Pi Mensae c Atmosphere

The XUV irradiation and likely atmospheric escape of the super-Earth π Men c
https://arxiv.org/abs/1901.01875

Quote :

π Men c was recently announced as the first confirmed exoplanet from the TESS mission. The planet has a radius of just 2 R⊕ 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 ROSAT and Swift observations of π 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 π Men has a similar level of X-ray emission to the Sun, with LX/Lbol=(4.84+0.92−0.84)×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 π Men to be four times brighter at Ly α 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 π Men c presents the best opportunity currently to determine the atmospheric composition of a super-Earth.

No Lyα detected:

García-Muñoz et al. "Is π Men c's atmosphere hydrogen-dominated? Insights from a non-detection of H I Lyα absorption"
https://arxiv.org/abs/1912.06913

Follow-up simulation indicates that escaping material from a hydrogen-dominated atmosphere would be detected.

Shaikhislamov et al. "Three-dimensional hydrodynamic simulations of the upper atmosphere of π Men c: comparison with Lyα transit observations"
https://arxiv.org/abs/2006.06959

Quote :

Since estimates indicate that the wind strength of π Men is close to solar (or even stronger; Vidotto & Cleary 2020), our simulations clearly suggest that it is very unlikely π Men c hosts an atmosphere dominated by hydrogen and helium, in agreement with the considerations of Gandolfi et al. (2018) and García Muñoz et al. (2020). Therefore, we argue that, despite the rather low bulk planetary density, π Men c’s atmosphere cannot be strongly hydrogen-dominated and that should thus contain a non-negligible amount of heavier elements, as suggested by García Muñoz et al. (2020). Future observations should focus on looking for elements, such as He, C, and O, to shed more light on the planetary atmospheric composition. Furthermore, additional observations are needed to check that the available measurements have not been made at non-typical and rare stellar conditions of either very low SW density or high XUV flux.