The Gathering Resonance

The standard ring model for terrestrial planet formation starts with a narrow annulus of material inside 1 AU that collapses into Mercury, Venus, Earth, and Mars. It explains why Mars is small — it formed at the ring's edge, where material was sparse. But it doesn't explain why the inner planets have systematically different oxidation states, or why Earth's bulk composition requires mixing from chemically distinct reservoirs.

Poon et al. (arXiv:2601.04032) propose that the inner Solar System had two sources, not one, and that the mechanism connecting them was a resonance that moved.

The setup: Jupiter and Saturn, locked in mean-motion resonance in the outer disk, impose a secular gravitational perturbation on the inner disk. As the gas disk dissipates, the location of this secular resonance shifts — it sweeps inward. And as it sweeps, it gathers outer planetesimals like a plow moving through loose soil, compressing them into a narrow ring that migrates toward the Sun.

The result is two reservoirs. A refractory-enriched ring inside 1 AU that formed Mercury, Venus, and proto-Earth. And an outer, more oxidized disk whose material was swept inward by the resonance to form Mars and deliver the oxidized component to Earth.

What makes this structural: the resonance isn't a disruption. It's a sorting mechanism. The same gravitational perturbation that could scatter planetesimals instead collects them — because the perturbation moves through the disk rather than sitting in one place. A stationary resonance would clear a gap. A moving resonance fills one.

The model simultaneously explains Mercury-to-Mars oxidation gradients, isotopic patterns in the asteroid belt, and the origin of aubrite meteorites. The through-claim is that planetary chemistry isn't determined by where material formed — it's determined by where a moving gravitational wave deposited it. The plow's path, not the soil's origin, decides where the garden grows.