Over the past decade, microlensing surveys in field stars and the Gaia-informed census of nearby star-forming regions have convincingly demonsrated the presence of a significant population of free-floating planetary-mass objects. The origin of these objects is still debated, with the main plausible scenarios being: the dynamical ejection from a planetary system, the collapse of extremely low-mass prestellar cloudlets, or the ejection of stellar embryos from a molecular cloud before it can accrete enough mass to form a more massive brown dwarf or a star. The relevance of these different scenarios can be tested using multiple observational appraches, such as the mass distribution of free-floating planets, their disk occurrence rate, and their kinematic and multiplicity properties.

In this contribution, we focus here on the multiplicity of planetary-mass objects. Until now, only a handful of extremely low-mass binaries have been identified in serendipitous studies. Here, we present the results of an HST imaging survey of 60 photometrically- and kinematically-confirmed planetary-mass members of the Taurus star-forming region and of the Upper Scorpius association. In combination with ground-based seeing-limited images, these observations are sensitive to companions with projected separations as close as 5 au. We identify one 15 au binary system for which we present resolved photometry to confirm the nature of the components. We discuss the results of our survey in the context of theoretical studies as well as the recent identification with JWST of similar candidate systems in the Orion Nebula Cluster.