Astronomers have detected helium escaping from the atmosphere of LHS 1140 b, a rocky, Earth-size planet sitting inside the habitable zone of its nearby M dwarf star. The finding, reported in Science, represents the first confirmed atmosphere on a temperate terrestrial world beyond our solar system. LHS 1140 b orbits roughly 40 light-years from Earth and was first identified as a transiting planet in 2017, but until now no team had produced direct spectroscopic proof that the planet holds gas above its surface.
Why a helium signal on LHS 1140 b changes the search for habitable worlds
Rocky planets orbiting M dwarf stars receive intense bursts of extreme ultraviolet and X-ray radiation, especially early in the star’s life. That bombardment strips away lighter gases, and most models predicted that Earth-size worlds in these systems would be left barren. The detection of helium streaming away from LHS 1140 b upends that expectation: the planet is actively losing atmosphere, yet enough gas persists to produce a clear spectroscopic signature during transit.
The result also introduces a natural test case. LHS 1140 c, a second terrestrial planet orbiting the same star, showed no helium absorption in the same observing campaign, according to the preprint detailing the 2024 observations. Because both planets share a host star but receive different levels of irradiation, the contrast between them points to a relationship between cumulative radiation exposure and atmospheric survival. If the helium escape rate scales inversely with the total XUV dose a planet has absorbed since its protoplanetary disk dispersed, then similar M-dwarf planets with lower lifetime irradiation should retain detectable helium envelopes at comparable surface gravities. That prediction is directly testable with the same near-infrared transit technique on other targets in the next few years.
Transit spectroscopy, helium absorption, and what the data show
The core evidence comes from near-infrared transit spectroscopy targeting the helium triplet absorption line at 10,833 angstroms. When LHS 1140 b passed in front of its star during 2024 transits, observers recorded a dip in starlight at that wavelength consistent with helium gas in the planet’s upper atmosphere. The team interprets the signal as a helium-dominated layer depleted in hydrogen, with other volatiles potentially present at lower altitudes, per the Science paper.
Earlier work using JWST’s NIRISS instrument had already attempted to characterize LHS 1140 b’s atmosphere through transmission spectroscopy. That effort, detailed in a separate study, found tentative evidence consistent with a nitrogen-rich atmosphere but flagged significant stellar contamination from faculae on the host star’s surface, which complicated the retrieval. The new helium detection sidesteps some of that contamination problem because the helium triplet line sits at a wavelength less affected by the star’s active regions.
The planet’s bulk properties add another layer of complexity. LHS 1140 b was originally characterized as rocky based on its mass and radius when it was discovered as a transiting super-Earth. Subsequent refinements of those measurements, however, suggest the planet may contain a low-density component such as a water layer or a thin volatile envelope. That ambiguity means the helium signal could be escaping from a primarily rocky body with a secondary atmosphere, or from a world with a more substantial water or gas component beneath the escaping helium.
Competing interpretations and the next observations to watch
Three lines of evidence now point in partially different directions. The helium detection indicates a gas layer exists and is escaping. The JWST/NIRISS data hint at nitrogen-dominated air closer to the surface. And the revised mass-radius analysis leaves open whether LHS 1140 b is a bare rocky planet with a thin atmosphere, a water world, or something in between. None of these findings directly contradict each other, but they have not yet been reconciled into a single coherent picture of the planet’s full atmospheric structure.
The raw near-infrared transit spectra and data reduction pipelines from the 2024 helium observations have not been deposited in public archives outside the preprint. Updated interior models incorporating the latest mass-radius posteriors also remain unpublished in open repositories. Until those datasets are available for independent reanalysis, the community is working from the team’s reported results rather than from fully reproducible data products.
What comes next is straightforward in principle but demanding in practice. Additional JWST transits at different wavelengths could distinguish between a nitrogen-rich lower atmosphere and a water-vapor-dominated one. Ground-based high-resolution spectrographs can monitor the helium escape rate over multiple epochs to determine whether it varies with stellar activity cycles. And applying the same helium triplet technique to LHS 1140 c and to other temperate rocky planets around M dwarfs will test whether the relationship between radiation history and atmospheric retention holds beyond this single system. For anyone tracking the search for habitable conditions on nearby exoplanets, the next round of scheduled JWST observations of LHS 1140 b is the development to follow.
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*This article was researched with the help of AI, with human editors creating the final content.
