GPI+SPHERE detection of a 6.1 $M_{\text{Jup}}$ circumbinary planet around HD 143811

TL;DR

This study reports the confirmation of a circumbinary exoplanet HD 143811(AB)b using a COBREX-driven reanalysis of SPHERE and GPI data, with three-epoch astrometry constraining a long-period, near-face-on orbit at ~63 au and a period of ~$319$ years. Atmospheric–interior modeling with the HADES grid places the planet at $M_p \approx 6.1\,M_{\text{Jup}}$ and $T_{ ext{int}} \approx 1000$ K, with a radius near $1.4\,R_{\text{Jup}}$; the nondetection in SPHERE-IFS is consistent with these properties given the system age. The discovery adds to the small cohort of imaged circumbinary planets and demonstrates the power of archival data reanalysis for informing planet formation and evolution in binary environments, while motivating ongoing dynamical and atmospheric follow-up.

Abstract

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. 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. 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. We report the discovery of a new exoplanet orbiting HD 143811 at a projected separation of 0.43" $\sim 60$ au. Based on a 9-year-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 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$_{\text{int}} = 1000 \pm 30$K), which corresponds to a mass of $6.1^{+0.7}_{-0.9} ~M_{\text{Jup}}$. 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.

Figures (8)

• Figure 1: S/N maps produced from PACO in the three epochs. The image scale is shown in the lower right corner, and the position of the planet is indicated by an arrow. The left color bar refers to the two GPI maps.
• Figure 2: Orbits of HD 143811 b. Left: 50 randomly drawn accepted orbits from the OFTI posterior plotted in their sky projection. The measured astrometry is plotted in red from GPI (2016 and 2019) and SPHERE/IRDIS (2025) data. Right: same randomly drawn accepted orbits plotted as a function of time. The red dots show the measured astrometry and associated errors.
• Figure 3: Spectrophotometry of the planet, normalized to $d=10$ pc, compared to the best-fit model from HADES (red line). The black circles show the GPI spectrum, the blue square shows SPHERE photometry, and the green arrow indicates the SPHERE-IFS $5 \sigma$ upper limit (all with 1$\sigma$ errors). The orange line shows $5 \sigma$ IFS upper limits in individual $\lambda$ slices. The predicted photometric points are shown with red markers.
• Figure 4: Model (red line) vs. observed (black line) spectrum compared in order 61. The dark blue and the cyan lines indicate the models of two individual components. If several of the lines, especially the deepest ones are well fitted, some other do not correspond well to the model.
• Figure 5: The 1D-cuts along the primary and secondary components radial velocities, of the 2D-CCF from JD-2458242.7673 (top) to JD-2458605.7720 (bottom).