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 MJup. 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 MJupand a secondary mass between 2.6– 5.2 MJup, 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 MJup between Upper Sco and Taurus. In Taurus, a binary fraction of 5.6−2.3+3.2% is found for objects with masses below 30MJup, and of 7.8−2.4+3.0% for objects with masses below 50MJup, 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.