Flyby-induced Second Accretion as a Pathway to Giant Planets Around M Dwarfs

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Abstract Classical pebble and core accretion theories face a severe challenge: they struggle to produce giant planets around low-mass M dwarfs, whose protoplanetary disks contain insufficient material. Yet, discoveries of such systems continue to mount. We propose flyby-induced second accretion (FISA), a new formation channel in which an M dwarf already hosting a terrestrial planet flies through a younger star’s protoplanetary disk, capturing gas and dust to build a second-generation disk. The preexisting planet then undergoes renewed pebble and gas accretion within this replenished reservoir and migrates inward to become a close-in giant. We performed hydrodynamic and core-accretion simulations to show that, under favorable encounter geometries, protoplanets around M dwarfs can grow into gas giants via this two-stage process. We apply the FISA framework to TOI-6894 b, a 0.168 Jupiter-mass planet orbiting a 0.207 solar-mass star at just 0.026 au, showing that it is a plausible outcome of such a flyby. An order-of-magnitude estimation shows that the probability for a close-in gas giant to form in an M dwarf system via the proposed disk-flyby mechanism is roughly 3 × 10 −5 .

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