I have a fictional system that is just under a billion years old, with a large metal-rich asteroid belt, two big jovian planets, a rocky Mars-like world, and a large icebal in the outermost orbit (no habitable worlds here). The star itself is an evolved A V star, now a K III giant in its Horizontal Branch stage.
What I'm a little stuck on is the amount of debris in the system. I suspect that after 986 million years most of the planet-forming is finished, and all the dust has been long since blown away by the stellar wind. In our own system the Heavy Bombardment stage took up roughly the first 500 million years after formation, right?
So would it be reasonably realistic to say that the system would be fairly stable (gravitationally) and that most of the asteroids and debris in dangerous planet-crossing orbits have either hit planets or been ejected from the system already? Or would there still be a lot of junk flying around? I just don't really have much of a clue what a one billion year old system would be like - data from our own system at that time is a bit sketchy .