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

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.

Published in A&A as a Letter to the Editor

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Cited by in corpus (13)

• The MUSCLES Treasury Survey IV: Scaling Relations for Ultraviolet, Ca II K, and Energetic Particle Fluxes from M Dwarfs
• On the Age of the TRAPPIST-1 System
• Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets
• Early 2017 observations of TRAPPIST-1 with $\text{Spitzer}$
• Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars: the TRAPPIST-1 System
• A temperate exo-Earth around a quiet M dwarf at 3.4 parsecs
• The Stellar Activity of TRAPPIST-1 and Consequences for the Planetary Atmospheres
• Is pi Men c's atmosphere hydrogen-dominated? Insights from a non-detecton of Hy Ly-alpha absorption
• Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?
• Time-variable electromagnetic star-planet interaction: The TRAPPIST-1 system as an exemplary case
• Stellar energetic particles in the magnetically turbulent habitable zones of TRAPPIST-1-like planetary systems
• Transit Ly-$\alpha$ signatures of terrestrial planets in the habitable zones of M dwarfs
• Comparative Climates of TRAPPIST-1 planetary system: results from a simple climate-vegetation model