If Planet X had a perihelion around 60 AU and an aphelion in the vicinity of Sedna's aphelion (940 AU), would it be considered to be responsible for both the high eccentricities of TNO's orbits and the Kuiper cliff, the abrupt outer end of the Kuiper belt at ~50 AU? Or isn't it expected that the planet would have a that close perihelion? If so, why not? Except Eris and Sedna, there's no other (dwarf) planet that would be at their current distances, after all. We would have already discovered it, right?

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    $\begingroup$ Is there an abrupt end to the Kuiper belt? Look at the discovery dates of Eris and Sedna.... what gives you the confidence to assume we've seen them all? Up to which distance? $\endgroup$ Jul 12 at 7:53
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    $\begingroup$ @planetmaker Eris and Sedna aren't KBOs, they don't have many bodies in their orbital neighbourhood and none approaching their size or mass. Eris and Sedna are scattered disk objects. Why didn't you just google Kuiper cliff? $\endgroup$
    – John
    Jul 12 at 8:03

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