Free-floating binary planets from ejections during close stellar encounters

The discovery of planetary systems beyond our Solar System has challenged established theories of planetary formation. Planetary orbits display a variety of unexpected architectures, and free-floating planets appear ubiquitous. The recently reported detection of candidate Jupiter-mass binary objects (JuMBOs) by the James Webb Space Telescope (JWST) has added another puzzling layer. Here, we demonstrate in direct few-body simulations that JuMBOs could arise from the ejection of two giant planets following a close encounter with a passing star, if the two planets are nearly aligned at closest approach. These ejected JuMBOs typically have an average semimajor axis approximately three times the orbital separation within their original planetary system and a high eccentricity, characterized by a superthermal distribution that sets them apart from those formed primordially. We estimate the JuMBO formation rate per planetary system in typical and densely populated clusters, revealing a significant environmental dependence. In dense clusters, this formation rate can reach a few percent for wide planetary systems. A comparative analysis of JuMBO rates and properties with current and forthcoming JWST observations across various environments promises to offer insights into the conditions under which these giant planets formed in protoplanetary disks, thereby imposing constraints on theories of giant planet formation.