Hint of curvature in the orbital motion of the exoplanet 51 Eridani b using 3 yr of VLT/SPHERE monitoring^★,★★

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
e-mail: almaire@uliege.be
³ CNRS, IPAG, Université Grenoble Alpes, 38000 Grenoble, France
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁵ INAF Catania Astrophysical Observatory, Via S. Sofia 78, 95123 Catania, Italy
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo, 8, 35121 Padova, Italy
⁸ INCT, Universidad de Atacama, calle Copayapu 485, Copiapó, Atacama, Chile
⁹ Department of Physics & Astronomy, College of Charleston, 66 George Street, Charleston, SC 29424, USA
¹⁰ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill View, Edinburgh, EH9 3HJ, UK
¹¹ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
¹² Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
¹³ Unidad Mixta Internacional Franco-Chilena de Astronomía CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁴ CRAL, UMR 5574, CNRS/ENS-Lyon/Université Lyon 1, 9 av. Ch. André, 69561 Saint-Genis-Laval, France
¹⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
¹⁶ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁷ Núcleo Milenio Formación Planetaria – NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁸ Department of Astronomy, University of Michigan, 1085 S. University Ave, Ann Arbor, MI 48109-1107, USA
¹⁹ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²⁰ Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²² Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²³ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²⁴ Université Côte d’Azur, OCA, CNRS, Lagrange, France
²⁵ NOVA Optical Infrared Instrumentation Group, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 7 January 2019
Accepted: 15 March 2019

Abstract

Context. The 51 Eridani system harbors a complex architecture with its primary star forming a hierarchical system with the binary GJ 3305AB at a projected separation of 2000 au, a giant planet orbiting the primary star at 13 au, and a low-mass debris disk around the primary star with possible cold and warm components inferred from the spectral energy distribution.

Aims. We aim to better constrain the orbital parameters of the known giant planet.

Methods. We monitored the system over three years from 2015 to 2018 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large Telescope (VLT).

Results. We measure an orbital motion for the planet of ~130 mas with a slightly decreasing separation (~10 mas) and find a hint of curvature. This potential curvature is further supported at 3σ significance when including literature Gemini Planet Imager (GPI) astrometry corrected for calibration systematics. Fits of the SPHERE and GPI data using three complementary approaches provide broadly similar results. The data suggest an orbital period of 32₋₉⁺¹⁷ yr (i.e., 12₋₂⁺⁴ au in semi-major axis), an inclination of 133₋₇⁺¹⁴ deg, an eccentricity of 0.45_−0.15^+0.10, and an argument of periastron passage of 87₋₃₀⁺³⁴ deg [mod 180°]. The time at periastron passage and the longitude of node exhibit bimodal distributions because we do not yet detect whether the planet is accelerating or decelerating along its orbit. Given the inclinations of the orbit and of the stellar rotation axis (134–144°), we infer alignment or misalignment within 18° for the star–planet spin-orbit. Further astrometric monitoring in the next 3–4 yr is required to confirm at a higher significance the curvature in the motion of the planet, determine if the planet is accelerating or decelerating on its orbit, and further constrain its orbital parameters and the star–planet spin-orbit.