2025ApJ...992...58H

Identifying Earth-like planets outside our solar system is a leading research goal in astronomy, but determining if candidate planets have atmospheres, and more importantly if they can retain atmospheres, is still out of reach. In this paper, we present our study on the impact of enhanced extreme-ultraviolet (EUV) flux on the stability and escape of the upper atmosphere of an Earth-like exoplanet using the Global Ionosphere and Thermosphere Model. We also investigate the differences between 1D and 3D solutions. We use a baseline case of EUV flux experienced at the Earth, and multiply this flux by a constant factor going up to 50. Our results show a clear evidence of an inflated and elevated ionosphere due to enhanced EUV flux, and they provide a detailed picture of how different heating and cooling rates, as well as the conductivity, are changing at each EUV flux level. Our results also demonstrate that 1D solutions are limited in their ability to capture a global atmosphere that is not uniform. We find that a threshold EUV flux level for a stable atmosphere occurs around a factor of 10 times the baseline level, where EUV fluxes above this level indicate a rapidly escaping atmosphere. This threshold EUV flux translates to about 0.3 au for a planet orbiting the Sun. Thus, our findings indicate that an Earth-like exoplanet orbiting its host star in a close-in orbit is likely to lose its atmosphere quickly.