HD 143811 AB b: A Directly Imaged Planet Orbiting a Spectroscopic Binary in Sco-Cen

We present confirmation of HD 143811 AB b, a substellar companion to spectroscopic binary HD 143811 AB through direct imaging with the Gemini Planet Imager (GPI) and Keck NIRC2. HD 143811 AB was observed as a part of the Gemini Planet Imager Exoplanet Survey (GPIES) in 2016 and 2019 and is a member of the Sco-Cen star formation region. The exoplanet is detected $\sim 430$ mas from the host star by GPI. With two GPI epochs and one from Keck/NIRC2 in 2022, we confirm through common proper motion analysis that the object is bound to its host star. We derive an orbit with a semi-major axis of $64 ^{+32}{-14}$ au and eccentricity ${0.23 ^{+0.24}{-0.16}}$. Spectral analysis of the GPI $H$-band spectrum and NIRC2 \textit{L’} photometry provides additional proof that this object is a substellar companion. We compare the spectrum of HD 143811 AB b to PHOENIX stellar models and Exo-REM exoplanet atmosphere models and find that Exo-REM models provide the best fits to the data. From the Exo-REM models, we derive an effective temperature of $1042^{+178}{-132}$ K for the planet and translate the derived luminosity of the planet to a mass of $5.6 \pm 1.1~M\textrm{Jup}$ assuming hot-start evolutionary models. HD 143811 AB b is the first directly imaged planet around a binary that is not on an ultra-wide orbit. Future characterization of this object will shed light on the formation of planets around binary star systems.

💡 Research Summary

The authors report the discovery and confirmation of a directly imaged exoplanet, HD 143811 AB b, orbiting the spectroscopic binary HD 143811 AB in the Scorpius‑Centaurus (Sco‑Cen) association. The host system is a young (13 ± 4 Myr) double‑lined spectroscopic binary with component masses of 1.30 ± 0.05 M⊙ and 1.16 ± 0.04 M⊙ and an orbital period of 18.59 days. The binary’s distance, derived from Gaia DR3, is 136.9 ± 0.4 pc.

Observations were carried out with the Gemini Planet Imager (GPI) on Gemini South and with NIRC2 on the Keck II telescope. GPI obtained two epochs in H‑band (1.50–1.80 µm) on 30 April 2016 (36 × 60 s exposures) and 11 August 2019 (45 × 60 s exposures), each providing > 174° of parallactic angle rotation for angular differential imaging (ADI). The data were reduced with the GPI Data Reduction Pipeline (v1.6.0) and PSF subtraction was performed using pyKLIP, which implements the Karhunen–Loève Image Projection (KLIP) algorithm. A third epoch was obtained in 2022 with Keck/NIRC2 in the L′ band (3.8 µm), delivering an independent astrometric point and photometry.

A point source was detected at a separation of ~430 mas (≈ 58 au) from the binary’s photocenter in all three data sets. Common proper motion analysis, comparing the measured motion to the expected trajectory of a distant background object, shows the source follows the host’s motion with > 3σ confidence, confirming it is bound.

Orbital fitting using a Markov Chain Monte Carlo (MCMC) approach yields a semi‑major axis of 64 au with asymmetric uncertainties (+32 /‑14 au) and an eccentricity of 0.23 (+0.24 /‑0.16). The inclination is loosely constrained to ~38° ± 16°, indicating a moderately inclined orbit relative to the line of sight. These orbital parameters place the planet at a “solar‑system‑scale” distance from the binary, far smaller than the ultra‑wide (> 500 au) companions known around other binaries (e.g., HD 106906 b, b Cen b).

Spectroscopically, the GPI H‑band low‑resolution spectrum combined with the NIRC2 L′ photometric point provides a spectral energy distribution (SED) for the companion. The authors compare the SED to two families of models: PHOENIX stellar atmosphere models and Exo‑REM exoplanet atmosphere models. The Exo‑REM grid provides the best fit, yielding an effective temperature T_eff = 1042 K (‑132 + 178 K) and a surface gravity consistent with a young giant planet. Using the derived luminosity (log L/L⊙ ≈ ‑5.2) and assuming hot‑start evolutionary tracks, the companion’s mass is estimated at 5.6 ± 1.1 M_Jup.

The detection is significant for several reasons. First, it is the first directly imaged planet around a spectroscopic binary at a modest orbital distance, demonstrating that high‑contrast imaging can succeed even when the host is a close binary, provided the binary separation is below the instrument’s inner working angle. Second, the planet’s location (≈ 60 au) is intermediate between the tight, core‑accretion‑friendly region (< 30 au) and the ultra‑wide regime where gravitational instability is often invoked. This makes HD 143811 AB b a valuable test case for competing formation scenarios in binary environments, including in‑situ core accretion within a circumbinary disk, disk fragmentation, or migration after formation at larger radii.

The paper also notes that the host binary’s orbital parameters are currently known only from radial velocities; future astrometric monitoring (e.g., with Gaia or long‑baseline interferometry) could reveal the three‑dimensional binary orbit, enabling a full dynamical model of the system. High‑resolution spectroscopy (e.g., with JWST/NIRSpec or ELT/METIS) could probe atmospheric composition (water, methane, CO) and rotation, while polarimetric imaging could map scattered light from any circumbinary debris disk.

In summary, HD 143811 AB b adds a new class of directly imaged planets—moderately separated companions to close binaries—expanding the parameter space of known exoplanets and offering a promising laboratory for studying planet formation and dynamical evolution in binary star systems. Future multi‑wavelength and time‑domain observations will be essential to fully characterize its atmosphere, orbit, and the dynamical interplay with its host binary.