VLT/CRIRES+ observations of warm Neptune WASP-107 b

Challenges in detecting molecules with ground-based transmission spectroscopy of cooler and cloudy exoplanets

¹ Observational Astrophysics, Department of Physics and Astronomy, Uppsala University, Sweden
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
⁶ Exzellenzcluster Origins, Boltzmannstrasse 2, 85748 Garching bei München, Germany
⁷ Namzitu Astro, 31130 Quint-Fonsegrives, France
⁸ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁹ Instituto de Astrofísica de Andalucía – CSIC, c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁰ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

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Received: 13 July 2025
Accepted: 24 April 2026

Abstract

Context. The atmospheres of transiting exoplanets can be studied spectroscopically using space-based or ground-based observations. Each has its own set of strengths and weaknesses, so there are benefits to both approaches. This is especially true for more challenging targets such as cooler, smaller exoplanets whose atmospheres most likely contain many molecular species and cloud decks.

Aims. We aim to study the atmosphere of the warm Neptune-like exoplanet WASP-107 b (T_eq ≈ 740 K). Several molecular species have been detected in this exoplanet in studies using the space-based JWST, and we aim to confirm and expand upon these detections by using the ground-based VLT and evaluating how well our findings agree with previously retrieved atmospheric parameters.

Methods. We observed two transits of WASP-107 b with VLT/CRIRES⁺ and created cross-correlation templates of the target atmosphere based on results from previous studies. We created different templates to investigate the impact of varying volume mixing ratios of species and the inclusion or exclusion of clouds. Considering this target’s observational challenges, we created simulated observations prior to evaluating our real data in order to assess our expected detection significances with the cross-correlation technique.

Results. We report cross-correlation signals of two molecular species, CO (~6σ) and H₂O (~4.5σ), in our K_p − v maps. This supports previous space-based detections and demonstrates, for the first time, the capability of VLT/CRIRES⁺ to detect species in targets cooler than hot Jupiters using transmission spectroscopy. We show that our analysis is sensitive to the inclusion of clouds but less so to different volume mixing ratios. Interestingly, our signal maximum deviates notably but consistently from its expected location in the K_p direction of our maps, and we speculate on the possible reasons for this effect. We demonstrate that the error budget for these relatively cool exoplanets is severely reduced in comparison to hotter exoplanets and emphasise the need for further work in the context of high-resolution spectroscopy.