Observational constraints on the chemical tracers of planet formation history

A systematic survey of 13 directly imaged low-mass companions with VLT/ERIS^★

¹ ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
² Max Planck Institute for Intelligent Systems, Max-Planck-Ring 4, 72076 Tübingen, Germany
³ National Center of Competence in Research PlanetS, Switzerland
⁴ University of Zurich, Rämistrasse 71, 8006 Zurich, Switzerland
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁶ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁷ Space Sciences, Technologies, and Astrophysics Research Institute, Université de Liège, 4000 Sart Tilman, Belgium
⁸ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁹ ETH Zurich, Department of Earth and Planetary Sciences, Sonneggstrasse 5, 8092 Zurich, Switzerland
¹⁰ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
¹¹ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹² Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
¹³ LIRA, Observatoire de Paris, Univ PSL, CNRS, Sorbonne Univ, Univ de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁴ Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA
¹⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
¹⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
¹⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
¹⁹ STFC UK ATC, Royal Observatory Edinburgh, Blackford Hill. Edinburgh, EH9 3HJ, UK
²⁰ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
²¹ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, 64100 Teramo, Italy

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 October 2025
Accepted: 18 February 2026

Abstract

Context. Constraining the link between the atmospheric composition of giant exoplanets and their formation history is a key goal of exoplanet studies. In particular, the atmospheric carbon-to-oxygen (C/O) ratio and metallicity – which are readily measurable with direct spectroscopic observations – are believed to be chemical tracers of the birth location of substellar and planetary companions.

Aims. We aim to collect observational constraints for planet formation theories by performing a large and systematic survey of the atmospheric C/O ratio and metallicity.

Methods. We collected new K-band moderate-resolution (R ~ 11 000) spectroscopic observations of 13 directly imaged planetary-mass companions spanning the molecular snowlines of CO₂, CO, and CH₄ and with masses 4–30 M_J with VLT/ERIS/SPIFFIER. Additionally, we gathered a large portion of the available archival observations for these targets, amounting to 40 spectra and 140 photometric fluxes. We performed spectral fits using self-consistent grid models and freely parametrisable models. We obtained robust estimates of the key atmospheric parameters by aggregating the results from the different models, thereby reducing modelling systematics.

Results. Using molecular mapping, we detected H₂O and CO in 12 of our targets as well as ¹³CO in HR 2562 B. With our multimodel spectral fitting strategy, we obtained stellar to superstellar C/O ratios – ranging between 0.3 and 0.8 – and predominantly superstellar metallicities – between −1 and 1 dex – across all targets. We measured a substantial enrichment of ¹³CO for HR 2562 B with ¹²CO/¹³CO= 12.0_−3.3^+4.5. If corroborated by independent observations, it could indicate that the companion might have formed beyond the CO snowline and later migrated inwards to its current location. We find an anti-correlation (R = −0.64) between the C/O ratio and the companion mass, consolidating a previous result.

Conclusions. Our work demonstrates the scientific potential of the ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of close-in substellar and exoplanet companions.