Title: Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet Authors: R. Allart1*, V. Bourrier1, C. Lovis1, D. Ehrenreich1, J.J. Spake2, A. Wyttenbach1,3, L. Pino1,4,5, F. Pepe1, D.K. Sing2,6, A. Lecavelier des Etangs7 Affiliations: 1Observatoire Astronomique de l’Université de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Versoix, SwitZerland. 2 Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK. 3 Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands 4 Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova I-35122, Italy 5 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands 6Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA. 7Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis boulevard Arago, F-75014, Paris, France *
[email protected] 1 Abstract: Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature – neutral hydrogen at ultraviolet wavelengths – is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08±0.05%. Interpreting absorption spectra with 3D simulations of the planet’s upper atmosphere suggests it extends beyond 5 planetary radii, with a large scale height and a helium mass loss rate ≲ 3´105 g‧s−1.