Variable X-ray flux tied to helium escape on LHS 1140b

A study submitted to arXiv on September 25, 2026, proposes that variable X-ray flux from LHS 1140 explains time-variable helium escape from LHS 1140b, according to Collin Cherubim, Tim Cunningham and Jason A. Dittmann.

The study carries the arXiv identifier 2609.31859 and sits in the astro-ph.EP category, with the manuscript in second-round review at ApJL. The paper calls atmospheric retention on potentially habitable M-star worlds a central exoplanet-science goal, since such planets are readily observable but face prolonged high-energy radiation that can erode atmospheres. Collin Cherubim, Tim Cunningham and Jason A. Dittmann wrote the manuscript, which was posted on September 25, 2026.

Researchers pointed the XMM-Newton space telescope at LHS 1140 for an 18-hour observing window in 2026 to collect X-ray and ultraviolet flux readings, testing whether stellar X-ray and extreme-ultraviolet (XUV) flux variability explains the helium absorption changes seen during transits. LHS 1140b is a rocky planet in the habitable zone, and that star is inactive.

The X-ray output swung between maximum and minimum by Fmax/Fmin = 8.5+9.4−4.4 during the window, with no variability detected in UV or visible light. The 2026 integrated X-ray flux matches the 2018 measurement, which points to stable XUV flux across years-long timescales. The earlier observation could not detect the variability level seen in 2026, though models fit both datasets.

The authors judge that short-term XUV variability and/or small flares explain the helium variability better than long-term XUV variability, based on fitting models to the 2018 and 2026 data. They say confirmation needs simultaneous near-infrared transit readings together with stellar activity markers such as XUV flux. That recommendation comes from the model comparison and the detected short-term X-ray variability, and it lays out the observational path for validating the proposed mechanism.

The helium signal appeared in 2024 but was absent in 2025, showing the atmospheric escape process varies. EarthSky and Tech Times say the identical setup yielded no helium absorption in 2025. Modeling suggests moderate changes in stellar high-energy output or upper-atmosphere temperature could drop the signal below detectability while leaving the atmosphere intact.

On July 16, 2026, a Harvard-led team reported in Science that LHS 1140 b, a rocky exoplanet in its star’s habitable zone, has a detected atmosphere. LHS 1140 b is among the targets chosen for the Rocky Worlds Director’s Discretionary Time Program, a joint James Webb Space Telescope and Hubble Space Telescope effort to find atmospheres on rocky worlds around dwarf stars.

The planet is a super-Earth roughly 48 light-years away, measuring 1.73 Earth radii across with a mass of 5.6 Earth masses. EarthSky and Tech Times give the 48-light-year distance, while Phys.org puts it at approximately 40 light-years, leaving the figure disputed across sources. Tech Times places the planet in the constellation Cetus, on a 24.7-day orbit while absorbing 42 percent of the stellar radiation Earth gets. Those measurements suggest an Earth-like interior with an added lower-density layer, perhaps an atmosphere or abundant water.

EarthSky puts the equilibrium temperature at 116 degrees Fahrenheit, or 47 degrees Celsius, while Tech Times reports around minus 47 degrees Celsius. Tech Times, citing a CfA press release, calls LHS 1140 an old, quiet M-dwarf well into its main-sequence life at more than three billion years. Phys.org calls LHS 1140 b rocky and says its temperature is similar to Earth’s for this kind of star, while placing LHS 1140 c outside the habitable zone. Red dwarfs are the most common stellar type, and they remain active with ultraviolet radiation and coronal mass ejections longer than Sun-like stars, which makes atmosphere survival around one noteworthy.

University of Florida astronomy professor Jason Dittmann found LHS 1140 b in 2016 through ground-based surveys and a machine-learning algorithm that sorts planetary transits from Earth weather signals. Tech Times separately describes the planet as discovered in 2017 and labels it among the most studied habitable-zone exoplanet candidates.

With WINERED at the Magellan Clay Telescope in Las Campanas, Chile, Cherubim and colleagues tested a model placing helium escape in the planet’s upper atmosphere. Harvard Department of Astronomy head David Charbonneau said Cherubim forecast the helium atmosphere, arranged the observing time and secured a statistically rock-solid detection. The same technique had appeared only on gas giant exoplanets before.

During September 2024, the team spent 6.5 hours watching the LHS 1140 system and gathered 70 spectra, with 23 taken as the planet crossed in front of its star. The two planets crossed in front of their star on the same night during the Magellan Clay run, giving researchers a direct internal comparison.

Helium in an energetic state absorbs starlight at 10,833 angstroms, a near-infrared wavelength linked to a triplet of metastable-helium lines, the quantum state atoms enter after stellar radiation ionizes them and they recombine under stellar radiation. Using this approach on a rocky world demanded a precise theoretical target and an instrument sensitive enough to catch the faint signal.

The helium cloud stretched to 1.52 times the planet’s physical radius, showing a stream ahead of the planet and tentative signs of a trailing stream. The signal showed up before the planet’s disk crossed the star, pointing to the leading stream, and the team ruled out interference from Earth’s atmosphere and from stellar activity.

The planet gets between 2.7 and 16 times the X-ray flux Earth receives from the Sun, depending on the solar activity level used for comparison. A competing process called core-powered mass loss predicted a helium escape rate far below what observations show. The team’s models favor hydrodynamic outflow driven by X-ray and extreme-ultraviolet radiation over core-powered mass loss as the explanation for the measured escape rate.

LHS 1140 b and LHS 1140 c sit on opposite sides of the cosmic shoreline, the conceptual boundary between atmosphere-retaining rocky planets and radiation-stripped bare ones. LHS 1140 c displayed no helium and is presumably airless, while LHS 1140 b keeps an atmosphere. Because the method works from the ground, it requires no JWST scheduling time and provides a model-validated template for choosing other rocky habitable-zone planets for space-telescope characterization.

The atmosphere probably survived for more than three billion years and likely no longer contains primordial hydrogen. Because the helium reservoir endures despite continuous escape, a bulk atmosphere must be replenishing the upper layers. Other gases and water would remain below the top helium layer, suggesting a more complex, evolved atmosphere like solar-system rocky planets.

Researchers do not know the lower atmosphere’s composition, though oxygen, nitrogen, carbon dioxide and water would stay at lower altitudes, with water vapor likely condensing in a cold trap below. Jason Dittmann said X-ray radiation from the host star accounts for the helium loss rate, meaning the rocky planet must replenish its helium or none would remain.

Jason Dittmann said JWST observations over the next four to five years will search for water in LHS 1140 b’s atmosphere to test whether stable and persistent. A planet needs an atmosphere to support life as we know it.

A central goal in exoplanet science is learning the conditions under which potentially habitable worlds retain atmospheres. Planets in M-star habitable zones are easier to observe, yet they face prolonged high-energy radiation capable of eroding planetary atmospheres. Red dwarfs are the most common stellar type, and they remain active with ultraviolet radiation and coronal mass ejections longer than Sun-like stars, which makes atmosphere survival around one noteworthy.

LHS 1140 b holds the first confirmed atmosphere on a rocky planet in a stellar habitable zone.