This is certainly a problem for any hypothesis that form planet 9 by ejecting it from the inner solar system. The issue (for those wondering) is that if you simply scatter an object out of the inner solar system, but it is retained in a bound elliptical orbit, then it should come back! The proposed planet 9 needs to have a perhelion distance beyond 200 au and at least a moderate eccentricity.
The solution might be found in a paper by Bromley & Kenyon (2016), who address this very point. In their model they scatter a "super-Earth" from 5-15 au in the inner solar system, at an early stage of the solar system formation when the Sun is still surrounded by a gaseous disc. They run through a suite of simulation with different mass planets, different gas surface densities and different rates of evolution and dissipation of the gas disc.
The outcome is that there are sets of parameters that lead to eccentric orbits with large perihelion distances that are caused by damping by dynamical friction from the gas disc. What is required are long-lived, low-density gas discs, or more short-lived, massive gas discs that clear from the inside-out.
The same paper also summarises a number of other possibilities that could explain the large perihelion distance and eccentricity of putative planet 9. These are: (i) in situ formation from a ring of solids - though the orbit would tend to be more circular; (ii) the nearby passage of another star could perturb the orbit, provide a boost to the perihelion distance and increase the semi-major axis, perhaps not by enough, but this mechanism could be more effective if the planet was in an eccentric orbit to begin with; (iii) tides, from the Galaxy, or more likely, from the birth cluster of the Sun could have influenced the orbit of a scattered planet and produced its high eccentricity and perihelion distance.
The gist of the discussion is that whilst the authors think these other things are possible, they favour their own gas-drag model (naturally!).
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