White dwarfs are polluted. Spectroscopic observations show heavy elements — calcium, magnesium, iron — in atmospheres that should be pure hydrogen or helium. These elements sink below the photosphere on timescales of days to millions of years, so their presence means something is actively delivering rocky material to the stellar surface. The puzzle is how. The star has lost half its mass during the red giant phase, destabilizing planetary orbits, but simply destabilizing orbits doesn't aim asteroids at a target the size of Earth.
Wood and Cummins propose a delivery mechanism: three-body mean motion resonance chains. When three surviving planets lock into orbital period ratios like 4:2:1 or 6:3:2, the overlapping resonances create chaotic zones that can pump asteroid eccentricities high enough to reach the white dwarf's Roche limit, where tidal forces shred the asteroid into an accretion disk.
The efficiency depends on specific parameters. The inner planet's mass controls delivery — a more massive inner planet creates stronger resonant perturbations that reach deeper into the asteroid belt. The outer planet's mass doesn't matter. The asymmetry makes physical sense: the inner planet is closer to the asteroids and to the white dwarf, so its gravitational influence dominates the delivery pathway.
The numbers work. An asteroid reaching the Roche limit every 13.8 million years sustains the observed accretion rate of 10^8 grams per second. The mechanism doesn't require special initial conditions — just surviving planets in resonance, which post-main-sequence dynamical evolution can produce naturally.
The dead star is fed by the clockwork of its surviving planets. The resonance that preserved their orbits destroys the asteroids between them.