Letter to the Editor

GPI+SPHERE detection of a 6.1 M_Jup circumbinary planet around HD 143811

¹ Department of Physics & Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
² LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, F-92190 Meudon, France
³ Univ. Grenoble Alpes, CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble F-38041, France
⁴ Centre de Recherche Astrophysique de Lyon (CRAL) UMR 5574, CNRS, Univ. de Lyon, Univ. Claude Bernard Lyon 1, ENS de Lyon, F-69230 Saint-Genis-Laval, France

^⋆ Corresponding author: v.squicciarini@exeter.ac.uk

Received: 4 September 2025
Accepted: 23 September 2025

Abstract

Context. Owing to its sensitivity to wide-orbit giant exoplanets, direct imaging is uniquely positioned to shed light on the interplay between protoplanetary disks and stellar hosts. In addition to constraining formation models, new detections are natural benchmarks for an atmospheric characterization.

Aims. The COBREX project performed an extensive reanalysis of archival observations from SPHERE and GPI using advanced post-processing techniques, that enhanced the detection sensitivity at close separation. Newly found companion candidates are being followed up to confirm new planets.

Methods. Following the detection of a companion candidate around the young (∼15 Myr) binary star HD 143811, we collected a new observation with SPHERE@VLT (0.95–1.67 μm) to confirm the presence of the source and to assess its physical bond to the target.

Results. We report the discovery of a new exoplanet orbiting HD 143811 at a projected separation of 0.43″ ∼ 60 au. Based on a 9-year-long baseline, we derive a mostly face-on and low-eccentricity orbit with a period of 320₋₉₀⁺²⁵⁰ years. The luminosity of the planet, constrained through the H-band spectrum from GPI, H-band photometry from SPHERE/IRDIS and YJ upper limits from SPHERE/IFS, allows us to place strong constraints on the intrinsic temperature of the planet (T_int = 1000 ± 30 K), which corresponds to a mass of 6.1_−0.9^+0.7 M_Jup.

Conclusions. HD 143811(AB)b is the second planet ever discovered by GPI. It joins the small cohort of circumbinary planets discovered through imaging and becomes a prime target for follow-up formation, dynamical, and characterization studies.