# Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-$\alpha$ line

## TL;DR

This study used Hubble observations to analyze the Lyman-$\alpha$ emission of TRAPPIST-1, revealing its stellar activity, hydrogen exospheres, and implications for planetary atmosphere retention and habitability.

## Contribution

First measurement of TRAPPIST-1's Lyman-$\alpha$ line, providing insights into stellar activity and potential exospheres affecting planetary atmospheres.

## Key findings

- TRAPPIST-1's Lyman-$\alpha$ line is the coldest measured for an exoplanet host.
- TRAPPIST-1 has similar X-ray emission to Proxima Centauri but lower Lyman-$\alpha$ emission.
- Inner planets may lose atmospheres within a few billion years due to XUV irradiation.

## Abstract

The TRAPPIST-1 system offers the opportunity to characterize terrestrial, potentially habitable planets orbiting a nearby ultracool dwarf star. We performed a four-orbit reconnaissance with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope to study the stellar emission at Lyman-$\alpha$, to assess the presence of hydrogen exospheres around the two inner planets, and to determine their UV irradiation. We detect the Lyman-$\alpha$ line of TRAPPIST-1, making it the coldest exoplanet host star for which this line has been measured. We reconstruct the intrinsic line profile, showing that it lacks broad wings and is much fainter than expected from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as Proxima Cen but a much lower Ly-$\alpha$ emission. This suggests that TRAPPIST-1 chromosphere is only moderately active compared to its transition region and corona. We estimated the atmospheric mass loss rates for all planets, and found that despite a moderate extreme UV emission the total XUV irradiation could be strong enough to strip the atmospheres of the inner planets in a few billions years. We detect marginal flux decreases at the times of TRAPPIST-1b and c transits, which might originate from stellar activity, but could also hint at the presence of extended hydrogen exospheres. Understanding the origin of these Lyman-$\alpha$ variations will be crucial in assessing the atmospheric stability and potential habitability of the TRAPPIST-1 planets.