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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.

Planetary 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.

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The primary difficulty in gathering up gasses is the matter of the velocity of the gas. So long as the relative velocity between the gas cloud and the relatively nearby planet or moon is small, there is a possibility of collecting some of the gas. Gary Walker's answer has a good first order approximation.

The difficulty is that the rogue planet is entering the system not at interplanetary velocity, but at interstellar speeds. The fastest an object can move in our Solar System and still remain bound to the Sun's gravity is 72 Km/s. An incoming body will be moving at at least that speed, if not even faster. Since we know that KE=1/2MV^2, it is trivial to see that increases in velocity have outsized effects on the kinetic energy delivered by the impacting body.

In practical terms, an object moving at mere orbital velocity (6-7 Km/s) already has enough energy to be extremely dangerous. The Space Shuttle needed replacement windows due to impacting paint chips in orbit, so an entire Jovian planet moving at interstellar speed produces more kinetic energy than my calculator has zeros...The impact will essentially flash the atmospheres of the two planets to vapour or plasma, and all the ejecta will be moving at fairly extreme velocities and have a tremendous amount of thermal energy. The high speed gasses coming from the collision is more likely to scour anything which it comes into contact with, rather than add to the body, and unless the body has an extremely strong gravitational field, or powerful magnetic field in the case of plasma, the atmospheric components will blow right past the various bodies. Protoplanetary bodies need to run into cold, slow moving clouds of gas in order to gravitationally gather the gasses up

As an incidental, you are still trying to add reactive gasses to the planets, but these elements will become strongly bound to other elements no matter what you do (injecting it as high velocity, high temperature vapour simply makes it even more reactive), which is why they are reactive and not found as free elements on Earth, for example.

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  • $\begingroup$ What viable ways are there to slow the incoming super-Jupiter down then? Have it orbit a few times? Have it crash into another body first? $\endgroup$
    – emo bob
    Commented May 7, 2016 at 1:20
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    $\begingroup$ There are very few things which could slow down in incoming interstellar rogue planet without destroying it first. Even passing through an interstellar dust cloud will strip it of its atmosphere as it loses energy through friction. The core might remain to crash into your system, but even then you are talking about massive objects moving at interplanetary speeds. The body that hit Earth and formed the Moon pretty much melted pro to Earth, and the core of the impacting body was melted and absorbed by our planet. $\endgroup$
    – Thucydides
    Commented May 7, 2016 at 1:27
  • $\begingroup$ Guess I'll have to give up on the extrasolar collision. Maybe I can just have it pass through the system and disturb orbits enough that a super-Jupiter already present in the system collides with another planet. $\endgroup$
    – emo bob
    Commented May 7, 2016 at 1:31
  • $\begingroup$ Read modern theories of planetary formation. The action of protoplanets crashing into each other, giant planetary bodies clearing their space and even migrating, the star igniting and blasting the nebula with its high energy radiation and stellar wind are actually pretty exciting without exotic additions. $\endgroup$
    – Thucydides
    Commented May 7, 2016 at 5:52
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It looks reasonable to me

but I'm not planetary physicist with only a rough understanding of orbital physics, mostly informed by Kerbal Space Program (take that for what it's worth). Note also, that there can be huge variations in outcomes when the masses of the planets involved vary by 10% or less.

It won't take too long for all those highly elliptical inner-system orbits to turn into more common near circular orbits we see in our solar system. The reason being that highly elliptical orbits tend to run into other things fairly quickly.

Test it yourself with Universe Sandbox$^2$

Any kind of orbital system can be modeled in Universe Sandbox$^2$, though I don't know if it will model planetary composition. And unlike my brain, it rigorously applies the laws of physics and thermodynamics. You can setup any kind of situation you want and watch it all explode in glorious planet destroying fire.

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  • $\begingroup$ Playing around in Universe Sandbox answered the orbit part (namely that I can do almost whatever I want, since the relative positions of the planets are the most important aspect), but didn't give much information regarding the dispersal of the victim Jupiter. $\endgroup$
    – emo bob
    Commented May 6, 2016 at 23:33
  • $\begingroup$ @emobob I'm surprised. Smashing two Jupiters into each other would have been the first thing I tried. My guess is that if your super Jupiter comes in wildly off orbital plane, that the resulting mess will go all over but off the orbital plane. If it comes in near the orbital plane from behind, then the smashed together planet will orbit farther out. If it comes in and hits the front of Jupiter, oh my the fireworks. I might try that myself. $\endgroup$
    – Green
    Commented May 6, 2016 at 23:38
  • $\begingroup$ @emobob At this point the accurate answer to your question will likely require specialized and custom written tools. Since this whole scenario is fabricated, you can make it fit your needs. How much chlorine and fluorine do you want to show up in the atmosphere? There's enough wiggle room in people's understanding of orbital mechanics that it won't break believability if a giant green cloud appears in the skies. (unless the reader is a planetary astronomer and they say "Yeah maybe, but..."). $\endgroup$
    – Green
    Commented May 7, 2016 at 15:53
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What is the result of the gas thrown off from the collision?

Even in an early solar system, Jovian target will have cleared its orbit of nearby planets -- If such were not the case, the Jovian could not have managed to reach a very large mass. In such a case, there is at least 10 million km separating the Jovian from the nearest planet in a stable orbit. Also note, that the duration of closest approach is always limited by the differences in orbital velocity around the star.

Assume the combined Jovian target and the Superjovian bullet both lose about 10 percent of their mass in the collision as ejecta. Also assume that the combined planetary mass is equal to 30 times that of Jupiter, i.e., a total of 3 Jupiter's worth of potential atmosphere is ejected. This atmosphere will necessarily be mostly hydrogen and most of the rest being helium. This represents a combined mass considerably higher than the average pair of gas giants.

Consider the ejecta as equivalent to a large explosion. How much of the ejecta would hit the Earth at the optimistically close distance of 10 million km. The area of a sphere 10 million km in diameter is about 1.25e15 km^2, the cross sectional area of Earth is about 1.27e8 km^2, so only about 1 part in 10 million of the ejecta would impact the Earth as directly in an explosion (or a single orbital passage through the ejecta cloud). So of the of 3 Jupiter masses of ejecta about a mass of 0.0000003 of Jupiter would impact the Earth. This is still a lot of mass, about 5.69e20 kg worth, esp. considering that this is about 100 times the total mass of Earth's atmosphere.

This is only a first order approximation, but clearly it is plausible that not only can you get an atmosphere, you could get quite of lot of it. Admittedly, Earth cannot retain a hydrogen/helium atmosphere over eons and smaller planets would lose it much faster. Dispersal of the ejecta will not be uniform at all, so you could expect less or more depending of the impact details. You do not have to bee particularly close to get a new atmosphere.

Due to solar wind, the ejecta will be driven out of the inner solar system fairly quickly, but you only need 1 pass in the fresh ejecta cloud to pick up a lot of atmosphere.

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