Helium Escape Reveals Atmosphere on Rocky Habitable-Zone Planet 48 Light-Years Away
Astronomers have detected helium escaping from LHS 1140 b, a rocky super-Earth orbiting within the habitable zone of a small red star about 48 light-years away. The signal is the first clear evidence of an atmosphere around a rocky planet in the habitable zone of another star. It closes the question of whether the planet has any air while leaving open what that air is made of.
The finding, reported on 16 July in Science by Collin Cherubim of Harvard University and an international team, does not reveal what the lower atmosphere is made of, whether liquid water is present, or whether the planet is habitable. The result is one detection, not a final word. The helium appeared strongly in observations made in 2024 but was not recovered when the team observed the planet again in 2025. The authors interpret that difference as possible variability in the escaping gas.
LHS 1140 b passes in front of its star once every 24.7 days, allowing starlight to filter through any surrounding gas during a transit. Cherubim and his colleagues used the WINERED near-infrared spectrograph on the 6.5-metre Magellan Clay Telescope at Las Campanas Observatory in Chile. A rare alignment in September 2024 let them observe LHS 1140 b and the system's hotter inner planet, LHS 1140 c, crossing the star on the same night. The spectrum of planet b contained helium absorption at high altitude, extending beyond the planet's solid radius. Planet c showed no comparable signal.
The team concluded that helium is escaping from the upper atmosphere as high-energy radiation from the star heats the gas. LHS 1140 b is thought to be more than three billion years old, so detectable helium at this age implies a continuing atmospheric reservoir. Current measurements put the planet at about 1.73 Earth radii and 5.6 Earth masses. Its stronger surface gravity improves its chance of retaining heavier atoms and molecules even while helium leaks into space. The host star, LHS 1140, is an old, comparatively quiet M dwarf at the low end of the X-ray output measured for its type, less hostile than many active red dwarfs.
The authors say the result changes the earlier discussion of the planet. An airless surface is now much harder to reconcile with the observations, while a heavier lower atmosphere and possible water layer remain open. A 2024 analysis suggested water could account for roughly 9 to 19 per cent of the planet's mass, and climate models allow an ocean near the point that permanently faces the star. Earlier James Webb Space Telescope observations ruled out a thick hydrogen-rich mini-Neptune atmosphere at high confidence and contained tentative, 2.3-sigma evidence consistent with nitrogen, too weak to count as a detection. One researcher described the planet as an exotic weirdo that defies prior expectations. There is no evidence of life, and no evidence yet of liquid water.
LHS 1140 b is included in the joint Hubble and Webb Rocky Worlds programme. Future transits can test whether the helium signal returns and search for carbon dioxide, water vapour and other constituents lower in the atmosphere. A second helium detection would address the quiet 2025 observation, and repeated Webb measurements will be needed to distinguish a nitrogen-rich atmosphere from other heavy compositions.