One possibility, the discoverers say, is that, in the infant days of the Solar System, the nascent Neptune lay much closer to the Sun.
Hmmmm...as did the Earth when the earth year had only 10 months of 30 days. Wonder what might have bumped into it?
> as did the Earth when the earth year had only 10 months of 30 days. Wonder what might have bumped into it?
Nothing. What has happened is that the sun has lost considerable mass over the last 5 billion years, whereas the planets have not lost much in the way of kinetic energy or angular momentum WRT their orbits. So as the sun lost mass, the planets sloooowly drifted out. A process which will continue.