Question

Why do we think that Neptune must have formed closer to the Sun, and later migrated outward to its present position?

Answer 1

Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km; 2.8 billion mi).

Later the Neptune migrated outward to its present position because sun gravitational affect Neptune atmosphere.

Note (The Neptune would indeed trap objects onto its 3 : 2 resonance if it were to migrate outward over a timescale 106 yr. But in order to avoid the concurrent capture of objects onto its 2 : 1 resonance, Neptune's migration timescale must be 107 yr.)