Before we begin, two things. This question was inspired by this one. I definitely don't want this to happen.

So, I have two very close tidally locked planets, each fills up about 2/3 of the sky of the other. They both have about .75g of gravity. There is also a moon orbiting the two planets at some distance (where it is stable). One planet is predominantly ocean with scattered topical islands, small forested islands, and some swamps. The other is mostly desert, in that it has little water.

Note - Assume these aliens have interstellar travel, and are able to travel just under the speed of light. If you think they'd have other kinds of technology as well, then feel free to include it. These alien races would like to help the planets come together gently, let them help if you can. (They can't stop the planets from colliding.)

Now, is there a way for the two planets to collide and not wipe out each other?

You can change some factors about the planets, but their sizes must be between Mars's and Earth's. As a last resort, the inhabitants can flee the planets, but I want to avoid this if possible.

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    $\begingroup$ I truly am agog that this is a serious question. Planets cannot collide, or merge, or probably even touch, without some serious catastrophe occurring. They'd have to be tidally locked, for sure! And think: according to classic science, the meteor that killed the dinosaurs was only 9 to 12 miles across (or maybe even just 2.5 to 3.7 miles across), it's clear that a planetary mass would probably kill you if it came even close, and at some close enough point everything would die really, really fast. $\endgroup$ – ErikE Jan 29 '16 at 18:00
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    $\begingroup$ If a 3-mile object killed everything on the planet... don't you think that two planets colliding, no matter how slowly, is going to be a problem? I also think that you're underestimating the power of tidal forces. Stop thinking of the two planets as separate objects. Imagine them as one barbell-shaped planet. There's a reason that planets are close to spherical! Centrifugal force only causes a slight eccentricity, but otherwise planets are spheres. Your two planets will smoosh, and faster than "really, really, slowly." And it will be catastrophic. $\endgroup$ – ErikE Jan 29 '16 at 18:29
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    $\begingroup$ Where did you get that collision gif? I've been trying to locate any kind of footage of what it would look like from a planetary surface of the planetary body being torn apart by the Rorche Limit gravitational forces. Whoever simulated that gif could probably do simulate what I want to see. $\endgroup$ – Draco18s no longer trusts SE Jan 29 '16 at 19:04
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    $\begingroup$ It's possible for two planets colliding to not kill anything, if there's nothing living on the planets in the first place. $\endgroup$ – KSmarts Sep 6 '17 at 19:04
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    $\begingroup$ "spinning faster and faster as they neared, minimizing the impact" -- won't work that way. At the very least you're releasing a horrifying amount of gravitational energy, which will become heat - more than enough to sterilize both biospheres. $\endgroup$ – LSerni Jun 4 '18 at 15:43

Everyone will burn to death.

This NASA page shows temperature changes during the collision between Earth and Theia in the Giant Impact Hypothesis. The temperature is on the x-axis, in Kelvin:

Your people are going to die at temperatures worse than hellish. This will be an inferno never before seen on Earth, or any terrestrial planet not subject to such an impact. Temperatures may be lower because the impact velocity will be slower than in the Earth-Theia collision, but they will still be very high.

So, I don't think that people can survive, unless they

  1. Escape somewhere else.
  2. Go to the orbiting moon, which is hopefully far away. The collision between the planets may eject material into an orbit around the resulting body (if there is one), so there's a chance the moon may be adversely affected.

If not, the intense temperatures will kill them, and even the oceans can't stop that.

Earlier simulations by the same author who created the above temperature simulation were also extreme.

  • $\begingroup$ I think the OP is NOT wanting to kill everyone :) Do he have to try again? $\endgroup$ – SE - stop firing the good guys Jan 29 '16 at 1:28
  • $\begingroup$ @Hohmannfan I know; I'm arguing that any collision will kill them because of the heat released. $\endgroup$ – HDE 226868 Jan 29 '16 at 1:28
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    $\begingroup$ @Hohmannfan No is a valid answer to the question. $\endgroup$ – PyRulez Jan 29 '16 at 1:53
  • $\begingroup$ @PyRulez I know, I was just joking $\endgroup$ – SE - stop firing the good guys Jan 29 '16 at 1:53
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    $\begingroup$ I would just like to say, that this collision is a lot tamer than that of Earth and Thea. These two planets have been spiraling together, slowly getting closer. It's the difference between throwing two watermelons at each other and slowly smashing them together. $\endgroup$ – Xandar The Zenon Jan 29 '16 at 3:06

You need to do everything you can to reduce the energy of the collisions. So lets start with a rocheworld pairing, two planets just outside the roche limit tidally locked with each other. One would be slightly larger. This is similar to the situation you already described so we've got a possibility here.

The two planets gradually approach closer and closer until the smaller one starts to break apart. This can actually be a gradual process as mass on the near and far sides of it starts to drift away from the smaller and go into orbit around the larger. The mass would rain down onto the larger planet but would mostly be limited to the orbital plane.

If the breakup was gradual enough the smaller planet would turn into a ring and then rain down as a steady sequence of low-energy meteor showers onto the larger. If the sections were small enough and the whole event spread out over a long enough time this would be survivable for at least some life on the surface.

Expect impact winters, flash fires, clouds of dust and ashe, amazing sunsets, and for at least some time a ring visible in the sky.

As the mass landing around the equator began to mount up the mass of the planet would start to increase, you would actually become heavier and heavier. That new mass would start to flow out and reshape the planet so expect some hefty volcanic activity as the planet starts to deform into a more spherical shape.

The poles (as far away from the impact plane as possible) would be the best place to survive. Even there though the increasing mass and reshaping of the planet would have massive effect even if you managed to dodge the rock falling from the sky.

Dangerous - without doubt. It would be possible for at least some life to survive though.

What can the aliens do? Very little. Maybe shelter the survivors, evacuate people from the smaller one to the larger. A lot would depend on how powerful they are but any mass they can remove or move from the smaller planet will reduce the impact.

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    $\begingroup$ Not necessarily the smaller one, the math favours it heavily but a smaller extremely dense world can pull a larger less cohesive one apart too. $\endgroup$ – Ash Sep 6 '17 at 15:33

If they are already orbiting each other they can. Imagine for example a scenario with two egg shaped planets almost touching each other, almost filling their Roche lobe. That is the setting of Robert L. Forward's novel Rocheworld. They can share an atmosphere and even an ocean, but you do not want them to merge. Then you are just killing everyone again.

flight of the dragonfly cover


Perhaps people from both plants escape to the moon or moons. Given enough time and significantly advanced technology they might be able to terraform the merged planet.


There are several things which are going to survive almost regardless of what you do to their world but none of them are really high up on the evolutionary ladder; high altitude radiophiles, tiny bacteriological lifeforms that live on the upper edges of the atmosphere, like Deinococcus radiodurans will keep going like nothing ever happened. At the other end of the altitude scale near-Asthenospheric thermophiles are going to experience a ridiculous amount of habitat disruption but they'll be okay in some areas. Large lifeforms are in trouble but I can think of a couple of ways to ride it out.

If you were using a gravity based drive technology, and it scaled, you could slow impact speed to almost nil and, having evacuated the population to the far sides of their respective worlds, allow a slow merger process under a governing gravity field. The tidal quakes etc... are still going to be murder but a reasonably large percentage will survive on what amounts to continental plate sized stone rafts that will settle onto there newly merged world. The same kind of drive could also potentially be used to pull the populated landmasses right off and maintain them with an independent atmosphere while the merger rages below.


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