H. Bouy, G. Duchêne, G. Strampelli, J. Aguilar, J. Olivares, D. Barrado, S. N. Raymond, N. Huélamo, M. Tamura, E. Bertin, W. Brandner, J. -C. Cuillandre, P. A. B. Galli, N. Miret-Roig

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. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects formed in these environments), these low-mass substellar binaries represent a most sensitive probe of the mechanisms at play during the star formation process. 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 5-33 M$_{\rm Jup}$. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9$\pm$0.4 mas, or $\sim$18 au assuming a distance of 160pc, with an estimated primary mass in the range between 3-6 M$_{\rm Jup}$ and a secondary mass between 2.6-5.2 M$_{\rm Jup}$, depending on the assumed age. This corresponds to an overall binary fraction of 1.8$^{+2.6}_{-1.3}$% among free-floating planetary mass objects over the separation range $\ge$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 $\sim$25 M$_{\rm Jup}$ between Upper Sco and Taurus. In Taurus, five companions were identified among 78 observed objects (4.9$^{+2.8}_{-2.0}$%), whereas none were found among 97 objects in Upper Sco ($\le$1.2%).}