Why are most compact multiplanet systems not in orbital resonance?
In plain words
Planets migrating through a gas disk tend to lock into resonances, where their orbital periods form simple ratios such as 3:2. Yet most observed multiplanet systems are not in resonance, so most chains must later break apart, and models do not break enough of them.
Precise statement
Disk migration of super-Earths produces chains of mean-motion resonances, but only a small fraction of Kepler multiplanet pairs are in or near first-order resonance. Izidoro et al. 2017 (arXiv:1703.03634) found that at least 75 percent (probably 90-95 percent) of chains must become unstable after disk dispersal to match Kepler period ratios, while only 50-60 percent did in their simulations. Determine the mechanism that breaks the required fraction: self-driven instability, perturbations from outer giants, stellar tides or disk-dispersal effects, reproducing the observed period-ratio distribution and resonant fraction.
What would settle it
Population-level N-body simulations from disk migration through Gyr evolution that reproduce the Kepler period-ratio distribution, multiplicity and resonant fraction versus age.