Split out from this question since it was too large.
I'm designing an exoplanetary system that was the victim of a drive-by super-Jupiter ejected from a nearby supernova that crashed into the system's largest gas giant and then left the system.
The planetary system is quite young; it has not yet finished forming terrestrial planets. It coalesced from the remains of several supernovas, which had remarkably high quantities of fluorine and chlorine. All bodies in the system possess notable levels of both of these elements. The recent formation of a pulsar in a neighboring system irradiated much of the system, as well as sending a super-Jupiter hurtling at it. The super-Jupiter collided with the system's largest gas giant on its way out of the system, spreading much of its victim's mass across the system and disturbing the orbits of most of the planets and protoplanets.
The inner system consists of around 35 protoplanets, many with somewhat eccentric orbits. There is a thin asteroid belt, but most asteroids are spread around the system instead of being concentrated. An ice giant is at the edge of the inner system, having migrated inwards. The outer system begins with the former second-biggest gas giant in the system, absorbing concentrations of mass from the former biggest. A mini-Neptune orbits near another gas giant and is likely to collide with it soon. Another mini-Neptune is at 65 AU, having been slingshotted away by the incoming super-Jupiter. It is now migrating out of the system.
The large gas giant was immediately following the asteroid belt when it was hit.
Two major questions here:
- Have I modeled the system-wide effects of a collision with a rogue super-Jupiter at all accurately? Universe Simulator (as recommended by Green) has mostly answered this one.
- Would the gas ejected from such a collision be dense enough for neighboring bodies (particularly moons) to pick up nontrivial atmospheres? I think it's plausible if the neighboring bodies are in a similar orbit and were close enough to it at the time of collision to receive large amounts of gas while being far enough away to not be majorly disrupted.
Note: I've left the planetary masses involved intentionally vague so as not to exacerbate any problems with the system. Feel free to assume whatever masses you would like.