The hypothetical fifth gas giant is an additional planet added by some theorists to recent versions of the Nice model.
Modern computer simulations show the Kuiper belt to have been strongly influenced by Jupiter and Neptune, and also suggest that neither Uranus nor Neptune could have formed in their present positions, as too little primordial matter existed at that range to produce objects of such high mass.
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The scattered disc was created when Neptune migrated outward into the proto-Kuiper belt, which at the time was much closer to the Sun, and left in its wake a population of dynamically stable objects that could never be affected by its orbit (the Kuiper belt proper), and a population whose perihelia are close enough that Neptune can still disturb them as it travels around the Sun (the scattered disc).
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All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its outward migration.
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Neptune's current influence is too weak to explain such a massive "vacuuming", though the Nice model proposes that it could have been the cause of mass removal in the past.
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