Multiplicity of young brown dwarfs and isolated planetary mass objects in Taurus and Upper Scorpius

Context. Free-floating planetary mass objects – worlds that roam interstellar space untethered to a parent star – challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation. Aims. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects that formed in these environments), these low-mass substellar binaries represent the most sensitive probe of the mechanisms at play during the star formation process. Methods. We use the Hubble Space Telescope and its Wide Field Camera 3 and the Very Large Telescope and its ERIS adaptive optics facility to search for visual companions among a sample of 77 objects, members of the Upper Scorpius and Taurus young nearby associations, with estimated masses in the range between approximately 6–66 M Jup . Results. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9±0.4 mas or ∼18 au assuming a distance of 160 pc, with an estimated primary mass in the range between 3–6 M Jup and a secondary mass between 2.6– 5.2 M Jup , depending on the assumed age. This corresponds to an overall binary fraction of 1.8 −1.3 +2.6 % among low-mass brown dwarfs and free-floating planetary mass objects over the separation range ≥7 au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below ∼30–50 M Jup between Upper Sco and Taurus. In Taurus, a binary fraction of 5.6 −2.3 +3.2 % is found for objects with masses below 30M Jup , and of 7.8 −2.4 +3.0 % for objects with masses below 50M Jup , while no binary was found among 80 objects over the matching luminosity range in Upper Sco, corresponding to an upper limit of ≤1.2%. Conclusions. This difference may point to intrinsically distinct formation conditions, with warmer parental molecular clouds originally present in Upper Sco potentially inhibiting fragmentation into the lowest-mass brown dwarfs and free-floating planets compared to cooler environments such as Taurus.