Astronomers have made a groundbreaking discovery by detecting helium in the atmosphere of a rocky exoplanet located in the habitable zone. What this finding reveals about LHS 1140 b.
In this artistic representation, the exoplanet LHS 1140 b is prominently featured, enveloped 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 crucial leap forward. Astronomers have, for the first time, detected an atmosphere around a rocky exoplanet in the habitable zone; the spotlight is on LHS 1140 b, located about 49 light-years away. This finding positions the planet as one of the most significant targets for future searches for potentially life-supporting conditions beyond our solar system.
Led by Collin Cherubim from Harvard University, an international research team has published their results in the journal Science. The conclusive detection involved helium escaping from the planet’s upper atmosphere into space.
Profile of LHS 1140 b: Between Super-Earth and Water World
LHS 1140 b orbits a cool red dwarf star in the constellation Cetus. Examining its physical parameters reveals the characteristics of this intriguing object:
- Size: 1.73 times Earth’s radius
- Mass: approximately 5.6 times Earth’s mass
- Orbital period: nearly 24.7 days
- Energy: receives about 42% of the radiative energy that Earth gets from the Sun
The measured density suggests a predominantly rocky core; however, astrophysical models indicate that the planet likely possesses a component of lower density, potentially in the form of an extensive gas envelope or a significant water content.
Important Practical Limitation: Being classified in the “habitable zone” does not guarantee life-supporting conditions. This zone merely defines the distance range from a star where water could be liquid under suitable atmospheric pressure. The theoretical equilibrium temperature is around −47 °C, not accounting for atmospheric greenhouse effects or actual heat distribution.
Detection Method: How to Observe an Atmosphere 49 Light-Years Away
The researchers utilized the WINERED spectrograph on the 6.5 m Magellan-Clay Telescope at Las Campanas Observatory in Chile. On September 23, 2024, they observed the planetary system for 6.5 hours while LHS 1140 b transited in front of its star.
During such a transit, part of the stellar light passes through the planet’s atmosphere. The atoms within absorb specific wavelengths, leaving characteristic traces in the light spectrum. The team discovered three closely spaced absorption lines typical of metastable helium, among a total of 70 high-resolution images taken, focused around 10,833 Å.
The signal was detectable both before and after the actual transit, suggesting an extended gas envelope with a trailing and leading helium tail. This indicates that the planet is likely losing gas along its orbit at least intermittently.
Hydrodynamic Escape: Why the Planet Loses Gas
The detection of helium is attributed to processes in the upper atmosphere. Intense X-ray and extreme UV radiation from the red dwarf heats the gas. This results in a phenomenon known as hydrodynamic atmospheric escape, where gas expands to such an extent that some of it escapes the planet’s gravitational field.
Over billions of years, this leads to atmospheric fractionation:
- Light hydrogen escapes relatively easily into space.
- Heavier helium remains behind and accumulates in the upper layers.
- Heavier elements like oxygen, carbon, or nitrogen do not, as per calculations, get carried along with the helium flow. They may exist in deeper layers but have not yet been measured.
The Mystery of the Second Measurement: Fluctuating Signal Strength
In a follow-up measurement on September 29, 2025 — despite comparable or better data quality — no helium was detected.
To rule out software errors, the data was assessed using two independent programs, yielding consistent results: helium was present in 2024, absent in 2025. Scientists hypothesize that the intensity of the gas outflow varies. Changes in stellar activity (e.g., UV flares from the star) could push the signal below the detection threshold. Thus, while the atmosphere isn’t vanished, the outflow behaves dynamically.
Next Steps: Planned Measurements with the James Webb Telescope
The helium signal provides no 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 indicated the presence of a nitrogen-rich gas envelope, albeit with insufficient statistical significance.
LHS 1140 b is now part of the joint “Rocky Worlds” program involving JWST and Hubble. Upcoming investigative steps are already in place:
- MIRI (JWST): Measurement of secondary eclipses in the infrared to determine daytime temperatures and search for absorption features of carbon dioxide (CO2).
- Hubble: Detailed 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 detected. However, the pioneering spectroscopic evidence of an atmosphere around a rocky planet in a habitable zone lays the necessary groundwork for future measurements.
