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

Vito Squicciarini, Johan Mazoyer, Christian Wilkinson, Anne-Marie Lagrange, Philippe Delorme, Alice Radcliffe, Olivier Flasseur, Flavien Kiefer, Evelyne Alecian

► To cite this version:

Vito Squicciarini, Johan Mazoyer, Christian Wilkinson, Anne-Marie Lagrange, Philippe Delorme, et al.. GPI+SPHERE detection of a 6.1MJup circumbinary planet around HD 143811. Astronomy & Astrophysics - A&A, 2025, 702, 10.1051/0004-6361/202557104 . hal-05279078

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LETTER TO THE EDITOR

GPI+SPHERE detection of a 6.1*M<sub>Jup</sub> circumbinary planet around HD 143811

V. Squicciarini¹,², J. Mazoyer², C. Wilkinson², A.-M. Lagrange²,³, P. Delorme³, A. Radcliffe², O. Flasseur⁴, F. Kiefer², and E. Alecian³

(Affiliations can be found after the references)

September 23, 2025

ABSTRACT

**Context.** 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. 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 (~15 Myr) binary star HD 143811, we collected a new observation with SPHERE@VLT (0.95-1.67 µm) 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" ~ 60 au. Thanks to a 9-yr-long baseline, we derive a mostly face-on and low eccentricity orbit with a period of $320_{-90}^{+250}$ 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 ± 30K), corresponding to a mass of $6.1_{-0.9}^{+0.7} M_{Jup}$.

**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.

**Key words.** Planets and satellites: detection – Planets and satellites: gaseous planets – Planetary systems – Techniques: high angular resolution – Stars: individual: HD 143811

1. Introduction

Understanding the physical processes behind planet formation and the intricate chemical and dynamical interplay of planets with parent disks – reflected in their internal structure and in their atmosphere – is a major challenge in modern astronomy. Compared to single stars, multiple systems provide additional challenges for planet formation (see Fontanive & Bardalez Gagliardi 2021; Thebault & Bonanni 2025, and ref. therein). Unlike transit and radial velocities, direct imaging (DI) probes wide-orbit giant planets around young ($t \le 1$ Gyr) stars, offering a complementary window on planet formation (Gaudi et al. 2021). Circumbinary planets are particularly compelling targets for DI: their disks evolve under non-standard initial conditions and their planets may either form in situ at large separations or be dynamically scattered outward (Nelson 2003; Pierens & Nelson 2008; Marzari & Thebault 2019; Gianuzzi et al. 2023). Despite dedicated searches (e.g. Asensio-Torres et al. 2018), only a few circumbinary planets have been imaged to date (HD 106906 b, Bailey et al. 2014; b Cen b, Janson et al. 2021; Ross 458 b, Goldman et al. 2010; DLR 1 b, Delorme et al. 2013).

The Gemini Planet Imager (GPI) was a second-generation high-contrast imaging instrument on the Gemini South Telescope active from 2014 to 2020 (Macintosh et al. 2014). The GPI Exoplanet Survey (GPIES) was a 5-year blind survey of 600 nearby young stars, the largest conducted to date, which resulted in the discovery of one new exoplanet (51 Eri b, Macintosh et al. 2015) and one new brown dwarf (HR 2562 B, Konopacky et al. 2016). Together with the 400-stars SpHere Infrared survey for Exoplanets (SHINE) conducted with the Spectro-Polarimetric

High-Contrast Exoplanet Research (SPHERE, Beuzit et al. 2019) at the Very Large Telescope, it provided the tightest statistical constraints to the occurrence of wide-orbit exoplanets (Nielsen et al. 2019; Vigan et al. 2021). In the last few years, new post-processing algorithms have been developed to enhance the signal-to-noise ratio (SNR) of existing observations, opening up a new detection window in the parameter space where previously undetected planets could reside. This is the main goal of the COupling data and techniques for BReakthroughs in EXoplanetary systems exploration (COBREX) project, that undertook a complete re-reduction of SHINE (Chomez et al. 2025) and GPIES (Squicciarini et al. 2025, hereafter S25) using the PAatch-COvariance algorithm (PACO, Flasseur et al. 2018).

The GPIES survey reanalysis by S25 achieved deeper detection limits than those previously obtained on a 150-star subsample (Nielsen et al. 2019), with up to a two-fold gain in minimum detectable mass, allowing us to identify new promising candidates. One of these, detected around the star HD 143811 = HIP 78663, was observed twice (in 2016 and 2019) and showed hints of common proper motion. However, the ~3.5σ detection in the 2019 observation did not meet our strict 5σ detection threshold required for confirmation. For this reason, we proposed it to be re-observed with SPHERE. In this letter, we report the confirmation of this candidate as a planet, the first GPI + SPHERE joint exoplanet detection.

2. The system

HD 143811 belongs to the young (5-30 Myr) Scorpius-Centaurus, the nearest OB association (de Zeeuw et al. 1999).
<table><caption>**Table 1.** Main astrometric, kinematic, photometric and astrophysical properties of the system.</caption><thead><tr><th></th><th>Value</th><th>Ref.</th><th></th><th>Value</th><th>Ref.</th></tr></thead><tbody><tr><td>α</td><td>240.8892°</td><td>1</td><td>H</td><td>7.784(47) mag</td><td>2</td></tr><tr><td>δ</td><td>-30.1372°</td><td>1</td><td>age</td><td>18 ± 3 Myr</td><td>3</td></tr><tr><td>d</td><td>136.86(38) pc</td><td>1</td><td>M<sub>1</sub></td><td>1.24 – 1.40 M<sub>☉</sub></td><td>3</td></tr><tr><td>μ<sup>*</sup><sub>α</sub></td><td>-14.850(26) mas/yr</td><td>1</td><td>M<sub>2</sub></td><td>1.08 – 1.28 M<sub>☉</sub></td><td>3</td></tr><tr><td>μ<sub>δ</sub></td><td>-24.984(16) mas/yr</td><td>1</td><td>T<sub>eff,1</sub></td><td>6250 – 6900 K</td><td>3</td></tr><tr><td>G</td><td>8.798(3) mag</td><td>1</td><td>T<sub>eff,2</sub></td><td>5750 – 6500 K</td><td>3</td></tr></tbody></table>

Notes. α, δ: RA, dec (J2016). d: distance. μ<sup>*</sup><sub>α</sub>, μ<sub>δ</sub>: proper motion. M<sub>1</sub>, M<sub>2</sub>, T<sub>eff,1</sub>, T<sub>eff,2</sub>: stellar masses and eff. temperatures. References: 1: Gaia Collaboration et al. (2023); 2: Skrutskie et al. (2006); 3: this work.

<table><caption>**Table 2.** Details of the observations of HD 143811.</caption><thead><tr><th>Date (UT)</th><th>Instrument</th><th>Int. time (min)</th><th>Seeing (arcsec)</th><th>τ<sub>0</sub> (ms)</th><th>ΔPA (°)</th></tr></thead><tbody><tr><td>2016/04/30</td><td>GPI</td><td>43.7</td><td>2.24</td><td>N.A.</td><td>174.4</td></tr><tr><td>2019/08/10</td><td>GPI</td><td>47.7</td><td>N.A.</td><td>N.A.</td><td>174.8</td></tr><tr><td>2025/07/24</td><td>SPHERE</td><td>90</td><td>0.33</td><td>4.8</td><td>52.1</td></tr></tbody></table>

Notes. ΔPA: total FoV rotation. Coherence time and seeing are averaged over the exposures. N.A.: values that could not be found.

Its membership is confirmed by BANYAN Σ (Gagné et al. 2018) using Gaia DR3 (Gaia Collaboration et al. 2023) data.

The star is classified as a spectroscopic binary in the literature (Zakhozhay et al. 2022; Grandjean et al. 2023). Although too close (< 1 au ~ 7 mas) to be detected in DI, the astrometric solution from Gaia shows the presence of a companion at a 2.8 σ level (Appendix A). The presence of an unresolved companion demands a dedicated fitting of the measured photometry to determine the age of the system and the two masses. The main stellar parameters are listed in Table 1; masses and effective temperatures are to be read as ranges encompassing both random and systematic (i.e., model-dependent) uncertainties. We refer the interested readers to Appendix A for a discussion on the derivation of these parameters and their uncertainties.

## 3. Observations and data reduction

Observed in GPI's H-band (1.50-1.78 µm) as part of GPIES in 2016 and 2019, HD 143811 was classified as an interesting target for follow-up in S25 after the detection of a dim source (SNR= 5.4) at about 0.4 arcsec from the central binary. Due to the low SNR of the second epoch (SNR= 3.5), we could not confirm the candidate — hereafter, CC1 — as a real companion. For these reasons, we requested a third epoch with SPHERE, that was obtained on July 24, 2025 (Program ID 114.27LG.002). The SPHERE observation was carried out in the IRDIFS mode: the Integral Field Spectrograph (IFS; Claudi et al. 2008) collected YJ-band (0.95-1.33 µm) spectroscopy over a 1.7" × 1.7" field of view, whereas the Infra-Red Dual-band Imager and Spectrograph (IRDIS; Dohlen et al. 2008) provided dual-band imaging in H23 (1.593 µm, 1.667 µm). The main details of the observations are provided in Table 2. A complete description of the pipeline that handles the data reduction from raw frames to PACO SNR map can be found in Chomez et al. (2023) for SPHERE data and in S25 for GPI data.

## 4. Results

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### 4.1. Confirmation of the planet

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SNR maps from PACO for the three epochs are reported in Figure 1. CC1 is clearly reddetected (SNR= 17.8) at the expected position in IRDIS data (sep = 431.1 ± 0.63 mas, PA = 21.71 ± 0.09 deg); conversely, no detection could be found in IFS data. Details about the astrometry and the photometry of the source are provided in Table 3. A good agreement between GPI (2016) and SPHERE/IRDIS (H2, H3) photometry<fnref n="1" /> can be seen; we consider GPI (2019) photometry as unreliable due to the low SNR.

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As is usual in direct imaging, the confirmation of CC1 involved the comparison of its astrometric displacements across the three epochs with the motion expected from a background interloper. As a first step, we verified that the relative proper motion of the source is compatible with a bound orbit around the central binary. Afterwards, we constructed a suitable sample of synthetic background stars using the Besançon Galaxy Model (Czekaj et al. 2014), and found that a proper motion such as the one of the source is ruled out at a C.L. = 3.6 σ. An additional argument against the background scenario is provided by the non detection of the source in IFS data, which sets a lower limit to the J - H2 color of the source, J - H2 > 1.4 mag. While such a red color is not atypical for young giant planets (see, e.g., HR 8799 planets; Bonnefoy et al. 2016), we found that no simulated source appears to be as red — in other words, every source from the synthetic sample would have been detected by IFS.

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Based on these two lines of evidence (described in more detail in Appendix B), we rule out the hypothesis of a background contaminant, confirming that CC1 — hereafter, HD 143811 (AB)b — is part of the HD 143811 system.

### 4.2. Orbital properties

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In order to constrain the orbital parameters of the planet, we used the **orbitize!** v3 package (Blunt et al. 2024) — specifically, the Orbits for the Impatient (OFTI) algorithm (Blunt et al. 2017). We included the three astrometric points (GPI IFS 2016, GPI IFS 2019, and SPHERE IRDIS 2025) in Table 3 and used a binary star mass of M = 2.44 ± 0.09 M<sub>☉</sub> (see Appendix A) and parallax as from Table 1. **orbitize!** default priors were used. The OFTI algorithm was run until we obtained 10⁶ accepted orbits. The planet's astrometry is consistent with a long-period orbit (T ~ 300 yr, corresponding to a ~ 60 au) with low eccentricity (e = 0.19 ± 0.12), seen almost face-on (i = 37°+11°). The impact of systematic stellar mass uncertainties is negligible (Δa/a < 2%, Δe/e < 10%, Δi/i < 5%, ΔT/T < 0.3%). A randomly drawn sample of orbits extracted from the posterior are plotted in their sky projection in Fig 2. Additional details on the priors and the posteriors can be found in Appendix C.

### 4.3. Physical properties

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Photometric data from Table 3 can be used to constrain the physical properties of the planet by comparison with atmospheric models. In the case of IFS, the wavelength-collapsed detection limit from PACO (enhancing the sensitivity by a factor ~√n<sub>wvl,IFS</sub> = √39 compared to individual λ slices) at the expected source separation was used as upper limits — adopting an effective λ<sub>IFS</sub> = 1.14 ± 0.10 µm. 2019 data were not used due to

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<footnote n="1">¹ SPHERE H2- and H3-band photometry was estimated from GPI data as described in S25.</footnote>
<table><caption>**Table 3.** Astrometric and photometric properties of CC1. H2 and H3 indicate absolute magnitude in IRDIS H23 bands.</caption><thead><tr><th>ObsID</th><th>SNR</th><th>Contrast (mag)</th><th>Separation (mas)</th><th>PA (°)</th><th>H2 (mag)</th><th>H3 (mag)</th><th>H2-H3 (mag)</th></tr></thead><tbody><tr><td>GPI H (2016)</td><td>5.4</td><td>11.9</td><td>427.8 ± 3.0</td><td>11.79 ± 0.46</td><td>14.14 ± 0.32</td><td>13.85 ± 0.29</td><td>0.29 ± 0.40</td></tr><tr><td>GPI H (2019)</td><td>3.5</td><td>12.5</td><td>427.3 ± 3.3</td><td>16.29 ± 0.38</td><td>15.00 ± 0.25</td><td>14.27 ± 0.19</td><td>0.73 ± 0.27</td></tr><tr><td>SPHERE H23 (2025)</td><td>17.8</td><td>12.0</td><td>431.1 ± 0.6</td><td>21.71 ± 0.09</td><td>14.31 ± 0.10</td><td>13.85 ± 0.10</td><td>0.44 ± 0.14</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** SNR maps produced from PACO in the three epochs. The image scale is shown in the lower right corner; the position of the planet is indicated by an arrow. The left colorbar refers to both GPI maps.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** The orbits of HD 143811 b. Left: 50 randomly drawn accepted orbits from the OFTI posterior are plotted in their sky projection. Measured astrometry is plotted in red from GPI (2016 and 2019) and SPHERE/IRDIS (2025) data. Right: the same randomly drawn accepted orbits plotted as a function of time, with red dots showing the measured astrometry and associated errors.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Spectro-photometry of the planet compared to the best-fit model from HADES (red line). Black circles show the GPI spectrum, the blue square SPHERE photometry, and the green arrow is the SPHERE-IFS 5σ upper limit (all with $1\sigma$ errors). The orange line shows 5σ IFS upper limits in individual $\lambda$ slices. Predicted photometric points are shown with red markers.</figcaption></figure>

the low SNR. The procedure to convert contrast data into absolute flux measurements is described in S25, with slight variations detailed in Appendix D. A binary template was adopted for the star; the impact of mass ratio and $T_{eff}$ uncertainties on the derived planet flux is negligible.

Spectro-photometric data were analyzed using a grid of atmosphere-interior models from the HADES model (Wilkinson et al. 2024). Six physical parameters were adjusted within a Monte Carlo Markov Chain (MCMC) framework. We adopt uniform (flat) priors for all fitted parameters, given in Table D.1. The age of the system, used as an additional constraint, was predicted from the other parameters adopting a medium entropy value per mass from Mordasini et al. (2017) equivalent to a "warm start". The system age upper limit (21 Myr) was treated as an upper limit, while a lower limit was set based on the estimated median disk life for an F-type star ($\tau$ = 5 Myr; Pfalzner et al. 2022).

Details on the implementation of the MCMC, on the age limits, and on the derived posteriors can be found in Appendix D.

The best fitting sample, with the highest log-likelihood, is shown in Figure 3. The GPI and SPHERE photometry are well fitted by the model. This result also explains, without any tension within the fit, why there is a non detection with SPHERE-IFS. Our analysis provides a strong constraint on the planet's intrinsic temperature: $T_{int}$ = 1000 ± 30 K. Combined with the system's age constraint, this temperature implies a mass of $6.1^{+0.7}_{-0.9} M_{Jup}$. The radius, derived from the other physical parameters at each MCMC sample, is $1.4 \pm 0.1 R_{Jup}$. The atmospheric metallicity is weakly constrained around a solar value, while the core mass and sedimentation efficiency ($f_{sed}$) remain poorly constrained. The fit also indicates vigorous vertical mixing, although this parameter
# Appendix A: Stellar parameters

According to BANYAN $\Sigma$, HD 143811 has a 70% membership probability to Upper Scorpius (US) and a 30% to Upper Centaurus-Lupus (UCL), two of the three regions in which Scorpius-Centaurus is classically divided (de Zeeuw et al. 1999). Indeed, the sky coordinates of the system are intermediate between the two subgroups, placing it in a region whose typical age is 13-15 Myr (Pecaut & Mamajek 2016).

As in S25, we estimated the reddening $E(B-V)$ by integrating the map by Leike et al. (2020) along the line of sight, finding $E(B-V) = 0.044$ mag. A larger value, $E(B-V) = 0.11$ mag, is suggested by Gontcharov & Mosenkov (2017); we allowed thus the parameter to vary between these two extremes.

The derivation of constraints on the mass ratio between the two stars is complicated by the lack of a direct detection of the secondary. As a first step, we used GaiaPMEX (Kiefer et al. 2024), a tool featuring a model of the Renormalized Unit Weight Error (ruwe) and of the Gaia-Hipparcos proper motion anomaly (Kervella et al. 2022)) expected for single stars, to try to constrain the properties of the companion. The ruwe, significant at a $2.8\sigma$ level, and the non-detection via PMa constrain the companion to a semi-major axis shorter than 10 au, and more probably < 3 au. However, no constraint could be found on the mass ratio owing to the large degeneracy between semi-major axis and companion mass. Fortunately, the SB2 signature detected by Grandjean et al. (2023) provides strong evidence for a flux ratio $\ge 0.5$, corresponding to a mass ratio $q \ge 0.9$.

We decided to undertake a more precise derivation of the mass ratio of the two components by exploiting the 5 spectra acquired by HARPS (Mayor et al. 2003). The spectra are separated in two main epochs (JD-2458243 and JD-2458606). Following the methodology explained in Kiefer et al. (2018), the spectra were fed to TODCOR (Mazeh & Zucker 1994), a tool that calculates a 2D cross correlation function based on a linear combination of two synthetic spectra and an observed spectrum.

Synthetic spectra were taken from the PHOENIX library (Husser et al. 2013). The stellar parameters defining the two synthetic spectra from the library used as input for TODCOR were optimized through an iterative $\chi^2$ minimization, iteratively fitting the stellar parameters ($T_{\text{eff},j}$, $\log g_j$, $\langle V \sin(i) \rangle_j$, where $j=A,B$; $\alpha = F_B/F_A$ the flux ratio at 5000 Å; [Fe/H] was set to solar) one after the other. We used a single order at $\sim 610$ nm (order 61 in HARPS) that contains many different, well separated, deep as well as shallow, absorption lines. The two SB2 components are well separated in all the 5 spectra of HD 143811. The five HARPS spectra were fitted independently, taking in the end the average and standard deviation of each parameter as our best-fit value.

The fit, shown in Figure A.1, yields $T_{\text{eff},A} = 6791 \pm 100$ K, $\log g_A = 4.34 \pm 0.01$, $v_A \sin i = 7.5 \pm 0.5$ km s$^{-1}$, $T_{\text{eff},B} = 6253 \pm 150$ K, $\log g_B = 4.41 \pm 0.02$, $v_B \sin i = 5.0 \pm 0.6$ km s$^{-1}$, $\alpha = 0.77 \pm 0.05$. If we assume that $F_B/F_A \sim q^4$ in the visible$^2$, this flux ratio corresponds to an expected mass ratio $\sim 0.94$.

Based on the optimized synthetic spectra determined above, we calculated the 2D-CCF of all the 5 spectra of HD 143811, removing beforehand the orders polluted by telluric lines. The 1D cuts of all the 2D-CCF along the primary and secondary velocities are shown in Figure A.2.

We derive relative radial velocities between the two epochs of $\Delta V_A = 21.7$ km/s and $\Delta V_B = 24.3$ km/s leading to $q = 0.89$. This is in good agreement with the mass ratio inferred from the flux ratio above. Therefore, we allowed for $q \in [0.85, 0.95]$ in our analysis.

<figure><img src="image_5.png" /><figcaption>**Fig. A.1.** 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.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. A.2.** 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).</figcaption></figure>

A solar metallicity, typical of young star-forming regions in the solar neighborhood (D’Orazi et al. 2011; Biazzo et al. 2012), is expected for Scorpius-Centaurus stars. However, in order to simultaneously account for the constraints on $E(B-V)$, age, and $q$ ratio, the metallicity of the star was allowed to vary between slightly subsolar and slightly super-solar values.

All the constraints on the system detailed above were provided as input to MADYS (Squicciarini & Bonavita 2022), so as to simultaneously derive the age of the system and the masses of the two components through isochrone fitting. As in S25, we opted for PARSEC v2.0 (Nguyen et al. 2022) isochrones. The analysis reveals a preference for [Fe/H] = -0.13 (similarly to Zakhozhay et al. 2022), as the system age would be pushed to values older than 20 Myr or to smaller $q$ for [Fe/H] = 0.00. We derive an age $t = 18 \pm 3$ Myr, masses $M_1 = 1.29 \pm 0.05$ M$_\odot$ and $M_2 = 1.15 \pm 0.07$ M$_\odot$, and temperatures $T_{\text{eff},1} = 6715 \pm 150$ K and $T_{\text{eff},2} = 6380 \pm 150$ K.

It is known that PARSEC isochrones tend to overestimate the effective temperature of young stars, but they typically pro-

<footnote n="2">2 Lindegren et al. 2022 technical note: https://dms.cosmos.esa.int/COSMOS/doc_fetch.php?id=1566327.</footnote>
vide a good match to the observed color–luminosity relations (see, e.g., Ruiz-Dern et al. 2018; Gaia Collaboration et al. 2018); this gives us confidence that our mass determination is more robust than the effective temperature. In order to estimate the parameter uncertainty due to systematic differences across models, we repeated the analysis using MIST (Dotter 2016; Choi et al. 2016) and BHAC15 (Baraffe et al. 2015) isochrones. MIST – linearly interpolated between [Fe/H] = −0.25 and [Fe/H] = 0.00 – yields similar results to PARSEC: $M_1 = 1.30 \pm 0.05 M_\odot$, $M_2 = 1.17 \pm 0.07 M_\odot$, $T_{\text{eff},1} = 6700 \pm 200 K$, $T_{\text{eff},2} = 6300 \pm 200 K$. Conversely, BHAC15 isochrones – only available at solar metallicity – are associated to higher masses and lower temperatures: $M_1 = 1.35 \pm 0.05 M_\odot$, $M_2 = 1.21 \pm 0.07 M_\odot$, $T_{\text{eff},1} = 6400 \pm 150 K$ and $T_{\text{eff},2} = 6000 \pm 250 K$. However, the large $\chi^2$ of the best-fit solution ($\chi^2 > 10$), similar to the one found with PARSEC and MIST isochrones of solar metallicity, suggests that a slightly subsolar metallicity – similar to the one found for the UCL member HD 139614 (Murphy et al. 2021) – might be a real feature of the system.

To summarize, we stress that the lack of a direct detection of the secondary, coupled with the intrinsic systematic uncertainties underlying models of pre-main sequence stars, does not allow us to determine in an unambiguous and accurate way the parameters of the system. Still, the total mass of the system ($M_{\text{tot}} \approx 2.50 M_\odot$) appears to be reasonably well constrained, and the temperature uncertainty does not impact in a significant way the derived planetary parameters (Appendix D).

## Appendix B: Ruling out the background scenario

Confirming a DI candidate requires the assessment of its proper motion, that must be compatible with a bound orbit and significantly different from the motion of background stars that might appear by chance at a short projected separation from the target.

The relative proper motion of CC1 with respect to the star can be estimated as the difference between J2025 and J2016 astrometry, divided by their time span: $\Delta\mu_\alpha^* = 8.74 \pm 0.32$ mas/yr and $\Delta\mu_\delta = -2.53 \pm 0.34$ mas/yr for right ascension and declination, respectively. These correspond to an heliocentric proper motion $\mu_{\alpha,CC1}^* = -6.11 \pm 0.32$ mas/yr and $\mu_{\delta,CC1} = -27.51 \pm 0.34$ mas/yr. The total proper motion difference, $\Delta\mu = 9.10 \pm 0.32$ mas/yr, can be easily converted into a physical velocity difference: $\Delta v = 5.9 \pm 0.2$ km/s. A planet at the projected separation of CC1 cannot be at a physical separation from the central binary smaller than $d = \text{sep}[\text{mas}]/\bar{\omega}[\text{mas}] \approx 60$ au, $\bar{\omega}$ being the parallax of the system. If $\Delta v$ were larger than the escape velocity from the system at a distance $d$, then no closed orbit could be possible and CC1 would have to be a passing-by object. Being $v_{\text{esc}}(d) = \sqrt{2GM_{\text{tot}}/d} = 8.5 \pm 0.1$ km/s larger than $\Delta v$, the motion of CC1 is compatible with that of a bound object.

In order to test the hypothesis that CC1 is a background interloper, we computed the probability that a background source be responsible for the observed astrometric shift (see, e.g., Squicciarini et al. 2022). We constructed therefore a sample of synthetic background stars using the Besançon Galaxy Model (Czekaj et al. 2014). The sample is defined by a distance [0, 50] kpc, an apparent H magnitude similar to the one of CC1 ($H \in [19.5, 20.1]$ mag), and a radius = 1° centered on the position of HD 143811. The sample is composed of 38825 stars, whose median and [16, 84] percentiles of the proper motion components are $\mu_{\alpha,gal}^* = -4.02_{-2.77}^{+2.94}$ mas yr$^{-1}$ and $\mu_{\delta,gal} = -3.44_{-2.51}^{+2.53}$ mas yr$^{-1}$. These values are significantly different from those of CC1, which crucially exhibits a $\mu_\delta$ that is even more negative – that is, even

farther away from the bulk of the background star population – than the one of HD 143811. Indeed, only 11 stars have a more extreme $\mu_\delta$ than the star and a total proper motion compatible with a bound orbit (Figure B.1). This corresponds to a false alarm probability of $3 \times 10^{-4}$, that is, a C.L. = 3.6 $\sigma$.

Additional information on the nature of the source is provided by its colors. We showed in S25 that the ($H2 - H3, H2$) position of CC1 is not decisive, as it can be compatible with both a planet and a background star; however, the non detection of the source in IFS data can be in principle a discriminating factor between the two scenarios. IFS detection limits correspond to an apparent $J > 21.34$ mag; coupled with the measured H-band flux for CC1, this sets a lower limit to the $J-H2$ color of the source, $J-H2 > 1.4$ mag. While such a red color is not atypical for young giant planets (see, e.g., HR 8799 planets; Bonnefoy et al. 2016), we do not expect to measure it in background stars. Using the same synthetic sample as above, we find a median apparent $J = 20.4 \pm 0.2$ mag, well within our detection capabilities; in fact, no source in the entire sample is dimmer than our detection limits – the dimmest source having $J = 20.9$ mag.

<figure><img src="image_7.png" /><figcaption>**Fig. B.1.** Astrometric displacements for CC1 in 2019 (green star) and 2025 (orange square) with respect to 2016. A static background star observed in 2025 would lie in the position of the empty star; a source with null relative motion compared to the target would lie where the filled star is. Simulated background sources from the Besançon model are shown as blue dots, with their median and (16th, 84th) percentiles shown as a blue cross. The area encompassing bound orbits at $d = 60$ au is filled in red.</figcaption></figure>

## Appendix C: Details on the orbital fitting

The priors adopted for the orbital fitting of the planet astrometry are provided in Table C.1. Default priors were used: a log-uniform prior for the semi-major axis; a sine-uniform distribution for the inclination; uniform priors for the other parameters (eccentricity, argument of periastron, position angle of nodes and epoch of periastron passage). The results of the analysis are presented in the same table, excluding those parameters that were
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not constrained by the analysis. The corner plot of the Bayesian analysis is shown in Figure C.1.

<table><caption>**Table C.1.** Priors and inferred parameters from the MCMC analysis obtained with **orbitize!**.</caption><thead><tr><th>Orbital parameters</th><th>Prior</th><th>Final estimate</th></tr></thead><tbody><tr><td>Semi-major axis (au)</td><td>[10<sup>-3</sup>, 10<sup>4</sup>] (log)</td><td>63<sup>+30</sup><sub>-12</sub></td></tr><tr><td>Eccentricity</td><td>[0,1]</td><td>0.19 ± 0.12</td></tr><tr><td>inclination (°)</td><td>[0,90] (sine)</td><td>37<sup>+11</sup><sub>-12</sub></td></tr><tr><td>Period (yr)</td><td></td><td>319<sup>+248</sup><sub>-86</sub></td></tr></tbody></table>

**Notes.** The reported values are the median of the marginalized posterior distribution, with errors corresponding to the 16th and 84th percentiles.

<figure><img src="image_8.png" /><figcaption>**Fig. C.1.** Posterior distributions of orbital parameters retrieved by OFTI (10⁶ accepted orbits). We did not plot unconstrained parameters with mostly flat posterior distributions.</figcaption></figure>

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## Appendix D: Derivation of planetary parameters

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Since spectro-photometric measurements from DI instruments are expressed in units of source-to-star contrasts, the first step of the analysis of a planet spectrum deals with the conversion of contrast data into absolute fluxes. Compared to the standard approach (see S25), where a synthetic stellar spectrum from the BT-Nextgen AGSS2009 library (Allard et al. 2011) is used, we built an unresolved binary spectrum by summing up two such spectra with $T_{eff} = 6715$ K and $T_{eff} = 6380$ K (see Appendix A). This spectrum was then, as in the standard procedure, renormalized so as to match the measured H-band magnitude from 2MASS (Skrutskie et al. 2006). We verified that the impact of the binary spectrum compared to the standard single-star spectrum on the planet flux is negligible (≈ 0.2%) across GPI wavelengths, increasing to 4% only in the case of the IFS upper limit. Even when accounting for the systematic uncertainties on the

effective temperature, the impact on the upper limit remains limited (< 7%). Hence, the spectral analysis is robust against uncertainties in the mass ratio $q = 0.90 \pm 0.05$ and the effective temperatures of the two stars.

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As mentioned in Section 4.3, we analyzed six physical parameters using a Markov Chain Monte Carlo (MCMC) framework applied to a grid of coupled atmosphere-interior models from HADES (Wilkinson et al. 2024). We employ the affine-invariant MCMC ensemble sampler emcee to explore the parameter space. We use 50 walkers and run the chain for 400,000 steps after a conservative burn-in phase of 2,000 steps, ensuring the run is approximately 50 times the estimated autocorrelation time for the best-constrained parameters. We adopt uniform (flat) priors for all fitted parameters, given in Table D.1.

<table><caption>**Table D.1.** Priors and inferred parameters from the MCMC analysis obtained with HADES codes.</caption><thead><tr><th>Planetary parameters</th><th>Prior</th><th>Final estimate</th></tr></thead><tbody><tr><td>Mass (M<sub>p</sub>/M<sub>Jup</sub>)</td><td>[0.1, 8.0]</td><td>6.1<sup>+0.7</sup><sub>-0.9</sub></td></tr><tr><td>Intrinsic Temp. (T<sub>int</sub>) [K]</td><td>[300, 1500]</td><td>1000 ± 30</td></tr><tr><td>Metallicity ([Fe/H]) [dex]</td><td>[-2, 3]</td><td>0.1<sup>+0.8</sup><sub>-0.4</sub></td></tr><tr><td>Core Mass (M<sub>core</sub>/M<sub>⊕</sub>)</td><td>[1, 100]</td><td>Unconstrained<sup>3</sup></td></tr><tr><td>Sedimentation (f<sub>sed</sub>)</td><td>[1, 6]</td><td>2.8<sup>+1.1</sup><sub>-0.8</sub></td></tr><tr><td>Vertical Mixing (log K<sub>zz</sub>)</td><td>[1, 12]</td><td>9.0<sup>+1.2</sup><sub>-1.5</sub></td></tr><tr><td>Radius (R<sub>Jup</sub>)</td><td>Model derived</td><td>1.4 ± 0.1</td></tr></tbody></table>

**Notes.** All priors are uniform in the given ranges. The reported values are the median of the marginalized posterior distribution, with errors corresponding to the 16th and 84th percentiles.

The age of the system is used as an additional constraint. For each step in the MCMC, the age is predicted from the physical parameters using the additional pre-computed HADES evolution model. This predicted age is then compared to the known system upper age of 21 Myr, which is treated as a soft upper limit. We use the upper bounds of the estimated median disk life for the stellar spectral type from Pfalzner et al. (2022) of 5 Myr as a soft lower limit to the planets age. The age is estimated considering a medium entropy value per mass from Mordasini et al. (2017) equivalent to a "warm start". The likelihood for the soft limits, as well as for the instrumental upper limit from the SPHERE-IFS data, is calculated using the cumulative distribution function of a Gaussian. This allows for uncertainty on the systems age estimation and for the noise in the SPHERE-IFS data.

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The derived posterior distributions are shown in Figure D.1, with the inferred planetary parameters summarized in Table D.1.

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<footnote n="3">³ The posterior for $M_{core}$ is bimodal and poorly constrained; see Fig. D.1.</footnote>
<figure><img src="image_9.png" /><figcaption>Fig. D.1. Posterior distributions of physical parameters retrieved using the HADES grid.</figcaption></figure>