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In my universe a race of humanoid beings caused 3 planets:

  • Earth
  • a larger rocky planet
  • and another earth sized rocky planet

to crash and collide into each other, but still allowed the humanoid beings, humans, and some microscopic plants and animals to survive.

The larger rocky planet is a red, dusty, hot, volcanic planet with little oxygen but has microscopic life inhabiting it, while the other earth sized planet is a blue, cold, oxygen rich planet that has a moderate amount of water and has larger life forms.

Also the two planets that aren't Earth come from a different solar system from earth, but these two planets come from the same solar system as each other (or possibly not if it causes the whole collided combined planets thing to become impossible), but they're all in the same galaxy.

After the three planets collide and merge, they become a large rocky planet with the highest part atmosphere having the qualities of the blue planet, the point closest to the planet's core having the qualites of the red planet, the surface of the planet to have the qualites of earth, and a point between the surface and the core to have a dramatic mixture of the three.

Soon after the collision, almost all of the planet's surface is water, but after a very long time, moss-like plants start to grow over it, and help shelter microscopic life from the harsh environments of the collision's aftermath, and soon the microscopic life would start to evolve into visible plants, animals, and eventually humanoids.

Also at some point after this point, humans from advanced technology underground bunkers are able to head back to the surface without all of them all dying instantly.

Is there a possibile scientific way for all of this to happen?

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    $\begingroup$ Please, use space after punctuation signs, proper capital letters at the beginning of sentences, etc. It will make your post much easier to read. $\endgroup$ – Mołot Dec 19 '17 at 19:31
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    $\begingroup$ My initial thought is not in a million years. The amount of energy involved with planetary collisions is huge. It's so huge planets don't event really collide so much as splash into eachother. If any life at all survives, it would only be microscopic. Waterbears might be the most complex life left alive only because they could hibernate through the catastrophe $\endgroup$ – Stephan Dec 19 '17 at 19:33
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    $\begingroup$ Welcome to WorldBuilding! Please use proper formatting to make your post easier to read. It will help to get more people to actually read your question and try to help you. You should also get used to markdown. The help is visible in the desktop version of the site when writing a question or answer. For example you need two linebreaks in the real text to get a paragraph in the result. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Dec 19 '17 at 19:45
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    $\begingroup$ Please do not edit your question in a way that invalidates already posted answers. It is considered rude, at least. $\endgroup$ – Mołot Dec 19 '17 at 20:11
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    $\begingroup$ Sorry, I'll avoid changing my question in the future. $\endgroup$ – BlueTangsRock Dec 19 '17 at 20:27
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The asteroid that killed the dinosaurs was around 10 km in diameter. You have three PLANETS colliding with each other. I don't think anything, even bacteria, would survive that. The earth would be turned into a massive sphere of molten rock for years.

Here's a simulation of a 500 km asteroid hitting the earth: https://www.youtube.com/watch?v=bU1QPtOZQZU

It's huge for an asteroid but still far from being considered a planet yet it still destroys absolutely everything on Earth.

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  • $\begingroup$ So if the two other planets were destroyed and multiple bits of debris in the form of metors that were smaller than 500km flew towards Earth, would there be a possible chance of survival then? Since giant planets wouldn't actually be colliding with Earth then, but more matter is added to Earth? I'ld be willing to tweak the factors a bit so it could be closer to being possible. $\endgroup$ – BlueTangsRock Dec 19 '17 at 19:43
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    $\begingroup$ @BlueTangsRock, watch that link. Zero chance of survival when you are talking about diameters the size of European countries. Full-on Mass Extinctions are caused by mere pebbles the size of a city. The biosphere of Earth is quite thin and fragile, and took millions of years to recover from the last such pebble. $\endgroup$ – user535733 Dec 19 '17 at 20:11
  • $\begingroup$ @BlueTansRock The problem with that is that two colliding planets would not break up into smaller pieces, rather they would merge and form an even bigger planet. Just for fun let's still make some calculations... $\endgroup$ – Esa Dec 19 '17 at 23:41
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    $\begingroup$ Let's assume the two planets broke up into pieces. To minimize impact energies let's assume they broke up into small pieces, say, roughly the size of tennis balls. Assuming the planets were similar in size to earth and mostly made of granite, that would give us around 27 000 000 000 000 000 000 000 000 asteroids that would then proceed to fall on to the earth. That would mean roughly 53 000 000 000 000 000 asteroids per square kilometer, or 53 000 000 000 asteroids per square meter. For some reason I have this funny feeling that nothing would survive that. $\endgroup$ – Esa Dec 19 '17 at 23:45
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    $\begingroup$ @BlueTangsRock The formation of the moon (still a hypothesis, but strongly supported by evidence) came from an impact with a planetoid smaller than any of the ones described - and it took a thousand years for the moon to settle out of the debris. Earth was molten for hundreds of thousands of years. Habitability would've been out of the question. $\endgroup$ – jdunlop Jan 25 at 20:07
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I really can't see any higher life survive this. Not animals, not plants, and neither humans. No matter how deep your bunker goes, the massive impact of a planet most likely will make it collapse and/or turn into an oven.

However, making microscopic life survive is rather easy. The enormous shock wave will send debris flying into space, some of it in orbits causing it to fall back to earth sooner or later. Make a massive rock with a core of water be thrown into a slightly higher orbit around the sun than the earth, and in a few ten or hundred thousand years it will fall back on the planet again.

However, given the end result you might as well just start a new evolution from zero - the crash will most likely provide a fertile ground for life to be created anew.

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  • $\begingroup$ So basically if it was only microscopic life that was supposed to survive from such an event,then it could work? Because Starting Evolution from basically Zero is the exact result I want! I could scrap the bunker idea,and make the humans that eventually come back to the planet come from somewhere out of Earth, such as a space station or Mars since technology would be more advanced to allow humans to move there. If at least microscopic lifeforms are able to survive and restart evolution is possible, it'll be a success for me. $\endgroup$ – BlueTangsRock Dec 19 '17 at 20:25
  • $\begingroup$ You don't even need any on Sun orbit. Germs in their spore forms can survive in low earth orbit. So can (probably) some seeds, moss spores etc. Of course most of them will die, but there is a hope in numbers, it is plausible enough to assume some of it will survive short space trip. $\endgroup$ – Mołot Dec 20 '17 at 21:14
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It honestly depends on how fast (or for that matter, how "slow") the planets merge. If they collide at any real speed then I would say that nothing could possibly survive except bacteria. Life on Earth would be permanently and irreparably extinct.

Conversely, arguing against what I just said, humans could survive, if they vacate the planet before the collision and have enough to survive while the planets cool and the "dust settles", literally and figuratively. However, any humans remaining on the planet's surface could not exist.

Since you mentioned that the 3 planets collide and become one big planet, then it would probably be safe to assume that in fact, the planets are not slowly inching toward each other and nearly zero speed, but instead rapidly collide to merge. In which case, again only humans (besides some bacteria) could survive, if the humans first vacate the planet and can survive in outer space until the "dust settles".

In this case, all life on Earth could also survive, if they build a "space ark" to contain samples from every known organism. Even the full animals would not be required if the civilization is advanced enough in cloning. In fact, if they are advanced enough, it's possible they could recreate life from merely the DNA genome and enough raw biomatter. Of course, we are headed in the direction of fantasy by this point, but you get the point.

Unfortunately, your theory about the underground bunkers I believe to be unsound. I do not believe that any bunker out of any material currently known to Man could possibly shield against the collision of a large asteroid, much less a planet -- not to mention two planets. Again I must point to the requirement to fully vacate the planet into outer space, in order to survive, unless you bring in a plot device in which there is a material which is not known to man to be used to build the bunker.

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