metastable helium triplet (1.0833 micrometers). This result conflicts with an earlier ground‑based claim of a clear helium signal during one transit.

LHS 1140 b is an important target because it is relatively large for a small planet: about 5.60 ± 0.19 times the mass of Earth and 1.730 ± 0.025 times the Earth’s radius, and it orbits its cool host star every 24.7 days. Those properties make it a promising place to look for an atmosphere. The ground‑based study reported a 1.24% absorption in the helium triplet, which should have appeared as roughly a 600 parts‑per‑million (ppm) signal in the lower‑resolution SOSS data. The JWST program measured the planet in four separate transits and had the sensitivity needed to detect a signal at that level, but saw nothing.

What the team did was focus on the narrow wavelength region around 1.07–1.095 micrometers where the metastable helium feature appears. Two of the JWST transits were archival (December 1 and December 26, 2023) and two were new (August 10, 2025 and July 23, 2026). All four visits used the SUBSTRIP256 detector configuration and standard readout patterns for high‑precision time series. The authors report that the ground‑based best‑fit helium model is statistically inconsistent with each JWST visit at greater than three sigma.

Why this matters: metastable helium is one of the clearest signs that a planet is losing gas from its upper atmosphere. A confirmed helium signal on LHS 1140 b would support ideas that the planet might have a helium‑rich envelope, or that it started with hydrogen and lost much of it, leaving heavier gases behind. That distinction matters for understanding whether this and similar planets are “mini‑Neptunes,” “water worlds,” or rocky planets with thin atmospheres. LHS 1140 b has already been debated: some interior and JWST studies favor a high‑mean‑molecular‑weight atmosphere or a water‑rich world rather than a hydrogen‑dominated mini‑Neptune.