![Orbital trajectories of two example scattering experiments. Left: An interaction that results in a free-floating planet-moon pair. Right: An interaction that results in a free-floating planet and a separate free-floating moon. The starting points of the bodies are indicated by the black dots. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/07/Planet-moon.png?fit=1200%2C676&ssl=1)
[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∼0.45RH. Planet-only ejection cross-sections instead decline steadily with ap. Io-like moons (am=0.008RH) survive, but survival falls beyond 0.4RH. Compared with planet-planet scattering, stellar encounters preserve moons more effectively across all separations.
Surviving moons at am≲0.4RH 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∼1.3au and ∼5.9 au for a solar-mass host.
Uncertainties remain large and the probability distribution is flat but we prefer ap∼5.2au. 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.
Yannick Badoux, Simon Portegies Zwart
Comments: Submitted to A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2607.16402 [astro-ph.EP] (or arXiv:2607.16402v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2607.16402
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Submission history
From: Simon Portegies Zwart
[v1] Fri, 17 Jul 2026 18:00:09 UTC (756 KB)
https://arxiv.org/abs/2607.16402
Astrobiology, exoplanet, interstellar,
