2025A&A...699A..78L

Context. Directly imaged exoplanets in wide orbits pose a challenge to current gas giant formation theories, as they need to form quickly and acquire enough material before the disc dissipates. These processes cannot be accommodated by in situ formation models based on core accretion.
Aims. We searched for wide separation (>100 au) planetary-mass companions with the Young Suns Exoplanet Survey (YSES). In this work, we present a planetary-mass candidate companion discovered as part of the survey.
Methods. We conducted follow-up observations of the candidate system after the first-epoch observations and obtained six epochs of observations for the candidate system between 2018 and 2024, along with the integral field spectroscopy of the stellar component.
Results. We report the detection of a candidate companion with H = 22.04 ± 0.13 mag at a projected separation of 730 ± 10 au from the primary star. High-angular-resolution-imaging observations of the central star show it is a visual binary. The acceleration data, orbital fitting, spectral energy distribution fitting, and radial velocity differences all suggest that there is at least one more unresolved low-mass stellar companion in this system. The planetary-mass candidate shows a significant proper motion comparable to that of the primary star. We have estimated an age of 19-28 Myr for the primary star. We cannot confirm the companionship of the candidate due to the unknown barycentre of the stars.
Conclusions. Long-term imaging and radial velocity monitoring of the central stars, along with spectroscopy of the candidate companion, are key to resolving the nature of this system. If confirmed, the candidate companion would be characterised by a mass of 3-5 M_J estimated with the ATMO evolutionary model. It would be another cold, low-mass planet resembling 51 Eri b and AF Lep b. Its extremely wide separation from the host star would challenge the formation theory of gas giant exoplanets.