The ExoGRAVITY survey: A K-band spectral library of giant exoplanet and brown dwarf companions

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
² LESIA, Observatoire de Paris, PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Janssen, 92195 Meudon, France
³ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁴ Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁷ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Department of Physics, University College Cork, Cork, Ireland
¹⁰ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
¹³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
¹⁴ Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
¹⁵ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
¹⁶ Astronomy Department, University of Michigan, Ann Arbor, MI 48109, USA
¹⁷ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
¹⁸ CENTRA – Centro de Astrofísica e Gravitaçâo, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹⁹ Départment d’astronomie de l’Université de Genève, 51 ch. des Maillettes Sauverny, 1290 Versoix, Switzerland
²⁰ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
²¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
²² STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
²³ Department of Astrophysical & Planetary Sciences, JILA, Duane Physics Bldg., 2000 Colorado Ave, University of Colorado, Boulder, CO 80309, USA
²⁴ 1. Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
²⁵ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
²⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, United Kingdom
²⁷ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
²⁸ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
²⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
³⁰ Advanced Concepts Team, European Space Agency, TEC-SF, ESTEC, Keplerlaan 1, NL-2201, AZ Noordwijk, The Netherlands
³¹ University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
³² Center for Space and Habitability, University of Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
³³ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
³⁴ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
³⁵ The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
³⁶ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
³⁷ Excellence Cluster ORIGINS, Boltzmannstraße 2, 85748 Garching bei München, Germany

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Received: 14 August 2025
Accepted: 8 October 2025

Abstract

Context. Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dualfield mode of VLTI/GRAVITY, provide unique measurements of the objects’ orbital motions and atmospheric compositions.

Aims. Here we compile a homogeneous library of all exoplanet and brown dwarf K-band spectra observed by GRAVITY thus far. This ExoGRAVITY Spectral Library is made publicly available online.

Methods. We re-reduced all the available GRAVITY dual-field high-contrast data in a uniform and highly automated way and, where companions were detected, extracted their ~2.0-2.4 μm K-band contrast spectra. We then derived stellar model atmospheres for all the employed flux references (either the host star or the swap calibrator), which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITY K-band flux spectra, we extracted spectral types, spectral indices, and bulk physical properties for all the companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most of the companions using evolutionary models.

Results. The resulting library contains R ~ 500 GRAVITY K-band spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH₄-dominated T-type dwarfs can be observed in the K-band. The GRAVITY spectra alone constrain the objects’ bolometric luminosity to typically within ±0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity.

Conclusions. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects’ atmospheres. It also constrains these objects’ bolometric luminosities, which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry.