In a major milestone for exoplanet research, astronomers have detected helium escaping from the upper atmosphere of LHS 1140b, a rocky exoplanet located 49 light-years away. Led by Collin Cherubim of the University of Chicago, researchers observed the planet passing in front of its host star, marking the first time scientists have detected an atmosphere on a potentially habitable terrestrial world in a stellar habitable zone.
The Discovery of LHS 1140b and Its Helium Envelope
LHS 1140b has been the subject of intense astronomical investigation. The exoplanet is approximately 70% larger than Earth and orbits a faint red dwarf star with a 25-day period. Because it resides within the star’s habitable zone, its surface could theoretically maintain liquid water.
During a transit event in September 2024, researchers used the Magellan-Clay telescope in Chile to observe the planet passing its host star. The team detected an absorption signature caused by excited helium atoms in the planet’s upper atmosphere. According to the research, the exoplanet is bleeding roughly 220 tons of helium into space every second—a mass equivalent to a blue whale. High-energy radiation from the host star strips away this helium, shaping it into a long tail streaming behind the planet.
Because the red dwarf star is more than 3 billion years old, LHS 1140b appears to be in the final stages of shedding its primordial hydrogen and helium envelope. The research team noted that the planet now retains almost no hydrogen.
Transient Signatures and Competing Atmospheric Models
Initial data collection yielded a complex puzzle. When the researchers captured follow-up measurements one year later, the helium absorption signature had vanished entirely. Stellar wind and radiation variability directly impact how quickly helium escapes, as well as how much of that escaped gas becomes ionized enough to absorb starlight. Re-examining the original data set confirmed that the initial helium detection originated from LHS 1140b itself, pointing to a thick, helium-rich mantle.
Charles Cadieux of the University of Montreal, who previously studied LHS 1140b and found evidence suggesting an ocean beneath the atmosphere, urged caution regarding the fluctuating helium detections because the element is also present in stars. Cadieux noted that further observations are necessary to fully understand the dynamics of the habitable-zone world.
Previous observations using the James Webb Space Telescope ruled out a puffed-up atmosphere. The new helium detections appear to conflict with those findings unless the planet features a distinct, helium-rich upper layer sheltering heavier compounds below. Low surface temperatures on LHS 1140b could cause water and similar compounds to condense into clouds and rain back onto the surface.
Implications for Terrestrial Planet Evolution
The discovery opens a new window into understanding rocky worlds across the galaxy. Robin Wordsworth of Harvard University, who served as Cherubim’s doctoral advisor, noted that the team may have identified a class of celestial objects previously restricted to theory: helium worlds.
Investigating these helium-rich planets offers comparative data for our own solar system. While rocky planets in our solar system were once thought to have formed after primordial hydrogen and gas dissipated, a 2023 study published in Nature suggested that Earth might have also possessed an early hydrogen and helium envelope that it eventually lost.
Astronomers are continuing their investigations. A 500-hour observation program utilizing both the James Webb Space Telescope and the Hubble Space Telescope launched last summer to analyze the atmospheres of planets orbiting red dwarfs. Meanwhile, the James Webb Space Telescope has already gathered nine additional observational datasets targeting LHS 1140b.
