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Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone

This paper reports the detection of time-variable helium escaping from the atmosphere of the habitable-zone rocky exoplanet LHS 1140b, suggesting a helium-dominated upper atmosphere with trapped volatiles, while no such signal was found for its smaller companion LHS 1140c.

Original authors: Collin Cherubim, Shreyas Vissapragada, Tim Cunningham, Annabella G. Meech, David Charbonneau, Robin Wordsworth, Aaron Householder, Johanna Teske, Leonardo A. Dos Santos, Nicole L. Wallack, William Mis

Published 2026-07-17

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CC BY 4.0

Original authors: Collin Cherubim, Shreyas Vissapragada, Tim Cunningham, Annabella G. Meech, David Charbonneau, Robin Wordsworth, Aaron Householder, Johanna Teske, Leonardo A. Dos Santos, Nicole L. Wallack, William Misener, Zifan Lin, Andrew McWilliam, Michael Zhang, Jason A. Dittmann, Mercedes López-Morales

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a vast, cosmic ocean where planets are like islands floating in space. For a long time, scientists have been trying to figure out which of these islands have "shields" around them—thick blankets of gas called atmospheres. These shields are crucial because they protect the surface from harmful space radiation, help keep water liquid, and regulate temperature, making life possible. While we've spotted these atmospheric blankets on giant, hot gas planets (like cosmic balloons), finding them on smaller, rocky worlds (like Earth) has been incredibly difficult. It's like trying to see a tiny, dim firefly buzzing next to a blindingly bright spotlight. The challenge is even harder around "red dwarf" stars, which are small and cool but often throw out fierce bursts of energy that can strip a planet's atmosphere away, leaving it a barren rock. The big question is: Can a rocky planet in the "Goldilocks zone" (where it's not too hot or cold) actually hold onto its atmosphere long enough to become a home for life, or does the star's energy blow it all away?

This paper tells the story of a detective team that finally caught a glimpse of an atmosphere on a rocky world named LHS 1140b. Using a powerful telescope equipped with a special "helium sniffer," the team watched this planet as it passed in front of its star. In 2024, they found something amazing: a faint signature of helium gas escaping from the planet's upper atmosphere, like steam rising from a hot pot. This wasn't just a steady leak; it was a dynamic event. The team saw a "leading tail" of gas stretching out in front of the planet and a "trailing tail" behind it, suggesting the atmosphere is being blown away by the star's wind. However, when they looked again in 2025, the helium was gone. This suggests the atmosphere is changing over time, perhaps because the star's energy output fluctuates.

The scientists also looked at a smaller neighbor in the same system, LHS 1140c, which gets hit by much more star energy. They found no helium there at all, suggesting that planet has likely lost its atmosphere completely. By analyzing the data, the team concluded that the atmosphere of LHS 1140b is very strange: it is mostly helium, with almost no hydrogen. This happens because the lighter hydrogen escapes first, leaving the heavier helium behind, while heavier elements like oxygen and water are trapped lower down. The paper rules out other explanations, such as the signal coming from the star itself or from Earth's atmosphere, confirming that this is a real, escaping planetary atmosphere. While the atmosphere is currently leaking, the fact that it still exists after billions of years suggests that rocky planets in these harsh environments might be able to keep their "shields" longer than we thought, offering a glimmer of hope for finding habitable worlds.

Technical Summary: Helium Escaping from the Atmosphere of a Nearby Rocky Exoplanet in the Habitable Zone

Problem Statement
While atmospheric escape has been well-documented in highly irradiated gas giant exoplanets, evidence for atmospheres on rocky exoplanets remains scarce. This scarcity may result from the complete loss of atmospheres over time, particularly around active M-dwarf stars where high-energy radiation drives atmospheric escape. The "cosmic shoreline" hypothesis suggests a boundary separating airless rocky worlds from those retaining atmospheres, yet observational constraints on rocky planets within habitable zones are limited. Detecting and characterizing the atmospheres of such planets is technically challenging due to the small size and brightness contrast between the planet and its host star.

Methodology
The authors conducted spectroscopic observations of the LHS 1140 system, which hosts two transiting rocky planets: LHS 1140b (a habitable-zone super-Earth) and LHS 1140c (a smaller, more heavily irradiated planet). Observations were performed using the WINERED spectrograph on the Magellan Clay telescope in September 2024 and September 2025.

Data Acquisition: In 2024, the team observed one transit of each planet in a single night, collecting 70 spectra. In 2025, an additional transit of LHS 1140b was observed.
Data Reduction: Spectra were reduced using the WINERED Automatic Reduction Pipeline (WARP) and an independent pipeline to ensure robustness. The data were continuum-normalized, corrected for telluric features (OH and H2O), and shifted into the stellar and planetary rest frames.
Analysis: The team searched for metastable helium absorption at the 10,833 Å triplet. They constructed time-series spectra and transmission spectra, modeling the helium feature using Gaussian profiles convolved with the instrumental line spread function. Correlated noise was modeled using a Gaussian Process (GP).
Physical Modeling: To interpret the observations, the authors utilized the p-winds code, a 1D hydrodynamic outflow model. This required an input stellar spectral energy distribution (SED). The team derived the host star's X-ray flux using archival XMM-Newton data and scaled the SEDs of two similar M dwarfs (GJ 1132 and GJ 699) to match LHS 1140's X-ray luminosity.
Comparative Analysis: The study included a non-detection analysis for LHS 1140c and a comparison of the 2024 and 2025 datasets for LHS 1140b to investigate temporal variability.

Key Results

Detection of Helium in 2024: The 2024 observations of LHS 1140b revealed a statistically significant absorption feature near 10,833 Å. The excess absorption depth was measured at $1.24^{+0.22}_{-0.23}\%$ for the blended red peaks and $0.25^{+0.14}_{-0.12}\%$ for the blue peak. The feature exhibited a Doppler shift of $2.0^{+2.0}_{-2.2}$ km s $^{-1}$ relative to the planet's velocity.
Temporal Variability: The helium absorption was not detected in the 2025 observations of LHS 1140b, nor in the 2024 observations of LHS 1140c. The non-detection in 2025 is consistent with the data's detection limit ( $\le 0.6\%$ ), suggesting the escape rate or atmospheric conditions varied significantly between the two epochs.
Atmospheric Composition: Modeling indicates the escaping wind is dominated by helium with a hydrogen-to-helium (H:He) atomic number ratio of $\sim 1 \times 10^{-3}$ . The low hydrogen abundance is necessary to prevent the attenuation of XUV radiation, which is required to ionize helium and produce the observed metastable state.
Outflow Characteristics: The inferred mass-loss rate is $2.03^{+0.67}_{-0.58} \times 10^8$ g s $^{-1}$ (based on the GJ 1132 SED), driven primarily by XUV irradiation rather than core-powered mass loss. The outflow temperature is estimated at $\sim 5160$ K.
Trailing and Leading Tails: Evidence suggests the presence of a leading tail (pre-ingress absorption) and tentative evidence for a trailing tail (post-egress absorption), indicative of interaction with stellar winds or magnetic fields.
Heavy Element Retention: The calculated escape rate is insufficient to drag heavier species (mass $\ge 9$ amu, such as atomic oxygen) into the outflow. This implies that while the upper atmosphere is helium-rich, heavier volatiles like water, carbon, and nitrogen may be retained at lower altitudes, potentially trapped by a cold trap at the tropopause.

Significance and Claims
The paper reports the first spectroscopic detection of escaping helium from a rocky exoplanet orbiting within the habitable zone of a nearby M dwarf. This finding challenges the assumption that habitable-zone rocky planets around M dwarfs are necessarily airless or have completely lost their primordial atmospheres.

The results support the "cosmic shoreline" hypothesis, as LHS 1140b (retaining an atmosphere) and LHS 1140c (showing no atmospheric signature) lie on opposite sides of the proposed boundary defined by irradiation and escape. The detection of a helium-dominated, hydrogen-depleted upper atmosphere provides observational evidence for atmospheric fractionation, where hydrodynamic escape strips lighter hydrogen while leaving heavier helium behind.

Furthermore, the time-variable nature of the detection (present in 2024, absent in 2025) highlights the dynamic nature of atmospheric escape in these systems, potentially driven by stellar activity cycles or variations in XUV flux. The study concludes that while the upper atmosphere of LHS 1140b is being stripped, the planet may still retain a bulk atmosphere rich in heavier volatiles, maintaining conditions that could be relevant for habitability.

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