Letter to the Editor

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

V. Detection of sodium on the bloated super-Neptune WASP-166b^⋆

¹ Observatoire Astronomique de l’Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
e-mail: julia.seidel@unige.ch
² University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Lund Observatory, Department of Astronomy and Theoretical Physics, Lunds Universitet, Solvegatan 9, 222 24 Lund, Sweden
⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
⁵ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
⁷ Department of Physics, University of Warwick, Coventry, UK
⁸ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile

Received: 26 May 2020
Accepted: 3 July 2020

Abstract

Planet formation processes or evolution mechanisms are surmised to be at the origin of the hot Neptune desert. Studying exoplanets currently living within or at the edge of this desert could allow disentangling the respective roles of formation and evolution. We present the High Accuracy Radial velocity Planet Searcher (HARPS) transmission spectrum of the bloated super-Neptune WASP-166b, located at the outer rim of the Neptune desert. Neutral sodium is detected at the 3.4σ level (0.455 ± 0.135%), with a tentative indication of line broadening, which could be caused by winds blowing sodium farther into space, a possible manifestation of the bloated character of these highly irradiated worlds. We put this detection into context with previous work claiming a non-detection of sodium in the same observations and show that the high noise in the trace of the discarded stellar sodium lines was responsible for the non-detection. We highlight the impact of this low signal-to-noise ratio remnant on detections for exoplanets similar to WASP-166b.