Is the high-energy environment of K2-18b special?

¹ Max Planck Institute for Extraterrestrial Physics Giessenbachstrasse 1 85748 Garching, Germany
² Department of Astrophysics, University of Vienna Türkenschanzstr. 17 1180 Vienna, Austria
³ Leibniz-Institute for Astrophysics Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
⁴ Potsdam University, Institute for Physics and Astronomy Karl-Liebknecht-Str. 24/25 14476 Potsdam-Golm, Germany
⁵ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen Sand 1 72076 Tübingen, Germany
⁶ European Southern Observatory Karl-Schwarzschild-Straße 2 85748 Garching near Munich, Germany
⁷ Meteorologisches Institut, Ludwig-Maximilians-Universität München Munich, Germany
⁸ European Southern Observatory Alonso de Córdova 3107 Vitacura Región Metropolitana, Chile
⁹ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS Université Côte d’Azur Nice, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 October 2025
Accepted: 12 January 2026

Abstract

Context. The exoplanet K2-18 b lies near the radius valley that separates super-Earths and sub-Neptunes, marking a key transitional regime in planetary and atmospheric composition. The system offers a valuable opportunity to study how M-dwarf high-energy stellar radiation influences atmospheric stability and the potential for sustaining volatile species, which is especially important in the context of the upcoming ELT and its ANDES spectrograph.

Aims. This study characterizes the high-energy environment of K2-18 with X-ray observations from eROSITA, the soft X-ray instrument on the Spectrum-Roentgen-Gamma mission, Chandra, and XMM-Newton.

Methods. We derived a representative 0.2–2 keV X-ray flux with an APEC thermal plasma model fit with the Bayesian X-ray Analysis (BXA). With the observed X-ray flux from the exoplanet host star, we estimated the photoevaporative mass loss of exoplanet K2-18b using the energy-limited model. In addition, we examined the thermal structure of the system based on a hydrodynamic model.

Results. In a 100 ks XMM-Newton observation, we identified K2-18 as a very faint X-ray source with F_X = 10⁻¹⁵ erg s⁻¹ cm⁻² and an activity level of (Lx/Lbol) ∼10⁻⁵. A small flare was detected during the observation. The planet is irradiated by an X-ray flux of F_pl,X = 12 ± 3 erg s⁻¹ cm⁻².

Conclusions. The X-ray flux measurement of K2-18 gives important limitations for the atmospheric escape and photochemical modeling of its exoplanets. Despite its near orbit around an M-dwarf star, K2-18b’s low-activity-level environment suggests that it can retain an atmosphere, supporting recent tentative detections of atmospheres.