X-ray irradiation of three planets around Hyades star K2-136

TL;DR

This study investigates the X-ray irradiation effects on three planets around K2-136, modeling their photoevaporation and suggesting that two are rocky while one retains a small gaseous envelope.

Contribution

It provides the first detailed analysis of X-ray driven photoevaporation for planets around K2-136, combining XMM-Newton data with advanced evaporation models.

Findings

Planets b and d are likely rocky and stripped of atmospheres.

Planet c probably retains a small gaseous envelope.

X-ray irradiation significantly influences planetary atmospheric evolution.

Abstract

We study the X-ray irradiation and likely photoevaporation of the three planets around the star K2-136. These are the Earth-sized K2-136 b, the mini-Neptune K2-136 c, and the super-Earth K2-136 d. XMM-Newton observations of the star indicate an X-ray luminosity of $(1.18\pm0.1)\times10^{28}$ erg s$^{-1}$ in the range 0.15 to 2.4 keV, resulting in an activity of $L_X/L_{\rm bol}=(1.80\pm0.68)\times10^{-5}$. The evaporation past of the planets were modelled using the XUV stellar tracks by Johnstone et al. (2021), the energy limited mass loss formulation (Lecavelier des Etangs et al. 2007, Erkaev et al. 2007), and the thermal evolution formulation by Lopez & Fortney (2014). Our results suggest that planets b and d are most probably purely rocky and have been stripped of their envelopes. On the other hand, planet c likely still has an envelope consisting of 0.8% of the total mass, with its core being relatively large (2.3 $R_{\rm E}$).