Microlensing surveys keep finding free-floating planets — objects with planetary masses drifting through interstellar space, unbound to any star. The standard explanation is that they formed normally around a star and were later ejected by planet-planet scattering. But the numbers are hard to reconcile: the ejection rates from known planetary systems don't easily produce the population observed.
Sheridan et al. (arXiv:2601.03820) propose a different origin. The planets weren't ejected from finished systems. They were ejected during the system's construction.
The mechanism begins with gravitational instability in a massive protostellar disk. The disk fragments, producing multiple clumps. The most massive fragment undergoes runaway accretion, growing into a secondary star — the system transitions from single to binary. As this happens, the two stars create a gravitational pincer: the primary and the growing secondary trap smaller fragments between their competing gravitational fields, then eject them.
The ejection efficiency reaches 50% when the secondary exceeds about 10% of the primary's mass. Half the small planets formed in the disk get expelled. The growing binary is both nursery and catapult — the same gravitational instability that formed the planets powers the mechanism that expels them.
The counterintuitive finding: these ejected objects are evidence for disk fragmentation, not against it. Disk fragmentation was thought to produce only massive objects — gas giants and brown dwarfs. But the process creates a range of masses, and the lighter ones get preferentially ejected because they're less gravitationally bound. The free-floating planets discovered by microlensing may be the strongest evidence that disk fragmentation forms objects far less massive than Jupiter.
The system that builds siblings also chooses which ones to keep.