HD 143811 AB b: Circumbinary Planet Imaging

• HD 143811 AB b is a directly imaged, young, planetary-mass companion orbiting a close spectroscopic binary in the Sco-Cen region, providing a benchmark for circumbinary planet studies.
• Multi-epoch high-contrast imaging and astrometric analysis across instruments confirmed its ~430 mas separation and stable orbital architecture, essential for dynamical modeling.
• Atmospheric fits indicate a cool, cloudy companion (T_eff ≈ 1042 K, ~6 M_Jup) that challenges and refines planet formation theories in binary star environments.

HD 143811 AB b is a directly imaged, young, planetary-mass companion orbiting the close double-lined spectroscopic binary HD 143811 AB, itself a member of the Scorpius–Centaurus (Sco-Cen) star-forming region. This object represents a benchmark system for empirical characterization of planet formation and dynamical evolution in circumbinary environments, due to its rare status as a directly imaged planet orbiting a compact spectroscopic binary at moderate separation (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025, Peck et al., 8 Sep 2025).

1. Host System: HD 143811 AB

HD 143811 AB is a well-characterized double-lined spectroscopic binary (SB2) comprising two solar-type stars:

• Primary: $M_A = 1.30^{+0.03}_{-0.05}\ M_\odot$, $T_{\rm eff,A} = 6439 \pm 132$ K,
• Secondary: $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$, $T_{\rm eff,B} = 5900 \pm 157$ K,
• Metallicity: $[M/H] = -0.18 \pm 0.10$,
• Orbital period: $P = 18.59098 \pm 0.00007$ days,
• Eccentricity: $e = 0.4935 \pm 0.0013$,
• Semimajor axis: $a = 0.1854^{+0.0014}_{-0.0024}$ au,
• Inclination: $i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$ (with retrograde mirror solution $156.1^{+0.2}_{-0.3}^\circ$).

The system's age, adopted from Sco-Cen subgroup studies, is $T_{\rm eff,A} = 6439 \pm 132$0 Myr, and the distance is $T_{\rm eff,A} = 6439 \pm 132$1 pc. Both components exhibit lithium absorption indicative of youth, with no strong chromospheric emission (Peck et al., 8 Sep 2025).

2. Discovery and Observational Confirmation

HD 143811 AB b was first identified via direct imaging by the Gemini Planet Imager (GPI) in 2016, confirmed at subsequent epochs with GPI (2019), Keck/NIRC2 ($T_{\rm eff,A} = 6439 \pm 132$2-band, 2022), and SPHERE/IRDIS ($T_{\rm eff,A} = 6439 \pm 132$3 dual-band, 2025). These multi-instrument detections leveraged advanced high-contrast post-processing (e.g., PACO, ADI) to achieve sensitivity at separations $T_{\rm eff,A} = 6439 \pm 132$4, crucial for circumbinary planet discovery (Squicciarini et al., 7 Sep 2025, Jones et al., 8 Sep 2025).

Key astrometric measurements: | Epoch | Instrument | Separation [mas] | Position angle [deg] | |---------------------------|-------------------|-----------------------|------------------------| | 2016-04-30 | GPI ($T_{\rm eff,A} = 6439 \pm 132$5) | $T_{\rm eff,A} = 6439 \pm 132$6 | $T_{\rm eff,A} = 6439 \pm 132$7 | | 2019-08-10/11 | GPI ($T_{\rm eff,A} = 6439 \pm 132$8) | $T_{\rm eff,A} = 6439 \pm 132$9 | $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$0 | | 2022-06-10 | NIRC2 ($M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$1) | $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$2 | $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$3 | | 2025-07-24 | SPHERE ($M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$4) | $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$5 | $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$6 |

All epochs yield a projected separation $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$7 mas ($M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$8 au), directly confirming the companion. A common proper motion analysis rejects the background hypothesis at $M_B = 1.15^{+0.03}_{-0.04}\ M_\odot$9; the companion’s motion is consistent with a bound orbit rather than a stationary background object (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025).

3. Orbital Architecture and Dynamical Configuration

Orbital parameters were derived via OFTI (“Orbits for the Impatient”) as implemented in orbitize! v3, incorporating astrometry, system mass ($T_{\rm eff,B} = 5900 \pm 157$0–$T_{\rm eff,B} = 5900 \pm 157$1), and Gaia parallax. The resulting solution:

• Semimajor axis: $T_{\rm eff,B} = 5900 \pm 157$2 au (Jones et al., 8 Sep 2025), $T_{\rm eff,B} = 5900 \pm 157$3 au (Squicciarini et al., 7 Sep 2025),
• Eccentricity: $T_{\rm eff,B} = 5900 \pm 157$4 (Jones et al., 8 Sep 2025), $T_{\rm eff,B} = 5900 \pm 157$5 (Squicciarini et al., 7 Sep 2025),
• Inclination: $T_{\rm eff,B} = 5900 \pm 157$6 (Jones et al., 8 Sep 2025), $T_{\rm eff,B} = 5900 \pm 157$7 (Squicciarini et al., 7 Sep 2025),
• Period: $T_{\rm eff,B} = 5900 \pm 157$8 yr (Jones et al., 8 Sep 2025), $T_{\rm eff,B} = 5900 \pm 157$9 yr (Squicciarini et al., 7 Sep 2025).

The orbit is moderately inclined, near-circular, and stable relative to the tight inner binary ($[M/H] = -0.18 \pm 0.10$0). Current data permit coplanarity but lack full 3D constraint on the mutual inclination; both the planet's and binary's node angles are poorly determined. Future astrometric and interferometric monitoring is required to resolve the orbital architecture definitively (Peck et al., 8 Sep 2025).

4. Photometric and Spectroscopic Characterization

The planetary companion's photometry and spectroscopy were derived from multi-band imaging and H-band spectroscopy:

• GPI $[M/H] = -0.18 \pm 0.10$1-band spectrum ($[M/H] = -0.18 \pm 0.10$2 $[M/H] = -0.18 \pm 0.10$3m, $[M/H] = -0.18 \pm 0.10$4, SNR up to 7.7),
• Keck/NIRC2 $[M/H] = -0.18 \pm 0.10$5-band ($[M/H] = -0.18 \pm 0.10$6),
• SPHERE/IRDIS $[M/H] = -0.18 \pm 0.10$7 ($[M/H] = -0.18 \pm 0.10$8, $[M/H] = -0.18 \pm 0.10$9),
• Non-detection in SPHERE/IFS ($P = 18.59098 \pm 0.00007$0).

Observed colors ($P = 18.59098 \pm 0.00007$1; $P = 18.59098 \pm 0.00007$2) and spectra are consistent with a cool, planetary-mass object. The flux calibration models the host as an unresolved binary, with negligible flux error propagation ($P = 18.59098 \pm 0.00007$3 in $P = 18.59098 \pm 0.00007$4-band) (Squicciarini et al., 7 Sep 2025, Jones et al., 8 Sep 2025).

Spectral comparison utilized both PHOENIX stellar templates and Exo-REM/HADES exoplanet atmosphere models. The data strongly favor cool Exo-REM-like atmospheres with thick cloud cover and vertical mixing, disfavouring stellar spectral templates, notably with the addition of $P = 18.59098 \pm 0.00007$5 data (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025).

5. Physical Properties and Atmospheric Modeling

Atmospheric fits yield:

• Effective temperature: $P = 18.59098 \pm 0.00007$6 K (Exo-REM, (Jones et al., 8 Sep 2025)), $P = 18.59098 \pm 0.00007$7 K (HADES, (Squicciarini et al., 7 Sep 2025)),
• Bolometric luminosity: $P = 18.59098 \pm 0.00007$8 (Stefan–Boltzmann consistent),
• Planetary mass: $P = 18.59098 \pm 0.00007$9 (hot-start models, (Jones et al., 8 Sep 2025)), $e = 0.4935 \pm 0.0013$0 (HADES, (Squicciarini et al., 7 Sep 2025)),
• Radius: $e = 0.4935 \pm 0.0013$1 (evolution models, (Jones et al., 8 Sep 2025)), $e = 0.4935 \pm 0.0013$2 (HADES, (Squicciarini et al., 7 Sep 2025)),
• Metallicity: $e = 0.4935 \pm 0.0013$3,
• Sedimentation parameter: $e = 0.4935 \pm 0.0013$4,
• Vertical mixing: $e = 0.4935 \pm 0.0013$5.

The companion is interpreted as a young, self-luminous, and likely cloudy planetary-mass object. Atmospheric fits indicate the presence of vertical mixing and potential chemical disequilibrium. The physical parameters are constrained assuming a $e = 0.4935 \pm 0.0013$6 Myr age and hot-start entropy initial conditions; core mass remains weakly constrained (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025).

6. Scientific Significance, Formation, and Future Prospects

HD 143811 AB b is one of a select group of directly imaged circumbinary planets and is unique at its moderate ($e = 0.4935 \pm 0.0013$7 au) separation from a tight ($e = 0.4935 \pm 0.0013$8 au), eccentric spectroscopic binary. This configuration provides a critical test for planet formation theories in multiple-star environments. The low-to-moderate eccentricity and separation are compatible with in situ formation by core accretion or disk instability, but post-formation migration or scattering remain possible. The system's architecture, notably the large planet-binary separation ratio, ensures dynamical stability (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025, Peck et al., 8 Sep 2025).

HD 143811 AB b joins a small cohort (HD 106906 b, b Cen b, WISPIT 1bc) of planets imaged in orbit around close binaries, but it occupies a previously underrepresented parameter regime. As both host binary and planet are sufficiently bright and well separated, the system is considered a prime candidate for:

• Continued high-precision astrometry,
• Interferometric monitoring (VLTI/GRAVITY),
• Spectral and photometric surveys with JWST and ELTs for atmosphere/composition studies,
• Further dynamical modeling to constrain mutual inclination and formation pathway.

A plausible implication is that discovery and further characterization of such systems will clarify the incidence and migration/formation histories of circumbinary planets in contrast to those around single stars, and constrain the conditions under which massive planets survive or form in multi-stellar disks (Jones et al., 8 Sep 2025, Squicciarini et al., 7 Sep 2025, Peck et al., 8 Sep 2025).

7. Summary Table: System and Planet Properties

Parameter	HD 143811 AB	HD 143811 AB b
Type	SB2, F+G	Directly imaged planet
$e = 0.4935 \pm 0.0013$9 ($a = 0.1854^{+0.0014}_{-0.0024}$0)	$a = 0.1854^{+0.0014}_{-0.0024}$1	—
$a = 0.1854^{+0.0014}_{-0.0024}$2 ($a = 0.1854^{+0.0014}_{-0.0024}$3)	$a = 0.1854^{+0.0014}_{-0.0024}$4	—
[M/H]	$a = 0.1854^{+0.0014}_{-0.0024}$5	$a = 0.1854^{+0.0014}_{-0.0024}$6
$a = 0.1854^{+0.0014}_{-0.0024}$7 (days)	$a = 0.1854^{+0.0014}_{-0.0024}$8	—
$a = 0.1854^{+0.0014}_{-0.0024}$9	$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$0	—
$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$1 (au)	$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$2	—
$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$3 (au)	—	$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$4
$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$5	—	$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$6
$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$7 (deg)	—	$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$8
$i_{AB} = 22.9^{+0.3}_{-0.2}^\circ$9 ($156.1^{+0.2}_{-0.3}^\circ$0)	—	$156.1^{+0.2}_{-0.3}^\circ$1 / $156.1^{+0.2}_{-0.3}^\circ$2
$156.1^{+0.2}_{-0.3}^\circ$3 (K)	—	$156.1^{+0.2}_{-0.3}^\circ$4 / $156.1^{+0.2}_{-0.3}^\circ$5
$156.1^{+0.2}_{-0.3}^\circ$6 ($156.1^{+0.2}_{-0.3}^\circ$7)	—	$156.1^{+0.2}_{-0.3}^\circ$8
Age (Myr)	$156.1^{+0.2}_{-0.3}^\circ$9	$T_{\rm eff,A} = 6439 \pm 132$00
Distance (pc)	$T_{\rm eff,A} = 6439 \pm 132$01	$T_{\rm eff,A} = 6439 \pm 132$02

All planet parameters reference (Jones et al., 8 Sep 2025) and (Squicciarini et al., 7 Sep 2025); host binary parameters are from (Peck et al., 8 Sep 2025).

• (Jones et al., 8 Sep 2025) HD 143811 AB b: A Directly Imaged Planet Orbiting a Spectroscopic Binary in Sco-Cen
• (Squicciarini et al., 7 Sep 2025) GPI+SPHERE detection of a 6.1 $T_{\rm eff,A} = 6439 \pm 132$03 circumbinary planet around HD 143811
• (Peck et al., 8 Sep 2025) Characterization of the Host Binary of the Directly Imaged Exoplanet HD 143811 AB b