HD 143811(AB)b: Directly Imaged Circumbinary Companion

Updated 10 July 2026

HD 143811(AB)b is a directly imaged circumbinary substellar companion orbiting a young spectroscopic binary in the Scorpius–Centaurus association, characterized by a wide (~64 au) orbit and a mass of ~5.6–6.1 M_Jup.
Detection involved multi-epoch high-contrast imaging with Gemini Planet Imager, Keck/NIRC2, and SPHERE, confirming common proper motion and refining orbital parameters (a ≈ 64 au, e ≈ 0.23, P ≈ 326 yr).
Atmospheric modeling indicates an effective temperature of ~1040 K with a red, dusty spectrum, offering key insights into planet formation and dynamical evolution in close binary systems.

HD 143811(AB)b, also written HD 143811 AB b, is a directly imaged substellar companion orbiting the young spectroscopic binary HD 143811 AB in the Scorpius–Centaurus association. The system lies at a Gaia DR3 distance of $136.85 \pm 0.38$ pc from a parallax of $7.3065 \pm 0.0204$ mas, and the age adopted for planet characterization is $13 \pm 4$ Myr. The companion was confirmed through multi-epoch high-contrast imaging and common proper motion analysis, and current analyses place it on a wide circumbinary orbit with a semi-major axis near $64$ au and a mass of order $5.6$– $6.1~M_\mathrm{Jup}$ , depending on the adopted atmospheric and evolutionary framework (Jones et al., 8 Sep 2025).

1. Host system and stellar architecture

HD 143811 AB is a double-lined spectroscopic binary (SB2) identified with HIP 78663 and associated with Sco–Cen. Probabilistic membership from BANYAN $\Sigma$ favors Upper Scorpius at $72.1\%$ , with a $27.8\%$ chance of Upper Centaurus–Lupus and $0.2\%$ field. The space motion is $7.3065 \pm 0.0204$ 0 km s $7.3065 \pm 0.0204$ 1. The adopted age used in the planet analyses is $7.3065 \pm 0.0204$ 2 Myr, synthesized from typical subgroup ages, although a stellar SED fit yielded a model-dependent age of $7.3065 \pm 0.0204$ 3 Myr (Peck et al., 8 Sep 2025).

The inner binary is unusually compact among hosts of directly imaged planets. Its spectroscopic orbit has period $7.3065 \pm 0.0204$ 4 days, eccentricity $7.3065 \pm 0.0204$ 5, systemic velocity $7.3065 \pm 0.0204$ 6 km s $7.3065 \pm 0.0204$ 7, and mass ratio $7.3065 \pm 0.0204$ 8. The radial-velocity semi-amplitudes are $7.3065 \pm 0.0204$ 9 km s $13 \pm 4$ 0 and $13 \pm 4$ 1 km s $13 \pm 4$ 2, and the argument of periastron for the primary is $13 \pm 4$ 3 deg. Combining the spectroscopic constraints with stellar evolutionary modeling gives component masses $13 \pm 4$ 4 and $13 \pm 4$ 5, with total mass $13 \pm 4$ 6. Kepler’s law then implies a binary semi-major axis $13 \pm 4$ 7 au.

The stellar properties are consistent with a near-equal-mass F+F pair. PHOENIX-based spectral fitting yielded $13 \pm 4$ 8, $13 \pm 4$ 9 K, $64$0 K, $64$1 km s$64$2, and $64$3 km s$64$4. A separate MIST+ATLAS9 SED fit to blended Gaia and 2MASS photometry gave $64$5, $64$6 mag, and somewhat hotter temperatures, $64$7 K and $64$8 K. The authors note that the SED-based temperatures are systematically higher than the spectral-fitting values and attribute this to extinction–temperature covariance and model systematics; the masses are described as more robust than $64$9.

The planet therefore resides in a hierarchical circumbinary configuration: an $5.6$0-day eccentric SB2 at $5.6$1 au, orbited by a giant planetary-mass companion at $5.6$2 au. This architecture is rare among directly imaged systems and provides the dynamical context for all subsequent interpretation.

2. Detection history and confirmation as a bound companion

The companion was initially detected in Gemini Planet Imager observations and later confirmed with Keck/NIRC2 and, in an independent analysis, with SPHERE. The primary discovery paper reports GPI integral-field spectroscopy in $5.6$3 band $5.6$4 on 2016 Apr 30 and 2019 Aug 11, together with Keck/NIRC2 $5.6$5-band imaging $5.6$6 on 2022 Jun 10. The GPI epochs comprised $5.6$7 s and $5.6$8 s integrations, while the NIRC2 data consisted of 175 frames of 60 coadds $5.6$9 0.5 s, for a total of 87.5 min (Jones et al., 8 Sep 2025).

Epoch	Instrument / band	Relative astrometry
2016	GPI $6.1~M_\mathrm{Jup}$ 0	$6.1~M_\mathrm{Jup}$ 1 mas, $6.1~M_\mathrm{Jup}$ 2 deg
2019	GPI $6.1~M_\mathrm{Jup}$ 3	$6.1~M_\mathrm{Jup}$ 4 mas, $6.1~M_\mathrm{Jup}$ 5 deg
2022	Keck/NIRC2 $6.1~M_\mathrm{Jup}$ 6	$6.1~M_\mathrm{Jup}$ 7 mas, $6.1~M_\mathrm{Jup}$ 8 deg

The 2016 GPI detection had signal-to-noise ratio $6.1~M_\mathrm{Jup}$ 9 using a forward model matched filter with an L-type template. Its measured $\Sigma$ 0-band flux ratio was $\Sigma$ 1, corresponding to apparent $\Sigma$ 2, absolute $\Sigma$ 3, and integrated $\Sigma$ 4-band flux $\Sigma$ 5 W m $\Sigma$ 6 $\Sigma$ 7m $\Sigma$ 8. The 2019 GPI epoch had SNR $\Sigma$ 9, flux ratio $72.1\%$ 0, and integrated flux $72.1\%$ 1 W m $72.1\%$ 2 $72.1\%$ 3m $72.1\%$ 4. The 2022 Keck/NIRC2 data gave an $72.1\%$ 5-band flux ratio $72.1\%$ 6, apparent $72.1\%$ 7, absolute $72.1\%$ 8, and flux $72.1\%$ 9 W m $27.8\%$ 0 $27.8\%$ 1m $27.8\%$ 2.

Common proper motion was established by comparison with the Gaia DR3 proper motion and parallax of the host. The expected trajectory of a stationary background source is strongly inconsistent with the measured astrometry, and the observed separations disagree with the background track at $27.8\%$ 3. Bound-companion orbital tracks generated by OFTI from the first epoch are consistent with the later epochs, confirming that HD 143811 AB b is gravitationally bound.

An independent SPHERE-based study reached the same conclusion with a longer temporal baseline. That analysis used 2016 and 2019 GPI data plus SPHERE/IRDIS and IFS observations obtained in 2025, reporting astrometry of $27.8\%$ 4 mas, $27.8\%$ 5 in 2016; $27.8\%$ 6 mas, $27.8\%$ 7 in 2019; and $27.8\%$ 8 mas, $27.8\%$ 9 in 2025. That work derived a relative proper motion $0.2\%$ 0 mas yr $0.2\%$ 1, equivalent to $0.2\%$ 2 km s $0.2\%$ 3 at projected separation $0.2\%$ 4 au, below the escape speed $0.2\%$ 5 km s $0.2\%$ 6, and reported a background false-alarm probability of $0.2\%$ 7 (Squicciarini et al., 7 Sep 2025).

3. Orbital solution and circumbinary geometry

The planet’s orbit has been modeled with Orbits For The Impatient (OFTI). In the discovery analysis, the priors were uniform in $0.2\%$ 8, $0.2\%$ 9, $7.3065 \pm 0.0204$ 00, $7.3065 \pm 0.0204$ 01, $7.3065 \pm 0.0204$ 02, and $7.3065 \pm 0.0204$ 03, with parallax $7.3065 \pm 0.0204$ 04 mas and total mass $7.3065 \pm 0.0204$ 05. Using three relative astrometry epochs, the fitted posterior summary yielded semi-major axis $7.3065 \pm 0.0204$ 06 au, eccentricity $7.3065 \pm 0.0204$ 07, inclination $7.3065 \pm 0.0204$ 08, and period $7.3065 \pm 0.0204$ 09 yr. The posterior medians were $7.3065 \pm 0.0204$ 10 au, $7.3065 \pm 0.0204$ 11, $7.3065 \pm 0.0204$ 12, $7.3065 \pm 0.0204$ 13 yr, $7.3065 \pm 0.0204$ 14, $7.3065 \pm 0.0204$ 15, and $7.3065 \pm 0.0204$ 16, with broad credible intervals for the angular elements (Jones et al., 8 Sep 2025).

The period estimate is consistent with Kepler’s third law. In astronomical units and solar masses, the relation is $7.3065 \pm 0.0204$ 17. Using $7.3065 \pm 0.0204$ 18 au and $7.3065 \pm 0.0204$ 19 gives $7.3065 \pm 0.0204$ 20 yr. The broad uncertainty in $7.3065 \pm 0.0204$ 21, rather than the small uncertainty in $7.3065 \pm 0.0204$ 22, dominates the reported period range.

Projected physical separations derived from $7.3065 \pm 0.0204$ 23 are $7.3065 \pm 0.0204$ 24 au in 2016, $7.3065 \pm 0.0204$ 25 au in 2019, and $7.3065 \pm 0.0204$ 26 au in 2022, fully consistent with a fitted semi-major axis near $7.3065 \pm 0.0204$ 27 au and modest eccentricity. A parallel orbit analysis using GPI plus SPHERE astrometry found closely similar values: $7.3065 \pm 0.0204$ 28 au, $7.3065 \pm 0.0204$ 29, $7.3065 \pm 0.0204$ 30 deg, and $7.3065 \pm 0.0204$ 31 yr. The agreement between the two studies indicates that, with present astrometric coverage, the outer orbit is already constrained to be wide, low-to-moderately eccentric, and mostly face-on, while $7.3065 \pm 0.0204$ 32, $7.3065 \pm 0.0204$ 33, and $7.3065 \pm 0.0204$ 34 remain weakly constrained (Squicciarini et al., 7 Sep 2025).

The relation between the inner and outer orbital planes remains unresolved. The host-binary characterization paper inferred a binary inclination $7.3065 \pm 0.0204$ 35 deg, with a spectroscopically degenerate retrograde alternative at $7.3065 \pm 0.0204$ 36 deg. Because spectroscopy does not constrain the inner node $7.3065 \pm 0.0204$ 37, and the planetary $7.3065 \pm 0.0204$ 38 posterior remains broad, the mutual inclination cannot yet be computed. The current estimates overlap within roughly $7.3065 \pm 0.0204$ 39, so coplanarity cannot be ruled out, but neither can it be established (Peck et al., 8 Sep 2025).

4. Photometry, spectroscopy, and atmospheric modeling

The planet’s spectro-photometric characterization currently rests on a GPI $7.3065 \pm 0.0204$ 40-band spectrum and thermal-infrared photometry. The 2016 GPI $7.3065 \pm 0.0204$ 41-band spectrum was extracted with KLIP-FM forward modeling and Bayesian PSF fitting, calibrated using satellite spots and the stellar SED, and was described as exhibiting strongly correlated spectral noise typical for GPI but robustly measured flux. The 2022 NIRC2 $7.3065 \pm 0.0204$ 42 photometry was calibrated to stellar $7.3065 \pm 0.0204$ 43 through the system SED model and was reported as consistent with a cool, substellar companion (Jones et al., 8 Sep 2025).

In color–magnitude space, the combined measurements yield $7.3065 \pm 0.0204$ 44 mag and $7.3065 \pm 0.0204$ 45 mag. On a color–magnitude diagram, the object lies near late-L/early-T field objects and close to the directly imaged companions HD 206893 B and 2M 1207 B. The discovery paper states that this positioning suggests a very red, dusty atmosphere may be plausible. This is explicitly framed as an interpretation of the photometric placement rather than a direct measurement.

Atmospheric model comparison was carried out against 842 PHOENIX synthetic stellar spectra and 9575 Exo-REM exoplanet atmosphere models, including clouds and disequilibrium chemistry, across $7.3065 \pm 0.0204$ 46 K. The initial goodness-of-fit statistic was

$7.3065 \pm 0.0204$ 47

For the GPI spectrum alone, PHOENIX and Exo-REM both matched moderately well, with PHOENIX reaching a maximum $7.3065 \pm 0.0204$ 48, and Exo-REM slightly preferred. For the joint GPI+NIRC2 fit, Exo-REM was strongly preferred, with best-fit $7.3065 \pm 0.0204$ 49, while the PHOENIX templates were rejected. A representative best-fit Exo-REM model had $7.3065 \pm 0.0204$ 50 K, $7.3065 \pm 0.0204$ 51, $7.3065 \pm 0.0204$ 52 dex, and $7.3065 \pm 0.0204$ 53, but the paper explicitly states that $7.3065 \pm 0.0204$ 54, metallicity, and C/O are not constrained by the data.

A more complete inference used nested sampling with dynesty and a Gaussian-process noise model for the correlated GPI spectral noise. That analysis yielded $7.3065 \pm 0.0204$ 55 K, radius $7.3065 \pm 0.0204$ 56, and luminosity $7.3065 \pm 0.0204$ 57. Surface gravity, metallicity, and C/O ratio remained unconstrained across the Exo-REM grid. Under hot-start evolutionary models of Baraffe et al. (2003), and adopting age $7.3065 \pm 0.0204$ 58 Myr, the derived luminosity corresponds to a mass $7.3065 \pm 0.0204$ 59, with an evolutionary radius $7.3065 \pm 0.0204$ 60, consistent with the atmospheric-fit radius within uncertainties.

A separate GPI+SPHERE analysis used the HADES coupled atmosphere–interior grid with an affine-invariant MCMC, including SPHERE/IRDIS $7.3065 \pm 0.0204$ 61 photometry and SPHERE/IFS $7.3065 \pm 0.0204$ 62 upper limits. That work reported $7.3065 \pm 0.0204$ 63 mag, $7.3065 \pm 0.0204$ 64 mag, $7.3065 \pm 0.0204$ 65 mag, and a $7.3065 \pm 0.0204$ 66 non-detection limit $7.3065 \pm 0.0204$ 67 mag, implying $7.3065 \pm 0.0204$ 68 mag. It derived intrinsic temperature $7.3065 \pm 0.0204$ 69 K, mass $7.3065 \pm 0.0204$ 70, radius $7.3065 \pm 0.0204$ 71, atmospheric metallicity $7.3065 \pm 0.0204$ 72 dex, cloud sedimentation efficiency $7.3065 \pm 0.0204$ 73, and vigorous vertical mixing $7.3065 \pm 0.0204$ 74. It interpreted the H-band shape and red $7.3065 \pm 0.0204$ 75 color as consistent with strong $7.3065 \pm 0.0204$ 76 absorption and muted $7.3065 \pm 0.0204$ 77 near the L/T transition in a young, cloudy, chemically disequilibrium atmosphere (Squicciarini et al., 7 Sep 2025).

5. Dynamical regime and formation significance

HD 143811(AB)b belongs to a small class of directly imaged planets orbiting binaries rather than single stars. The discovery paper explicitly places it among few such systems and compares it with HD 106906 b, b Cen b, and WISPIT 1bc, while also noting borderline planetary-mass companions around binaries such as Ross 458C, SR 12 C, 2MASS J01033563–5515561 (AB)b, and ROXs 42B b. Within that group, HD 143811 AB b is distinguished by a moderate separation of about $7.3065 \pm 0.0204$ 78 au, modest eccentricity near $7.3065 \pm 0.0204$ 79, and youth at $7.3065 \pm 0.0204$ 80 Myr (Jones et al., 8 Sep 2025).

The inner binary is sufficiently tight that the planet resides deep in the stable circumbinary regime. The host-binary study invokes the Holman & Wiegert (1999) critical semimajor axis framework, writing $7.3065 \pm 0.0204$ 81 with $7.3065 \pm 0.0204$ 82. Using $7.3065 \pm 0.0204$ 83 au, $7.3065 \pm 0.0204$ 84, and $7.3065 \pm 0.0204$ 85, it concludes that $7.3065 \pm 0.0204$ 86 remains orders of magnitude smaller than the planet’s $7.3065 \pm 0.0204$ 87 au orbit, so the system is comfortably within the stable circumbinary regime. A separate analysis, adopting a compact binary with $7.3065 \pm 0.0204$ 88 au, similarly states that the planet’s orbit at $7.3065 \pm 0.0204$ 89 au comfortably exceeds any plausible critical stability radius (Peck et al., 8 Sep 2025).

The formation interpretation remains open. The discovery paper states that the planet’s moderate separation, modest eccentricity, and youthful age provide a valuable data point for understanding planet formation and dynamical evolution in circumbinary environments where core accretion is challenged but not excluded. The host-binary study adds that the tight inner binary likely truncated and structured the primordial circumbinary disk, setting initial conditions for planet formation and migration, and that the large binary–planet separation ratio of $7.3065 \pm 0.0204$ 90 is consistent with long-term stability. It further states that the observed configuration suggests formation in a circumbinary disk at tens of au, or post-formation outward migration or scattering, but does not distinguish among these scenarios.

The system may also include circum-system debris. Spitzer detections indicate a cold debris belt with fractional infrared luminosity between $7.3065 \pm 0.0204$ 91 and $7.3065 \pm 0.0204$ 92, inferred temperatures of $7.3065 \pm 0.0204$ 93 K, and blackbody radii of $7.3065 \pm 0.0204$ 94 au, although the host-binary paper emphasizes that grain properties and the then-unknown binary nature can shift radii outward and that a 70 $7.3065 \pm 0.0204$ 95m non-detection was reported. This suggests that the architecture may eventually permit joint analysis of binary, planet, and disk dynamics, but current disk parameters remain too uncertain for strong conclusions.

6. Open questions and future characterization

Several central parameters remain weakly constrained. For the planet, the current three-epoch orbit fits already limit $7.3065 \pm 0.0204$ 96, $7.3065 \pm 0.0204$ 97, $7.3065 \pm 0.0204$ 98, and $7.3065 \pm 0.0204$ 99, but $13 \pm 4$ 00, $13 \pm 4$ 01, and $13 \pm 4$ 02 remain broad. For the inner binary, spectroscopy fixes the period, eccentricity, and mass ratio with high precision but cannot determine the longitude of the ascending node, and radial velocities alone leave a mirror degeneracy between prograde and retrograde orbital inclinations. Consequently, the full three-dimensional architecture and mutual inclination are not yet known (Peck et al., 8 Sep 2025).

Continued high-precision relative astrometry is expected to refine the planetary orbital parameters $13 \pm 4$ 03, $13 \pm 4$ 04, $13 \pm 4$ 05, $13 \pm 4$ 06, $13 \pm 4$ 07, and $13 \pm 4$ 08, and may eventually permit dynamical mass constraints when combined with absolute astrometry and a well-determined binary orbit. The discovery paper specifically notes that Hipparcos–Gaia accelerations could contribute once the host-binary solution is improved. The host-binary paper recommends VLTI/GRAVITY, since the binary apastron corresponds to approximately $13 \pm 4$ 09 mas at $13 \pm 4$ 10 pc, potentially allowing recovery of the inner $13 \pm 4$ 11 and true $13 \pm 4$ 12, and cites Gaia DR4 individual-scan astrometry as another route to constraining the $13 \pm 4$ 13 mas photocenter wobble.

Additional spectroscopy across broader wavelength coverage is a major priority. The discovery analysis recommends further observations in $13 \pm 4$ 14, $13 \pm 4$ 15, and $13 \pm 4$ 16 bands and medium-resolution spectroscopy to constrain $13 \pm 4$ 17, $13 \pm 4$ 18, and CO, as well as cloud properties, metallicity, and C/O, while reducing the impact of correlated-noise systematics. It explicitly recommends follow-up with upgraded high-contrast imagers such as Gemini Planet Imager 2.0, along with complementary Keck observations. The GPI+SPHERE study further identifies JWST NIRSpec/G395M and MIRI/LRS–MRS as promising for resolving molecular and cloud signatures across $13 \pm 4$ 19m, and also points to variability, polarimetry, and ELT-class high-resolution spectroscopy as routes to probing cloud patchiness, particle size, winds, rotation, and disequilibrium chemistry (Squicciarini et al., 7 Sep 2025).

The scientific importance of these follow-up programs is twofold. First, they can test whether the apparent consistency of the binary and planetary inclinations reflects genuine coplanarity expected from formation in a common circumbinary disk. Second, they can determine whether HD 143811(AB)b is best interpreted as a planet formed in situ in a circumbinary disk, a product of migration, or an object whose present orbit encodes later dynamical evolution. Present data suggest several of these possibilities, but do not yet isolate a unique formation pathway.