Letter to the Editor

The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b^⋆

¹ Observatoire Astronomique de l’Université de Genève, 51 Chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: Leonardo.dosSantos@unige.ch
² Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera, 2850 Concepción, Chile
³ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, Université Paris 6 Boite Postale 99, 75252 Paris Cedex 05, France

Received: 16 December 2019
Accepted: 13 January 2020

Abstract

K2-18 b is a transiting mini-Neptune that orbits a nearby (38 pc), cool M3 dwarf and is located inside its region of temperate irradiation. We report on the search for hydrogen escape from the atmosphere K2-18 b using Lyman-α transit spectroscopy with the Space Telescope Imaging Spectrograph instrument installed on the Hubble Space Telescope. We analyzed the time-series of fluxes of the stellar Lyman-α emission of K2-18 in both its blue- and redshifted wings. We found that the average blueshifted emission of K2-18 decreases by 67% ± 18% during the transit of the planet compared to the pre-transit emission, tentatively indicating the presence of H atoms escaping vigorously and being blown away by radiation pressure. This interpretation is not definitive because it relies on one partial transit. Based on the reconstructed Lyman-α emission of K2-18, we estimate an EUV irradiation in the range 10¹ − 10² erg s⁻¹ cm⁻² and a total escape rate on the order of 10⁸ g s⁻¹. The inferred escape rate suggests that the planet will lose only a small fraction (< 1%) of its mass and retain its volatile-rich atmosphere during its lifetime. More observations are needed to rule out stellar variability effects, confirm the in-transit absorption, and better assess the atmospheric escape and high-energy environment of K2-18 b.