Astronomers have reported the first detection of an atmosphere around a truly rocky exoplanet that orbits within its star’s habitable zone. The result marks a milestone for the search for life beyond the Solar System by showing that an Earth-like world can retain atmospheric gas over geological timescales.
The planet, LHS 1140 b, is located about 48 light-years away and circles a cool red dwarf star. Because the habitable zone is defined by the temperature range where liquid water could exist on a surface, the finding elevates this target from “potentially rocky” to “atmospherically active.”
The study, published in Science, uses observational evidence of helium escaping from the planet’s upper atmosphere. Helium loss is a direct physical signature: when atmospheric particles are accelerated beyond the planet’s escape energy, they can stream away and imprint subtle absorption features in starlight during transits.
To test theoretical expectations, the researchers employed the Warm Infrared Echelle (WINERED) Spectrograph on the Magellan Observatory in Chile. The approach hinged on a rare alignment: LHS 1140 b and another planet transited the same star on the same night, allowing the team to compare atmospheric signatures between worlds under nearly identical observing conditions.
In the resulting spectra, one planet showed no clear atmospheric evidence, while LHS 1140 b displayed the helium-related signal. The detection supports the interpretation that the planet maintains an atmosphere and that helium is gradually leaking into space.
Crucially, the helium atmosphere was not identified by assumption alone. It followed from a prior theoretical model predicting that LHS 1140 b could develop and sustain a helium-rich upper layer that escapes slowly. The observations provide the first strong empirical confirmation of that kind of atmospheric retention in a habitable-zone rocky planet.
The researchers estimate the atmosphere has likely persisted for more than three billion years. That longevity suggests the planet’s environment has remained stable enough for an atmosphere to endure, making it a compelling candidate for follow-up studies of surface conditions and possible ocean worlds.
Lead author Collin Cherubim emphasized that an atmosphere is essential for habitability as we understand it. He and collaborators now aim to determine the atmosphere’s broader composition and use their model to identify additional exoplanets with similar atmospheric escape signatures.
The work also points to a practical strategy for future surveys: monitoring escaping gases from ground-based telescopes may become a powerful way to characterize atmospheres on rocky planets that are otherwise difficult to probe.
