Revisiting the atmosphere of the exoplanet 51 Eridani b with VLT/SPHERE^★

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117, Heidelberg, Germany
e-mail: brown@mpia.de
² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁵ Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
⁸ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁰ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹² Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹³ CRAL, UMR 5574, CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France
¹⁴ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁵ DOTA, ONERA, Université Paris Saclay, 91123 Palaiseau, France

Received: 29 August 2022
Accepted: 16 November 2022

Abstract

Aims. We aim to better constrain the atmospheric properties of the directly imaged exoplanet 51 Eri b using a retrieval approach with data of higher signal-to-noise ratio (S/N) than previously reported. In this context, we also compare the results from an atmospheric retrieval to using a self-consistent model to fit atmospheric parameters.

Methods. We applied the radiative transfer code petitRADTRANS to our near-infrared SPHERE observations of 51 Eri b in order to retrieve its atmospheric parameters. Additionally, we attempted to reproduce previous results with the retrieval approach and compared the results to self-consistent models using the best-fit parameters from the retrieval as priors.

Results. We present a higher S/N YH spectrum of the planet and revised K1K2 photometry (M_K1 = 15.11 ± 0.04 mag, M_K2 = 17.11 ± 0.38 mag). The best-fit parameters obtained using an atmospheric retrieval differ from previous results using self-consistent models. In general, we find that our solutions tend towards cloud-free atmospheres (e.g. log τ_clouds = −5.20 ± 1.44). For our 'nominal' model with new data, we find a lower metallicity ([Fe/H] = 0.26 ± 0.30 dex) and C/O ratio (0.38 ± 0.09), and a slightly higher effective temperature (T_eff = 807 ± 45 K) than previous studies. The surface gravity (log g = 4.05 ± 0.37) is in agreement with the reported values in the literature within uncertainties. We estimate the mass of the planet to be between 2 and 4 M_Jup. When comparing with self-consistent models, we encounter a known correlation between the presence of clouds and the shape of the P–T profiles.

Conclusions. Our findings support the idea that results from atmospheric retrievals should not be discussed in isolation, but rather along with self-consistent temperature structures obtained using the best-fit parameters of the retrieval. This, along with observations at longer wavelengths, might help to better characterise the atmospheres and determine their degree of cloudiness.