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I'm trying to run a few ideas on an alien species that live on one planet and has a twin planet that is slightly smaller in size and had a small number of other beings living on it underground. They are the only planets in the solar system that can support life and both inhabitants have visited each other's worlds.

But the first planet is hit by a large asteroid and destroyed by the impact. I want to know, would this affect the other inhabitable, twin planet? If so, How? Would the inhabitants survive under the ground if any major damage is done to the surface?

If the other planet was not fully destroyed fully but fractured into large dead pieces, what orbit would it stick to?

Would some of the planet debris become a moon for the twin if the debris gets caught into the twin's orbit?

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closed as unclear what you're asking by Brythan, L.Dutch, sphennings, Vincent, RonJohn Apr 3 '18 at 16:12

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    $\begingroup$ Hit by an asteroid and destroyed by the impact seems to be more questionable than what would happen if it were somehow fragmented. You might want to do a reality-check, because that part of the premise seems really questionable. You also might consider restating the question in terms of what you want rather than what you have considered to get there. It might be easier for us to help you get where you want to go than to comment on this particular path. $\endgroup$ – Brythan Apr 3 '18 at 13:01
  • $\begingroup$ What do you mean by "more or less close enough to have a stable orbit with each other"? $\endgroup$ – RonJohn Apr 3 '18 at 13:22
  • $\begingroup$ Twin planet as in Mars and Earth? How close are they? Also, you need a freaking huge asteroid to shatter a planet. $\endgroup$ – Vincent Apr 3 '18 at 14:23
  • $\begingroup$ I agree with @Vincent: such an impact could only occur during the early stages of solar system formation, with the planets being uninhabitable anyways during that period. $\endgroup$ – JavaScriptCoder Apr 3 '18 at 18:07
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Yes, any impact big enough to destroy one of a pair of twin planets would render the other uninhabitable.

The way the question is asked, it appears that the destroyed plant is not merely rendered uninhabitable, but is actually completely disassembled. (You talk of planet debris possibly forming a new moon.) Additionally, the planets would appear to be roughly the same mass and big enough to hold atmospheres.

The energy required to do disrupt a planet would not just pulverize it, but would sent bits of all sizes flying in all directions. The undisrupted twin would get a large amount of matter hitting at just under escape velocity. The upshot would be conditions that would make the K-T impact look like a picnic. It is very unlikely that any life would survive on the surface.

Without detailed simulations, and without knowing the direction, speed and mass of the impact I can't estimate the amount of material impacting the twin, but it would probably be large. (It would be huge if the impact was "head-on" and the twin lost a lot of its orbital angular momentum.)

Having 1% of the mass of an earthlike planet rain down on Earth at 80% of escape velocity would probably melt most of Earth's surface, though it would not disrupt Earth. (You can get this estimate by remembering that 5 miles/second turned into heat is enough to melt the impacting body. Adding 1% of the Earth's mass in lava to the Earth's surface all at once makes a pretty thorough mess.)

In most scenarios, a substantial amount of material would remain in orbit forming a disk which would in turn form one or more new satellites. (You can actually get a very good idea of what happens by looking at the scientific literature about the formation of the Moon due to a giant impact on Earth.)

Bottom line: If you live in a twin planet system, do not use a giant impactor to disrupt your twin planet, no matter how much they annoy you.

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Disclaimer: I am neither a planetary scientist nor an astronomer

With that out of the way...

An asteroid large enough to destroy a planet to a moon is large enough to fracture it. Through the law of conservation of mass, matter cannot be created or destroyed. Therefore, all that planet and asteroid has to go somewhere. Impact earth is a good simulation for this, and it predicts that the planet would become a ring around the other planet.

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Yes, a twin planet impact would almost certainly end up with one bigger planet and one moon, BUT! You're better off focusing on making aliens that could survive that than focusing on the planetary physics of it all. An impact of that nature would screw up the atmospheric content, actual amount of atmosphere, gravitational pull, would make a dust cloud dense and long-lasting enough to screw over photosynthesis, and thus anything more complex than plants would almost certainly die if they didn't have access to climate control (atmosphere generation, food generation, gravity controls, etc). So you're either talking about a space-faring race who's going to have a really rough time, or tardigrades.

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