# The XUV irradiation and likely atmospheric escape of the super-Earth $\pi$ Men c

## TL;DR

This study assesses the high-energy irradiation of exoplanet $ ext{ extpi}$ Men c, finding it likely experiences significant atmospheric escape due to intense XUV radiation, making it a prime candidate for atmospheric characterization.

## Contribution

First analysis of $ ext{ extpi}$ Men c's X-ray and UV irradiation levels, demonstrating potential for atmospheric escape and identifying it as an ideal target for atmospheric studies of super-Earths.

## Key findings

- $ ext{ extpi}$ Men has Sun-like X-ray emission levels.
- High-energy irradiation is 2000 times Earth's, driving atmospheric escape.
- $ ext{ extpi}$ Men c is four times brighter at Ly $ ext{ extalpha}$ than GJ 436.

## Abstract

$\pi$ Men c was recently announced as the first confirmed exoplanet from the TESS mission. The planet has a radius of just 2 R$_{\rm\oplus}$ and it transits a nearby Sun-like star of naked-eye brightness, making it the ideal target for atmospheric characterisation of a super-Earth. Here we analyse archival $\textit{ROSAT}$ and $\textit{Swift}$ observations of $\pi$ Men in order to determine the X-ray and extreme-ultraviolet irradiation of the planetary atmosphere and assess whether atmospheric escape is likely to be on-going. We find that $\pi$ Men has a similar level of X-ray emission to the Sun, with $L_{\rm X}/L_{\rm bol} = (4.84^{+0.92}_{-0.84})\times10^{-7}$. However, due to its small orbital separation, the high-energy irradiation of the super-Earth is around 2000 times stronger than suffered by the Earth. We show that this is sufficient to drive atmospheric escape at a rate greater than that readily detected from the warm Neptune GJ 436b. Furthermore, we estimate $\pi$ Men to be four times brighter at Ly $\alpha$ than GJ 436. Given the small atmospheric scale heights of super-Earths, together with their potentially cloudy atmospheres, and the consequent difficulty in measuring transmission spectra, we conclude that ultraviolet absorption by material escaping $\pi$ Men c presents the best opportunity currently to determine the atmospheric composition of a super-Earth.