There’s an author, name of Sitchin I believe, who would say this extra planet was the destroyed Tiamat, and it happened fairly recently.
Also I don’t quite understand how Jupiter can scatter little asteroids, if it’s close enough to affect them it should attract them, no? In some cases, like the Trojan asteroids, they get locked into Jupiter’s orbit. How many, of those on the same plane more or less, are going to “scatter?”
What ever happened to the conjecture that the asteroid belt was caused from the disintegration of a planet between Mars and Jupiter? Also, how big would such a planet have been? Might it have been damaged when the 5th giant planet was ejected, or could it have been that 5th planet?