A team led by the Center for Astrophysics at Harvard detected helium escaping from the atmosphere of LHS 1140 b, a rocky planet 48 light-years away in the constellation Cetus. The results, published in Science on July 17, represent the first confirmed atmosphere on any rocky exoplanet orbiting within a star's habitable zone. Thirty years of searching for air on a world that could hold liquid water ended not with a spectrum of what remains, but with a measurement of what is leaving.
LHS 1140 b is 5.6 times Earth's mass and 1.73 times its radius. It orbits a red dwarf every 24.7 days, receiving 42 percent of the stellar radiation Earth absorbs from the Sun. The equilibrium temperature is 226 kelvin, roughly minus 47 degrees Celsius. A sufficient greenhouse effect could raise surface temperatures above freezing, but that requires an atmosphere, and until this paper, no one had confirmed one existed.
The detection used the WINERED spectrograph on the Magellan Clay telescope at Las Campanas Observatory in Chile. When LHS 1140 b transits its star, helium atoms in the planet's upper atmosphere absorb starlight at a precise near-infrared wavelength: 10,833 angstroms. The absorption signature appeared in observations taken in 2024. The team repeated the measurement in 2025. The signal was gone.
The Fractionation
The upper atmosphere of LHS 1140 b is dominated by helium and depleted in hydrogen. Hydrogen, the lightest element, has already escaped. Helium, the next lightest, is escaping now. The atmosphere is stratified by loss. What sits at the surface, potentially nitrogen, potentially water vapor, is the residue after billions of years of selective winnowing. The 2024 JWST study using the NIRISS instrument ruled out hydrogen-rich atmospheres to more than ten sigma and found tentative evidence of a nitrogen-dominated lower atmosphere at 2.3 sigma. The ground-based helium detection confirms what JWST could only suggest.
The escape is hydrodynamic, driven by X-ray and extreme-ultraviolet radiation from the host star. The same stellar energy that defines the habitable zone is the force stripping the atmosphere away. The star is more than five billion years old. The atmosphere has survived at least that long, but the variable detection, present one year and absent the next, suggests the loss is not steady. It pulses. The atmosphere breathes, and each breath is thinner than the last.
The Proof
For three decades, exoplanet science accumulated partial evidence. Thousands of planets cataloged. Dozens in habitable zones. Atmospheres confirmed on gas giants and mini-Neptunes where thick hydrogen envelopes produce unmistakable spectral signatures. But the question that started the search, whether a rocky world at the right distance from its star could hold gas against the forces trying to strip it away, had no observational answer.
The answer arrived as a leak. Not a stable blanket of nitrogen detected in absorption, but helium streaming into space, caught at one wavelength in one transit observed from the ground with a spectrograph that cost a fraction of what JWST demanded. The $10 billion telescope provided the constraint. The telescope on the mountain provided the confirmation.
The discovery that closes the longest-standing gap in exoplanet science simultaneously opens a clock. LHS 1140 b has an atmosphere, and that atmosphere is on a timer. The helium that proved the air exists is the same helium proving the air is finite. The leak is the proof.