LHS 1140 b has the first confirmed atmosphere on any rocky planet in a habitable zone

Helium is escaping from LHS 1140 b at a rate that tells astronomers exactly what they needed to know: the super-Earth orbiting a red dwarf 48 light-years away in the constellation Cetus has an atmosphere. That detection — made via near-infrared spectroscopy at the Magellan Clay telescope in Chile — is the first confirmed atmosphere on any rocky planet in another star’s habitable zone, closing a question that exoplanet science has been unable to answer for two decades.

The result does not mean LHS 1140 b is inhabited. It means the planet passed the most basic prerequisite for surface habitability: holding onto gas against the forces trying to strip it away. Until now, scientists had found atmospheres on gas giants and a handful of sub-Neptunes, but never on a solid world of the type considered most likely to support life.

How they found it

The technique that confirmed LHS 1140 b’s atmosphere had a specific history. Detecting helium escaping from exoplanet upper atmospheres via near-infrared spectroscopy was developed for gas giants, where the signal is large. Applying it to a rocky planet required a precise target and an instrument sensitive enough to resolve a far weaker signal.

The Harvard & Smithsonian team led by Collin Cherubim chose LHS 1140 b because it orbits a red dwarf — a class of star that bombards its planets with ultraviolet and X-ray radiation intense enough to erode atmospheres. If helium were escaping, the Magellan Clay’s WINERED Spectrograph had a plausible chance of detecting it. What gave the team an additional edge was geometry: on a single night, both LHS 1140 b and an adjacent planet transited their host star simultaneously, allowing the team to compare the two signals in parallel and isolate the helium signature from stellar background noise.

The signal matched predictions for a helium-rich upper atmosphere under sustained stellar bombardment. No competing explanation fit the data. Cherubim summarized the result without ambiguity: “This is the first time anyone has found an atmosphere on a rocky planet in the habitable zone of another star.”

What the atmosphere contains — and doesn’t

The detection identifies helium in the upper atmosphere specifically. It does not characterize the bulk composition at lower altitudes. Helium is detectable via this escape method precisely because it is light enough to leak upward when heated by stellar radiation; heavier gases stay lower and leave no detectable imprint on the transit signal at this instrument’s sensitivity.

Whether the lower atmosphere contains nitrogen, carbon dioxide, or water vapor — the greenhouse gases that would determine surface temperature and the prospect of liquid water — cannot be established from this dataset. An atmosphere dominated by helium with minimal greenhouse contribution would produce surface temperatures far below what LHS 1140 b’s orbital position alone would imply. An atmosphere with significant CO₂ or N₂ could push temperatures toward habitable range. The difference between a frozen rock and a water world lies in those as-yet-unmeasured lower layers.

Three billion years of survival

LHS 1140, the host red dwarf, is approximately 30% the mass of the Sun. Red dwarfs emit intense radiation during an extended pre-main-sequence phase lasting hundreds of millions to over a billion years — a period hostile enough to strip thin atmospheres entirely. The fact that LHS 1140 b retains a detectable atmosphere implies it either started with a very large atmospheric reservoir, was resupplied over time by volcanic outgassing, or both.

The team estimates the atmosphere has persisted for more than three billion years. For comparison, Earth’s current oxygen-rich atmosphere is itself less than 2.5 billion years old. The previous consensus held that rocky planets orbiting red dwarfs were poor candidates for long-term atmosphere retention precisely because of this radiation exposure. LHS 1140 b contradicts that consensus with direct observational evidence.

The planet’s position in the habitable zone of LHS 1140 means it receives energy roughly comparable to what Earth receives from the Sun — when averaged across its orbital geometry. That energy balance is a necessary condition for liquid water, assuming an atmosphere thick enough to prevent its immediate evaporation or freezing.

What this discovery doesn’t settle

The uncertainties remaining are substantial. The planet’s interior composition is still contested. Previous mass and radius measurements are consistent with either a rocky super-Earth or an ocean world — a planet whose bulk is partially composed of water ice — with different implications for surface conditions and habitability. Its bulk density sits between what pure rock would produce and what a volatile-saturated composition would suggest.

The temperature profile of the atmosphere is unknown. The helium escape signal constrains the upper atmosphere, not the lower levels where surface climate is determined. A helium-rich outer layer could be capping a much thinner atmosphere than the signal implies, or a much thicker one.

Replication will also be difficult. The simultaneous transit of two planets around LHS 1140 provided an unusual calibration opportunity; without that geometric coincidence, the background noise makes independent repeat measurements harder to interpret cleanly. A second confirmation from a different technique — or from the James Webb Space Telescope’s transmission spectroscopy — would significantly strengthen the case.

Finally, the star’s own flare activity continues to bombard LHS 1140 b with energetic particles. Whether the current atmospheric mass is in steady-state equilibrium or in slow ongoing depletion cannot be determined from a single-epoch detection.

Common questions about LHS 1140 b

Could LHS 1140 b support life? The atmosphere confirmation establishes one necessary condition for surface habitability, not a sufficient one. Scientists need to measure whether the lower atmosphere includes greenhouse gases, whether it sustains surface pressure high enough for liquid water, and whether any liquid water exists on the planet’s surface. LHS 1140 b is now the highest-priority target in habitable-zone exoplanet research; it is not yet a confirmed candidate for life.

Why does detecting helium confirm an atmosphere? Helium escaping from a planet implies a continuous atmospheric reservoir. Without a substantial atmosphere, there would be no helium supply to sustain a detectable escape rate over geological time. The signal is indirect, but it is well-constrained: the observed rate matches atmospheric escape models with no competing explanation currently proposed by the community.

How far away is LHS 1140 b? The planet is 48 light-years from Earth. At that distance, no spacecraft built today or envisioned for the near future could reach it. All measurements of LHS 1140 b come from light — the imprint the planet leaves on the spectrum of its host star as it passes in front of it during transit.

What is the habitable zone? The habitable zone around a star is the orbital distance range where a planet with an Earth-like atmosphere could maintain liquid water on its surface. It is defined by the star’s luminosity and the planet’s albedo. Being in the habitable zone is a necessary but not sufficient condition for habitability — a planet’s atmosphere, interior, and magnetic field all also matter.

The next step is transmission spectroscopy with the James Webb Space Telescope during Cycle 4, targeting the lower atmospheric composition of LHS 1140 b. Those observations are scheduled for 2027.

Reference: Cherubim et al., “Helium Escape Detection Confirms an Atmosphere on the Habitable-Zone Super-Earth LHS 1140 b,” Science, 2026. DOI: 10.1126/science.aea9708

Tags: astronomy, James Webb Space Telescope, exoplanet, habitable zone, LHS 1140, LHS 1140 b