On the feasibility of studying the exospheres of Earth-like exoplanets by Lyman-alpha monitoring. Detectability constraints for nearby M stars

TL;DR

This paper explores the potential for detecting Earth-like exoplanet exospheres via Lyman-alpha monitoring, demonstrating feasibility around nearby M stars with current or near-future space telescopes despite absorption challenges.

Contribution

It assesses the detectability of Earth analogues' exospheres around M stars using Lyman-alpha flux monitoring, highlighting the capabilities of 4-8 m and 2 m class space telescopes.

Findings

Detectability of Earth's exosphere from 1.35 pc with an 8 m telescope.

Feasibility of observing Earth-like planets around Proxima Centauri with a 2 m telescope.

Monitoring Lyman-alpha flux can reveal planetary atmosphere robustness under stellar activity.

Abstract

Observations of the Earth's exosphere have unveiled an extended envelope of hydrogen reaching further than 10 Earth radii composed of atoms orbiting around the Earth. This large envelope increases significantly the opacity of the Earth to Lyman-alpha (Lya) photons coming from the Sun, to the point of making feasible the detection of the Earth's transit signature from 1.35 pc if pointing with an 8~meter primary mirror space telescope through a clean line of sight (NH < 1e17 cm-2), as we show. In this work, we evaluate the potential detectability of Earth analogues orbiting around nearby M-type stars by monitoring the variability of the Lya flux. We show that, in spite of the interstellar, heliospheric and astrospheric absorption, the transit signature in M5 V type stars would be detectable with a dedicated Lya flux monitor implemented in a 4-8 m class space telescope. Such monitoring programs would enable measuring the robustness of planetary atmospheres under heavy space weather conditions like those produced by M-type stars. A 2-m class telescope, such as the World Space Observatory, would suffice to detect an Earth-like planet orbiting around Proxima Centauri, if there was such a planet or nearby M5 type stars.