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

TL;DR

Is the high-energy environment of K2-18b special? The paper characterizes the host star's high-energy irradiation using multi-mission X-ray data, finding a faint X-ray source with $L_X/L_{bol} \sim 10^{-5}$. It integrates Bayesian differential-emission-measure coronal modeling with an energy-limited mass-loss framework and VPLanet simulations to assess whether K2-18b can retain its atmosphere over Gyr timescales. The results indicate the present-day XUV flux is low enough to permit a hydrogen-rich atmosphere to persist, though uncertainties in early stellar activity and EUV conversions remain relevant. These constraints refine atmospheric escape and photochemical modeling for temperate sub-Neptunes around M-dwarfs and inform planning for future observations with ELT/ANDES and next-generation XUV missions.

Abstract

K2-18b 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, especially important in the context of the upcoming ELT and its ANDES spectrograph. 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 (SRG) mission, Chandra, and XMM-Newton. We derive a representative 0.2-2 keV X-ray flux with an APEC thermal plasma model fitted with the Bayesian X-ray Analysis (BXA). With the observed X-ray flux from the exoplanet host star, we estimate the photo-evaporation mass loss of exoplanet K2-18b using the energy-limited model. In addition, we examine the thermal structure of the system based on a hydrodynamic model. In 100 ks XMM-Newton observations we identified K2-18 as a very faint X-ray source with $\mathrm{F_X = 10^{-15}\ erg\,s^{-1}\,cm^{-2}}$, with an activity level of (Lx/Lbol) $\sim 10^{-5}$. A small flare has been detected during the observation. The planet is irradiated by an X-ray flux of $\mathrm{F_{pl,X} = 12\pm3\ erg\,s^{-1}\,cm^{-2}}$. The X-ray flux measurement of K2-18 gives important limitations for 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.

Figures (10)

• Figure 1: Orbital positions of the K2-18 planets (colored circles) are shown relative to the habitable zone (HZ) Kopparapu2013. K2-18b lies near the inner edge of the conservative HZ. The yellow dotted line marks the Recent Venus limit, while the region between the cyan dash-dotted line and the conservative HZ corresponds to orbital distances where water would likely remain frozen. The white regions beyond all HZ boundaries represent environments where surface liquid water is not expected to persist.
• Figure 2: The K2-18 source detected on PN-CCD, MOS1, and MOS2 is marked by the black dashed circle region of 20 arcsec with its coordinates, while the dark region below corresponds to a quasar. The image is displayed with a smoothing parameter of 1.5$\sigma$. Note that the efficiency/sensitivity of MOS2 is lower than MOS1 Mineo2024. In PN, the source lies in a chip gap, while it is unobstructed in MOS1 & MOS2.
• Figure 3: K2-18 XMM-Newton light curves from left to right: PN, MOS1, and MOS2, showing quasi-quiescent activity. The yellow background marks the good time interval (GTI) from each detector. Two light curves are plotted with different time binning (grey 0.5 ks bin and brown 1.5 ks bin).
• Figure 4: X-ray spectrum of K2-18 analyzed over the 0.2–2.0 keV range, extracted from the PN-CCD (top), MOS1 (middle), and MOS2 (bottom) on XMM-Newton during a 100 ks observation. The black solid line is the spectral model through BXA-Plasma, the orange dashed line is the source model, and the grey dotted line is the background model with PCA routine in BXA.
• Figure 5: The reconstructed continuous Differential Emission Measure (DEM) distribution from MOS1. The black curve shows the median of the posterior DEM predictions, and the gray band contains 68% of the distribution. The colorful dotted lines are ten random posterior samples of possible DEM distributions. Note that the kT values from PN and MOS2 are not well constrained and are not shown here.