Letter to the Editor

Eccentric and cool: A high-spectral-resolution view of 51 Eri b with VLT/HiRISE^★

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
² Max-Planck-Institut für Astronomie, Königstuhl 17 69117 Heidelberg, Germany
³ Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
⁴ Laboratoire J.L. Lagrange, Université Côte d’Azur, Observatoire de la côte d’Azur, CNRS, 06304 Nice, France
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁶ Department of Physics & Astronomy, John Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁹ NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹¹ Institute for Astrophysics und Geophysik, Georg-August University, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹² Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstraße 1, 81679 München, Germany
¹³ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
¹⁴ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁵ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁶ Millennium Nucleus on Young Exoplanets and their Moons, Santiage, Chile
¹⁷ Department of Physics and Astronomy, University of Texas-San Antonio, San Antonio, TX, USA
¹⁸ Center for Advanced Instrumentation, Durham University, Durham DH1 3LE, United Kindgom
¹⁹ Dept. of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
²⁰ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
²¹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
²² Physics & Astronomy Dpt, University of Exeter, Exeter EX4 4QL, UK
²³ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA

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

Received: 5 January 2026
Accepted: 31 January 2026

Abstract

Discovered almost ten years ago, the giant planet 51 Eridani b is one of the least separated (≈0.2″) and faintest (J ≈ 19.74 mag) directly imaged exoplanets known to date. Its atmospheric properties have been thoroughly investigated through low- and medium-resolution spectroscopic observations, enabling the robust characterization of the planet’s bulk parameters. However, the planet’s intrinsically high contrast renders high-resolution spectroscopic observations difficult, despite their potential to yield key measurements essential for a more comprehensive characterization. This study sought to constrain the planet’s radial velocity, enabling a full 3D orbital solution when integrated with previous measurements. We obtained four high-contrast, high-resolution (R ≈ 140 000) spectroscopic datasets of the planet, collected over a two-year interval with the HiRISE visitor instrument at the VLT to derive the planet’s radial velocity. Using self-consistent models of atmosphere, we were able to derive the radial velocity of the planet at each of the four epochs. These radial-velocity measurements were then used in combination with all existing relative astrometry in order to constrain the orbit of the planet. Our radial velocity measurements allowed us to break the degeneracy along the line of sight, making the unambiguous interpretation of the phase curve of the companion possible. We further constrained the orbital parameters, and particularly the eccentricity, for which we derive e = 0.55^+0.03_−0.07. The relatively high eccentricity indicates that the system has experienced dynamical interactions induced by an external perturber. We place constraints on the mass and semimajor axis of a hypothetical, unseen outer planet capable of producing the observed high eccentricities.