The mid-infrared spectrum of β Pictoris b

First VLTI/MATISSE interferometric observations of an exoplanet^★

¹ Univ. Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ LESIA, Observatoire de Paris, PSL, CNRS, Sorbonne Univ., Univ. de Paris, 5 place Janssen, 92195 Meudon, France
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Institute of Astronomy, Univ. of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Université Paris-Saclay, Univ. Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁸ Department of Physics & Astronomy, Johns Hopkins Univ., 3400 N. Charles Street, Baltimore, MD 21218, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Leiden Observatory, Leiden Univ., PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Vitacura, Santiago, Chile
¹² Univ. of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
¹³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁴ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
¹⁶ CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
¹⁷ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹⁸ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern Univ., Evanston, IL 60208, USA
¹⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

^★★ Corresponding author: mathis.houlle@univ-grenoble-alpes.fr

Received: 6 December 2024
Accepted: 25 August 2025

Abstract

Few spectra of directly imaged exoplanets have been obtained in the mid-infrared (>3 μm). This region is particularly rich in molecular spectral signatures, whose measurements can help recover atmospheric parameters and provide a better understanding of giant planet formation and atmospheric dynamics. In recent years, exoplanet interferometry with the VLTI/GRAVITY instrument has provided medium-resolution spectra of a dozen sub-stellar companions in the near-infrared. The 100 meter interferometric baselines enable the stellar and planetary signals to be efficiently disentangled at close angular separations (<0.3″). We aim to extend this technique to the mid-infrared using MATISSE, the VLTI’s mid-infrared spectro-interferometer. We take advantage of the fringe tracking and off-axis pointing capabilities recently brought by the GRA4MAT upgrade. Using this new mode, we observed the giant planet β Pictoris b in L and M bands (2.75–5 μm) at a spectral resolution of 500. We developed a method to correct chromatic dispersion and non-common path effects in the fringe phase and modelled the planet astrometry and stellar contamination. We obtained a high-signal-to-noise spectrum of β Pictoris b, showing the planet continuum in the L (for the first time) and M bands, which contains broad absorption features of H₂O and CO. In conjunction with a new GRAVITY spectrum, we modelled it with the ForMoSA nested sampling tool and the Exo-REM grid of atmospheric models, and found a solar carbon-to-oxygen ratio in the planet atmosphere. This study opens the way to the characterization of fainter and closer-in planets with MATISSE, which could complement the JWST at angular separations too close for it to obtain exoplanet spectra. Starting in 2025, the new adaptive optics system brought by the GRAVITY+ upgrade will further extend the detection limits of MATISSE.