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

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${}_{\oplus }$ 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 $\text{ROSAT}$ and $\text{Swift}$ 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 $L_{\mathrm{X}}/L_{\mathrm{bol}}=(4.84_{-0.84}^{+0.92})\times 10^{-7}$. 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.

arXiv comment5 pages, 3 figures, 3 tables; accepted for publication in MNRAS Letters

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