  We explore conventional Neptune migration model with one additional planet of
mass at 0.1-2.0 Me. This planet inhabited in the 3:2 mean motion resonance with
Neptune during planet migration epoch, and then escaped from the Kuiper belt
when Jovian planets parked near the present orbits. Adding this extra planet
and assuming the primordial disk truncated at about 45 AU in the conventional
Neptune migration model, it is able to explain the complex structure of the
observed Kuiper belt better than the usual Neptune migration model did in
several respects. However, numerical experiments imply that this model is a
low-probability event. In addition to the low probability, two features
produced by this model may be inconsistent with the observations. They are
small number of low-inclination particles in the classical belt, and the
production of a remnant population with near-circular and low-inclination orbit
within a = 50-52 AU. According to our present study, including one extra planet
in the conventional Neptune migration model as the scenario we explored here
may be unsuitable because of the low probability, and the two drawbacks
mentioned above, although this model can explain better several features which
is hard to produce by the conventional Neptune migration model. The issues of
low-probability event and the lack of low-inclination KBOs in the classical
belt are interesting and may be studied further under a more realistic
consideration.
