Variable X-ray flux may explain variable helium escape from the habitable-zone exoplanet LHS 1140b

Abstract

A central goal in exoplanet science is to understand the conditions under which potentially habitable worlds retain atmospheres. This question is of special interest for worlds orbiting M stars, whose habitable-zone planets are more readily observable, but which also emit prolonged high-energy radiation that can erode planetary atmospheres. For one such world, LHS 1140b, a rocky planet orbiting in the habitable zone of an inactive star, time-variable helium absorption has been observed as the planet transited in front of its host star, consistent with atmospheric escape. We present X-ray/ultraviolet flux observations with the XMM-Newton space telescope, which serve to test whether the observed helium absorption variability can be explained by stellar XUV flux variability. We detect X-ray flux variability with an amplitude of $F_\mathrm{max}/F_\mathrm{min} = 8.5^{+9.4}_{-4.4}$ within the 18-hour observing window. We do not detect variability in the UV or visible flux. The integrated X-ray flux is fully consistent with the X-ray flux measured in 2018, suggesting stable XUV flux over years-long timescales. We find that the 2018 observation was not sensitive to the level of variability observed in 2026. We explore several models that fit both datasets and conclude that short-term XUV variability and/or small flares are more likely to explain the helium variability than long-term XUV variability. Simultaneous observations of the planet-transit in the near-infrared and stellar activity indicators like XUV flux are critical to confirm the link between the observed helium signal variability and the observed X-ray variability.