NIRPS detection of delayed atmospheric escape from the warm and misaligned Saturn-mass exoplanet WASP-69 b^★

¹ Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
² Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ Observatoire du Mont-Mégantic, Québec, Canada
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada (Madrid), Spain
⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁸ Department of Physics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁹ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, Brazil
¹⁰ Department of Physics & Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
¹¹ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
¹² Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 0E8, Canada
¹³ Centre Vie dans l’Univers, Faculté des sciences de l’Université de Genève, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland
¹⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁶ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁷ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁸ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
¹⁹ Bishop’s Univeristy, Department of Physics and Astronomy, Johnson-104E, 2600 College Street, Sherbrooke, QC, J1M 1Z7, Canada
²⁰ Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON, Canada
²¹ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
²² Departamento de Física da Faculdade de Ciências da Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
²³ Centre of Optics, Photonics and Lasers, Université Laval, Québec, Canada
²⁴ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²⁵ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
²⁶ Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, Fortaleza, Brazil
²⁷ European Southern Observatory (ESO), Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
²⁸ Planétarium de Montréal, Espace pour la Vie, 4801 av. Pierre-de Coubertin, Montréal, Québec, Canada
²⁹ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
³⁰ York University, 4700 Keele St, North York, ON M3J 1P3, Canada
³¹ Herzberg Astronomy and Astrophysics Research Centre, National Research Council of Canada, Canada
³² University of British Columbia, 2329 West Mall, Vancouver, BC V6T 1Z4, Canada
³³ Western University, Department of Physics & Astronomy and Institute for Earth and Space Exploration, 1151 Richmond Street, London, ON N6A 3K7, Canada
³⁴ Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio, s/n Guimar, 38500 Tenerife, Canarias, Spain
³⁵ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
³⁶ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles, 08193, Spain
³⁷ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
³⁸ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, Nice, France

^★★ Corresponding author: romain.allart@umontreal.ca

Received: 8 October 2024
Accepted: 17 December 2024

Abstract

Context. Near-infrared high-resolution échelle spectrographs unlock access to fundamental properties of exoplanets, from their atmospheric escape and composition to their orbital architecture, which can all be studied simultaneously from transit observations.

Aims. We present the first results of the newly commissioned ESO near-infrared spectrograph, Near-InfraRed Planet Searcher (NIRPS), from three transits of the well-studied warm Saturn WASP-69b. Our goals are to measure the orbital architecture of the planet through the Rossiter-McLaughlin (RM) effect and its atmospheric escape through the 1083 nm helium triplet.

Methods. We used the RM Revolutions technique to better constrain the orbital architecture of the system. We extracted the high-resolution helium absorption profile to study its spectral shape and temporal variations. Then, we made 3D simulations from the EVE code to fit the helium absorption time series.

Results. We measure a slightly misaligned orbit for WASP-69 b (3D spin-orbit angle of 28.7_−5.3^+6.1 ∘). We confirm the detection of helium with an average excess absorption of 3.17±0.05% (maximum of 4.02%). The helium absorption is spectrally and temporally resolved, extends to high altitudes and has a strong velocity shift up to −29.5±2.5 km s⁻¹ 50 minutes after egress. The signature cannot be explained by a thermosphere alone and thus requires 3D modeling of the thermosphere and exosphere. EVE simulations put constraints on the mass loss of 2.25 · 10¹¹ g s⁻¹ and hint at reactive chemistry within the cometary-like tail and interaction with the stellar winds that allow the metastable helium to survive longer than expected.

Conclusions. Our results suggest that WASP-69 b is going through a transformative phase of its history by losing mass while evolving on a misaligned orbit, similar to a growing number of Neptunian worlds. This work shows how combining multiple observational tracers such as orbital architecture, atmospheric escape, and composition is critical to understand exoplanet demographics and their formation and evolution. We demonstrate that NIRPS in the near-infrared can reach precisions similar to HARPS in the optical for RM studies, and the high data quality of NIRPS leads to unprecedented atmospheric characterization. Therefore, the addition of NIRPS to HARPS on the ESO 3.6 m makes it the driving force of such new studies. The high stability of NIRPS combined with the large Guaranteed Time Observation (GTO) available for its consortium enables in-depth studies of exoplanets as well as large population surveys.