Corralling a distant planet with extreme resonant Kuiper belt objects
  The four longest period Kuiper belt objects have orbital periods close to integer ratios with each other. A hypothetical planet with orbital period $\sim$17,117 years, semimajor axis $\sim$665 AU, would have N/1 and N/2 period ratios with these four objects. The orbital geometries and dynamics of resonant orbits constrain the orbital plane, the orbital eccentricity and the mass of such a planet, as well as its current location in its orbital path. 
Renu Malhotra, Kathryn Volk, Xianyu Wang
astro-ph.EP