Astronomers have detected helium in the atmosphere of a rocky planet within the habitable zone for the first time. What the discovery reveals about LHS 1140 b.
In this artistic representation, the exoplanet LHS 1140 b is depicted in the foreground, surrounded by a helium-rich atmosphere. A nearby rocky planet orbits the same cool red dwarf star in the distance.
Photo: Melissa Weiss/CfA
The quest for a second Earth has taken a significant leap forward. For the first time, astronomers have confirmed the presence of an atmosphere in a rocky exoplanet situated in the habitable zone. At the heart of this discovery is LHS 1140 b, a world located approximately 49 light-years away. This finding makes the planet one of the most important targets in the ongoing search for potentially habitable conditions beyond our solar system.
The international research team, led by Collin Cherubim from Harvard University, published their results in the journal Science. The specific detection was achieved through the analysis of helium that escapes from the upper atmosphere of the planet into space.
Between Super-Earth and Water World: The Profile of LHS 1140 b
LHS 1140 b orbits a cool red dwarf star located in the constellation Cetus. A glance at its physical parameters reveals the dimensions of this intriguing object:
- Size: 1.73 times the radius of Earth
- Mass: about 5.6 times the mass of Earth
- Orbital Period: approximately 24.7 days
- Energy: receives about 42% of the solar energy that Earth receives from the Sun
The measured density suggests a predominantly rocky core; however, astrophysical models indicate that the planet must possess a lower density component—potentially an extended gas envelope or a significant amount of water.
Important Practical Limitation: Placement within the “habitable zone” does not guarantee life-supporting conditions. This definition merely indicates the distance range from a star where water could exist in liquid form given the right atmospheric and pressure conditions. The theoretical equilibrium temperature is about −47 °C, and atmospheric greenhouse effects or actual heat distribution have not been accounted for yet.
The Measurement Method: How to “See” an Atmosphere 49 Light-Years Away
For their measurements, the researchers used the WINERED spectrograph at the 6.5-meter Magellan-Clay telescope at the Las Campanas Observatory in Chile. They monitored the planetary system over a period of 6.5 hours on September 23, 2024, while LHS 1140 b transited in front of its star.
During such a transit, a portion of the starlight passes through the planet’s atmosphere. The atoms within absorb specific wavelengths, leaving characteristic traces in the light spectrum. In total, the team identified three closely spaced absorption lines at around 10,833 Å, a pattern typical for metastable helium.
The signal was detectable both before and after the actual transit. The researchers interpret this as evidence of an extended gas envelope with a trailing and leading helium tail. The planet appears to lose gas along its orbit at least temporarily.
Hydrodynamic Outflow: Why the Planet Loses Gas
The detectable helium is a result of processes occurring in the upper atmosphere. The intense X-ray and extreme UV radiation from the red dwarf heats the gas, leading to a phenomenon known as hydrodynamic atmospheric outflow. This gas expands to such an extent that some escapes the planet’s gravitational field.
Over billions of years, this results in atmospheric fractionation:
- The lighter hydrogen escapes comparatively easily into space.
- The heavier helium remains and accumulates in the upper layers.
- Heavier elements like oxygen, carbon, or nitrogen are not significantly carried away by the helium flow. They might exist in deeper layers but haven’t been measured yet.
The Mystery of the Second Measurement: Variable Signal Strength
In a follow-up measurement on September 29, 2025—despite comparable or better data quality—helium could not be detected.
To rule out software errors, data were analyzed with two independent programs. The results remained consistent: helium was present in 2024 but not in 2025. Scientists’ most likely explanation is that the intensity of the gas outflow varies. Changes in stellar activity (e.g., UV flares from the star) can decrease the signal below the detection limit. Thus, the atmosphere hasn’t disappeared, but the outflow behaves dynamically.
What’s Next? Planned Measurements with the James Webb Telescope
The helium signal does not provide information about the lower atmosphere near the surface. Previous data from the James Webb Space Telescope (JWST) largely ruled out a pure hydrogen atmosphere and vaguely suggested a nitrogen-rich gas envelope, though without sufficient statistical significance.
LHS 1140 b is now part of the joint “Rocky Worlds” program involving JWST and Hubble. Upcoming research steps have already been scheduled:
- MIRI (JWST): Measurement of secondary eclipses in the infrared range to determine daytime temperatures and seek absorption features for carbon dioxide (CO2).
- Hubble: Accurate analysis of the UV radiation from the parent star to more accurately model atmospheric loss rates.
Conclusion: LHS 1140 b is not a second Earth. It is larger, more massive, and orbits a completely different type of star. Water or biological activity has not been confirmed. Nevertheless, the first spectroscopic detection of an atmosphere in a rocky planet located within a habitable zone lays the essential groundwork for future measurement campaigns.
