Resolving the degeneracy between eccentric planets and 2:1 mean motion resonances

The dynamical state of a multiple planet system reflects the underlying processes of planet formation and orbital evolution. Radial velocity planet searches have yielded several systems that appear to be in or near a low-order mean-motion resonance, including several near a 2:1 mean-motion ratio. Still more systems apparently contain one eccentric planet. We demonstrate that many of these systems are indistinguishable from a system with two planets in a 2:1 mean-motion ratio, and outline a framework for determining how often the observed, apparently-single planets are likely to have such a second planet present.