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Why does the migration model suggest that Neptune once orbited closer to the Sun?

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Kuiper belt binary object — Kuiper belt An artist's rendition of a binary object in the Kuiper belt. The two objects depicted orbit each other at the edge of the solar system.

In the migration model of the solar system, it is suggested that Neptune formed at a closer distance to the Sun—approximately 23 astronomical units—and gradually moved outward to its current position of about 30 AU.1

This model helps explain the observed orbital structure of the Kuiper belt. As Neptune slowly migrated outward, its gravitational influence swept through the surrounding disk of planetesimals. This process caused many of these small bodies to become trapped in mean motion resonances with the planet, effectively shaping the distribution of objects found in the Kuiper belt today.1