Astronomers have detected an atmosphere around a rocky planet in another star’s habitable zone for the first time. The planet, LHS 1140b, orbits a quiet red dwarf in the constellation Cetus, and the signal that revealed its atmosphere came from helium escaping into space.
The finding addresses a question astronomers have been trying to answer for years: whether small rocky worlds orbiting red dwarfs can retain atmospheres for billions of years despite prolonged exposure to high-energy radiation.
Three important properties in one planet
Exoplanet researchers have long searched for rocky planets that orbit at temperatures where liquid water might be possible and that also possess atmospheres. Gas giants and larger, gas-rich planets have revealed atmospheres before, but detecting one around a small rocky world is much harder.
LHS 1140b is classified as a super-Earth. It has about 5.6 times Earth’s mass and roughly 1.7 times Earth’s radius. It circles its star every 24.7 days and receives about 42 percent as much stellar energy as Earth receives from the Sun.
Those measurements place it within the star’s habitable zone, the range of distances where temperatures could permit liquid water under suitable atmospheric conditions. That does not establish that oceans or life exist there. It does make the planet one of the strongest available targets for studying whether a temperate rocky world can retain air.
How helium revealed the atmosphere
The team used the WINERED infrared spectrograph on the Magellan Clay telescope at Las Campanas Observatory in Chile. They watched LHS 1140b pass in front of its star and examined how the planet affected the starlight reaching the telescope.
During the 2024 transit, the researchers detected absorption at wavelengths associated with metastable helium. The signal extended beyond the planet’s solid body, indicating that helium was flowing from its upper atmosphere.
The researchers interpret the observation as a hydrodynamic atmospheric outflow driven largely by the star’s X-ray and extreme-ultraviolet radiation. Their modeling suggests that the escaping upper atmosphere is rich in helium and severely depleted in hydrogen.
Heavier volatile compounds may remain trapped at lower altitudes because the observed flow is not powerful enough to carry many heavier atoms and molecules into space. The exact composition and pressure of the lower atmosphere, however, have not yet been measured.
The signal disappeared in 2025
When the researchers observed another transit of LHS 1140b in 2025, they did not detect the same helium absorption.
The team reprocessed the observations using different reduction methods and considered possible contamination from Earth’s atmosphere, changes in the star, and other sources of error. The researchers concluded that the 2024 signal was planetary, while the difference between the two observing periods points to time-variable atmospheric escape.
The mechanism behind that variation is not yet settled. Changes in stellar radiation or interactions involving the planet’s atmosphere, magnetic environment, and stellar wind could affect how much detectable helium surrounds the planet at a particular time.
Rather than weakening the discovery, the changing signal gives astronomers another phenomenon to investigate. Continued monitoring could reveal how the upper atmosphere responds to its environment and how quickly the planet is losing gas.
Why red dwarfs matter
Red dwarfs are the most common type of star in the Milky Way. They are smaller, cooler, and longer-lived than the Sun, which makes their nearby habitable zones attractive places to search for rocky planets.
They also create difficult conditions. Young red dwarfs can produce powerful flares and intense high-energy radiation capable of driving atmospheric escape. Because planets in their habitable zones orbit close to their stars, they can experience this radiation more intensely than Earth does.
LHS 1140 is relatively old and inactive for a red dwarf. The planet still receives more X-ray radiation than Earth, but the star’s present activity is low compared with many stars of the same class.
The survival of LHS 1140b’s atmosphere for at least several billion years shows that atmospheric loss around red dwarfs is not always total. Under some combinations of planetary gravity, composition, history, and stellar activity, a rocky world can keep a substantial reservoir of gas.
What the detection does and does not mean
The helium was detected in the planet’s extended upper atmosphere, where gas is escaping. It does not provide a direct measurement of conditions at the surface or identify the dominant gases close to the ground.
The finding therefore does not amount to evidence of life. It does not confirm an ocean, breathable air, oxygen produced by biology, or an Earth-like climate.
The team also examined LHS 1140c, a smaller planet closer to the same star, and found no helium signal around it. LHS 1140c receives substantially more stellar radiation, which may have caused it to lose an atmosphere or may make any remaining atmosphere harder to detect by this method.
LHS 1140b is expected to be tidally influenced by its close orbit, and climate studies have explored the possibility that it is synchronously rotating, with one side facing its star. Models have also proposed that it may contain a large quantity of water. Neither its precise rotation state nor the presence and distribution of surface oceans has been established by the helium observations.
The bigger picture
Thousands of exoplanets have been confirmed since the first discoveries in the early 1990s. Several rocky planets have been found within the habitable zones of their stars, but researchers have struggled to determine whether any of them retained substantial atmospheres.
LHS 1140b provides the first clear observational answer. At least one rocky planet in a habitable zone has held onto an atmosphere for billions of years while continuing to lose a relatively small amount of gas from its upper layers.
That result also demonstrates a new way to investigate rocky exoplanets. Detecting escaping helium from the ground may help astronomers identify other worlds with atmospheres even when current telescopes cannot directly characterize the gases closer to their surfaces.
Future observations can test whether the helium signal returns and determine how its strength changes. Space telescopes may also probe deeper atmospheric layers and search for heavier molecules, although separating planetary signals from stellar effects will remain challenging.
LHS 1140b is not a confirmed second Earth. It is larger, colder by simple equilibrium estimates, exposed to a different radiation environment, and still largely hidden from direct observation.
What astronomers now know is narrower but historically important: a rocky planet in another star’s habitable zone can retain an atmosphere for billions of years. The gas escaping from LHS 1140b is evidence that the atmosphere is there, and it may provide one of the best tools available for learning what lies beneath it.