The Helium Breath of a Rocky Planet: Atmospheric Escape from LHS 1140b ✛ ✦
Context
Studying exoplanet atmospheres of Earth-like worlds is key to understanding their habitability. Of particular interest are rocky planets in the habitable zones of red dwarfs, where intense stellar radiation can completely strip away the gaseous envelope, leaving a lifeless desert. The 'cosmic shoreline' concept divides worlds into those that have kept and those that have lost their atmospheres, and observations of such systems are crucial for testing it.
Methods
The team led by David Charbonneau used high-resolution spectroscopy with the WINERED spectrograph (Magellan Clay) in transit mode to observe the LHS 1140 system. A key element was applying the Doppler effect to isolate the planetary signal and construct transmission spectra. The analysis involved modeling the signal with Gaussian profiles and Gaussian processes to account for noise, as well as hydrodynamic modeling with the p-winds code.
Results
In 2024, an absorption line of helium was detected at a depth of 1.24%, along with a preceding tail indicating outflow. Models with the p-winds code yielded a mass loss rate of about 2·$10^8$ g/s, an outflow temperature of ~5160 K, and an H:He ratio of only ~$10^{-3}$. This points to a helium-dominated upper atmosphere, where water and other volatiles may be trapped at lower levels due to a cold trap. For the neighboring planet LHS 1140c, no signal was detected, consistent with its lack of an extended atmosphere. In 2025, the signal at LHS 1140b also did not appear, indicating variability in the escape, likely linked to stellar activity.
Implications
The results confirm that rocky planets around quiet red dwarfs can retain atmospheres, even if heavily fractionated. This shifts the proposed 'cosmic shoreline' in favor of the possible existence of secondary atmospheres. However, the variability of the escape shows that even old stars can periodically increase their impact on a planet, which must be accounted for in evolutionary models.
Future development
Further observations with high-resolution spectrographs, as well as with JWST, will make it possible to search for similar helium leaks on a larger number of exoplanets, statistically delineating the 'shoreline'. Three-dimensional hydrodynamic modeling that includes magnetic fields and X-ray emission will provide a more accurate picture of atmospheric evolution.
Impact
The discovery impacts our understanding of atmospheric evolution of rocky worlds, habitability criteria, and the interpretation of transit spectroscopy data from future missions.
Next steps
Long-term monitoring of LHS 1140b is planned to identify the connection between leak variability and stellar cycles, as well as searches for helium in other candidates using high-resolution spectroscopy.
Key open problems
The work is directly connected to the problem of the 'cosmic shoreline' — how and which planets retain atmospheres. It also touches on issues of fractionation and the role of helium in the evolution of planetary envelopes, which is important for interpreting data from future telescopes.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
Observations of LHS 1140b with WINERED/Magellan in 2024 and 2025 revealed variable helium absorption during transit, interpreted as outflow from a helium-dominated upper atmosphere with a mass loss rate of about 2×10^8 g/s and low hydrogen abundance. p-winds modeling indicated a cold trap for water and other volatiles in the lower layers.
Where its strength is
Direct detection of helium outflow from a rocky planet in the habitable zone and demonstration of its variability is a rare observational fact that allows studying the mechanisms of atmospheric retention and fractionation under stellar radiation.
Where it could go next
- It might be interesting to compare the detected outflow variability with models of planetary wind interaction with the stellar wind and magnetic field, as considered for the formation of double tails in hot Jupiters.based on: A Scorching Planet Grew Two Tails arXiv:2510.09809
- Related is the study of escaping atmospheres in the helium line, which analyzes the glow of an extended envelope; perhaps a similar approach could help estimate the large-scale structure of the outflow from LHS 1140b.based on: The Glow of an Escaping Atmosphere arXiv:2606.23899
- The question of whether volcanic degassing can sustain a hydrogen-rich secondary atmosphere has been examined for hot planets; for LHS 1140b, it would be interesting to assess the contribution of internal sources to the observed helium composition.based on: Hydrogen Atmospheres on Scorching Planets: The Volcano Secret arXiv:2510.07378
Worth trying
- Search for a correlation between the variability of the helium signal and indicators of stellar activity (flares, X-ray emission) to refine the mechanism modulating the outflow.
- Model the observed leading tail, accounting for radiation pressure and stellar wind, to distinguish between hydrodynamic outflow and magnetospheric interaction scenarios.
- Compare upper limits on hydrogen abundance with predictions from atmospheric evolution models that include fractionation and diffusive separation to assess the initial atmospheric composition.
Why this may matter
Confirmation of helium atmospheres on rocky planets in habitable zones of red dwarfs shifts the cosmic shoreline in favor of the possibility of volatile retention, and the variability of outflow underscores the need to account for episodic losses in habitability models.
Nearby works in our archive
- The Glow of an Escaping Atmospheresimilarity 0.65
- Hydrogen Atmospheres on Scorching Planets: The Volcano Secretsimilarity 0.65
- How a Planet's Breath Shapes Its Fatesimilarity 0.64
- A Scorching Planet Grew Two Tailssimilarity 0.62
- Dark, Chilled Planet Without Volcanoes: JWST’s Gazesimilarity 0.62
- Chemical Portrait of an Invisible Worldsimilarity 0.61
The closest work in the archive sits at 0.65 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2607.14326
Observations of highly irradiated gas giant exoplanets have shown helium escaping from their atmospheres. There is limited evidence for atmospheres on rocky exoplanets, perhaps because they have already escaped. We report spectroscopic observations of LHS 1140b, a rocky exoplanet that orbits in the habitable zone of a nearby low-mass star. The near-infrared transit spectra show absorption by helium escaping from the planet's atmosphere. Helium absorption is detected in 2024 but not in 2025, indicating time-variable atmospheric escape. We interpret these results as indicating an upper atmosphere dominated by helium and depleted in hydrogen, with other volatile species trapped at lower altitudes, consistent with atmospheric fractionation models. No helium absorption is detected for LHS 1140c, a smaller and more heavily irradiated exoplanet in the same system.
The upper atmosphere temperature of LHS 1140b reaches 5000 K, comparable to the surface of the Sun, despite the planet's cold climate (equilibrium temperature just 226 K).
Key numbers
- mass_loss_rate: 2.03 × 108 g/s (for SED GJ 1132)
- H_He_ratio: ~10-3
- absorption_depth_2024: 1.24%
- outflow_temperature: 5160 K
- X_ray_luminosity_ratio: L_X/L_bol = 6.5 × 10-6