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

Abstract: Context. Owing to its sensitivity to wide-orbit giant exoplanets, direct imaging is uniquely positioned to shed light on the interplay between proto-planetary disks and stellar hosts. Besides constraining formation models, new detections are natural benchmarks for atmospheric characterization. Aims. The COBREX project performed an extensive reanalysis of archival observations from SPHERE and GPI using advanced post-processing techniques, enhancing 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 ($\sim 15$ Myr) binary star HD 143811, we collected a new observation with SPHERE@VLT ($0.95-1.67 μ$m) 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 $0.43" \sim 60$ au. Thanks to a 9-yr-long baseline, we derive a mostly face-on and low eccentricity orbit with a period of $320^{+250}_{-90}$ years. The luminosity of the planet, constrained through 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 planet's intrinsic temperature ($T_{int} = 1000 \pm 30$K), corresponding to a mass of $6.1^{+0.7}_{-0.9}$ MJup. Conclusions. HD 143811(AB)b is the second planet ever discovered by GPI; it joins the small cohort of circumbinary planets discovered through imaging, becoming a prime target for follow-up formation, dynamical, and characterization studies.