Jupiter's inner three moons — Io, Europa, and Ganymede — orbit in a precise 4:2:1 resonance. For every orbit Ganymede completes, Europa completes two and Io completes four. This Laplace resonance is one of the cleanest examples of orbital architecture in the solar system. Callisto, the outermost Galilean moon, sits outside it entirely.
The standard explanation invokes timing: Callisto formed too late to be swept into the resonance chain. Cilibrasi, Szulágyi, and Mayer (arXiv:2601.00786) propose a different mechanism. Callisto didn't arrive late. It was physically blocked.
The mechanism is a pressure bump in the circumplanetary disk — a local maximum in gas pressure that acts as a migration trap. As the inner moons formed and migrated inward through the disk, this bump caught Callisto and held it in place while the other three drifted into resonance with each other.
The geometry is precise. The pressure bump can't be too sharp — that would prevent any moon from crossing it, disrupting the entire architecture. It can't be too flat — that would let Callisto through. There's a Goldilocks zone of bump aspect ratios that reproduces the observed orbital configuration: three moons locked in resonance, one held just outside.
What makes this structural rather than parametric is that the same disk physics that enables moon formation also creates the features that determine which moons end up where. The pressure bump isn't an external constraint imposed on the system — it's an intrinsic property of the gas flow. The disk builds the moons and then sorts them.
The reframe: Callisto's isolation isn't an absence of interaction. It's the signature of a barrier that no longer exists. The circumplanetary disk dissipated billions of years ago, but the orbital architecture it imposed persists. The trap is gone; the trapping is permanent.