2026/07/17 by Yannick Badoux, Simon Portegies Zwart
#astro-ph.EP
Free-floating planet-moon pairs (FFPMs) may form when planetary systems experience close stellar encounters. We quantify moon-retention probabilities and ejection cross sections for planets and planet-moon systems subject to close stellar encounters, and to dentify the orbital conditions most favorable for forming FFPMs. We conducted extensive numerical scattering experiments to compute ejection cross-sections on a grid in the orbital separation of the planet (ap) and the moon (am), marginalizing over all other initial parameters. The simulations tracked both planetary and lunar fates across a wide range of encounter geometries. FFPM cross-sections depend on the moon semi-major axis am, decreasing gradually until a drop near am∼0.45 RH. Planet-only ejection cross-sections instead decline steadily with ap. Io-like moons (am = 0.008 RH) survive, but survival falls beyond 0.4 RH. Compared with planet-planet scattering, stellar encounters preserve moons more effectively across all separations. Surviving moons at am\lesssim 0.4 RH retain near-circular, low-inclination orbits, while wide-orbit moons show stronger dynamical excitation. Applying our cross-sections to the microlensing system MOA-2011-BLG-262Lb suggests possible progenitor semi-major axes between ap ∼ 1.3 au and ∼ 5.9 au for a solar-mass host. Uncertainties remain large and the probability distribution is flat but we prefer ap ∼ 5.2 au. Stellar-encounter ejections constitute a viable channel for producing FFPMs whose orbital properties differ from those formed by planet-planet scattering. Moon retention and orbital excitation provide promising diagnostics of ejection history. Current and upcoming microlensing and direct-imaging surveys may be capable of detecting Galilean-mass moons around rogue planets, offering new tests of dynamical formation pathways.