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An Atmosphere Around a Rocky Neighbor
Astronomers have detected an atmosphere surrounding LHS 1140b, a rocky exoplanet orbiting within the habitable zone of a nearby M dwarf star. The findings, published in Science on July 16, confirm that the planet possesses a molecular fingerprint consistent with a gas-rich envelope rather than a barren, airless surface.
Defying Stellar Radiation
The discovery settles a long-standing debate over whether rocky planets orbiting volatile M dwarf stars can retain their gaseous layers. Astronomers long feared that the intense radiation and harsh stellar flares typical of these stars would strip such planets bare. Yet, observations using the Magellan Clay telescope at the Las Campanas Observatory reveal that LHS 1140b has defied these odds.
By tracking the planet as it transited its host star, researchers identified a specific molecular signature of excess helium absorbing starlight. This signature serves as a clear proxy for a gaseous envelope streaming away from the planet, confirming the atmosphere’s existence.
A Super-Earth’s Exotic Composition
LHS 1140b is a “super-Earth,” boasting a mass 5.6 times that of our planet and a radius 1.73 times larger. Its density profile suggests a composition that could support either a thick, gaseous atmosphere or a global liquid water ocean. Because the planet sits at a distance where surface temperatures could potentially allow for liquid water, it has emerged as a primary target for astrobiological study.
Unlike “mini-Neptunes,” which are rapidly shedding their gases, LHS 1140b is losing mass at a significantly slower rate. The research team calculates the planet has maintained its atmosphere for more than 3 billion years, with projections indicating it will remain stable for at least another billion.
The Search for Life-Supporting Chemistry
While the initial detection is confirmed, the scientific community is now turning to the James Webb Space Telescope to sharpen the focus. Future observations aim to quantify the helium content and hunt for other molecular components, such as nitrogen or carbon dioxide, to map the planet’s chemical makeup.
For Laura Kreidberg of the Max Planck Institute for Astronomy, the discovery is unexpected. The atmospheric thickness observed on LHS 1140b exceeds what is typically anticipated for a rocky planet. Researchers now classify it as a unique, “exotic” world that challenges existing models of planetary evolution. Confirming these atmospheric features remains a critical step in determining whether such environments can truly host conditions conducive to life.
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