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Let's suppose we live in different star system with two similar-sized planets (with a mass of three and a half Earths.) We live in Planet A, which is habitable. For some reason, Planet 2, our neighbor, enters in collision orbit with our Planet 1. But us, Planet-1ers are not happy with this, so we somehow catch Planet 2 and keep it close together. Both planets end up as a double planet.

Now, what would it take to catch Planet 2 and keep it orbiting along with ours? I'm aware that with current technology it is not possible, but let's suppose for a moment we have an advanced level of technology, but still not going off the laws of nature. Double Planet

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    $\begingroup$ Answer: a lot of handwavium $\endgroup$ – Rob Watts Oct 9 '15 at 20:10
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    $\begingroup$ Someone can probably calculate the required energy...which will have a lot of digits, but if you are asking about a method that would probably fall under idea generation and would be off topic. Interesting idea though. Welcome to the site. $\endgroup$ – James Oct 9 '15 at 20:21
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    $\begingroup$ What Rob Watts said. You may want to start at Dealing with sense of scale in space, then follow the links in that question as well as the comments to it. We get questions every now and then about doing things to planets, and the answers pretty much boil down to "it's hard. like, really, really, really, really, REALLY freaking hard." For this question, a good start would be to consider what sent Planet 2 onto a collision course with Planet 1 to begin with. $\endgroup$ – a CVn Oct 9 '15 at 20:21
  • $\begingroup$ Well, I guess if we narrow down the possibilities to current technology, it wouldn't make any sense. I'll read that thread. Probably will end up building some kind of Handwavitor Device or something, heh. $\endgroup$ – A.C Louis Oct 9 '15 at 20:24
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    $\begingroup$ @A.CLouis Indeed he did. The planets are egg shaped (pulling planet A or B into an egg shape would likely have some catastrophic nature). Each is carefully balanced within the roche lobe. But again, note the shape of the planet. The closest you could possibly get is 2.4 planet radiuses apart. Though going to the hard math to get the 'what is the closest two earth mass planets could orbit a common barycenter?" would be one way to do it. $\endgroup$ – user487 Oct 9 '15 at 22:07
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It would be far easier to just move one of the planets a little so they don't collide if there's a large velocity difference.

If the planets are just barely going to collide, it seems like it might make sense to try to catch one. The problem is once you've caught it, the two planets will destroy each other from gravity. Unless your magical tech device can magically prevent gravity from working, you'll get something like this: Planets melting onto each other from gravity.

The only way you're going to avoid this is if the planets are far enough apart that they don't tear each other apart. At this point, the best bet is just to get the new planet into orbit around the old planet.

I thought there was a question about how to move planets around, but I'm not finding it right now.

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  • $\begingroup$ Yeah, maybe the best way to handle it is to somehow get Planet B in orbit around Planet A. They would end up in a Pluto-like setting. Assuming they don't collide with each other. Also, pretty awesome illustrations! $\endgroup$ – A.C Louis Oct 9 '15 at 21:58
  • $\begingroup$ Try this: worldbuilding.stackexchange.com/questions/21605/… $\endgroup$ – Thucydides Oct 10 '15 at 6:58
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    $\begingroup$ @A.CLouis Note that technically, Charon does not orbit Pluto, nor does Pluto orbit Charon! Instead, the barycenter of the Pluto-Charon system is outside of either body, with the two orbiting a common center of mass that is actually in between them. That makes the Pluto-Charon system unique (as far as I am aware) in our solar system. If you have two similarly sized planets capturing each other and surviving, that's the effect you will get. The barycenter is only inside either body if there is a large difference in mass between the two. $\endgroup$ – a CVn Oct 10 '15 at 23:17
  • $\begingroup$ See also Can a nonspherical planet exist and can it be habitable?. $\endgroup$ – a CVn Oct 10 '15 at 23:20
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There are 2 separate tasks: avoiding collision and forming double planet.

  1. To avoid collision you need to give incoming planet lateral push. If collision is far ahead then this push can be relatively small - for 1 year and 30K km miss you need only 1 m/s. Though, of course it is hard to change planets speed even by 1 m/s since we are talking about trillions of trillions kgs. But you can put engines on asteroid, and make it "hang" to the side of the planet so that gravitational pull from asteroid would change planets trajectory.

  2. To form double planet on other hand is much harder. Two passing bodies don't just start rotating around each other on it's own. They need to lose speed to do that. Otherwise, as they come closer, they would speed up just enough to reach escape velocity(because there is conservation of energy - binary planet has less potential energy than 2 separate planets). And we are talking about losing some km/s. So this task is 3 orders of magnitude harder than the first one.

So, if civilization thinks about creating double planet, then collision is not a threat, if it can barely escape a collision, then creation of double planet is out of question.

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You'd probably need an artificial black hole as a stopgap measure to do the capturing but in theory you could get your two worlds orbiting a mutual centre of gravity like Pluto and Charon. I'm pretty sure that having done this the two worlds will be tidally locked to each other as they orbit that centre though so you're going to lose a lot of habitat on both worlds. If you actually have the technology to capture a world in this fashion you can probably produce a better outcome.

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  • $\begingroup$ It'd be kinda neat to use that black hole AS the barycenter. I mean, gravity and tides would still have troubles, but... $\endgroup$ – Pyrotrain Aug 16 '17 at 3:53
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    $\begingroup$ @Pyrotrain That gives you a three-body system orbiting a star. And of course all the issues with having a black hole nearby. You don't need an actual celestial body at the location of the barycenter; just a common center of mass. As I discussed in (much) earlier comments, look at the Pluto/Charon system. $\endgroup$ – a CVn Aug 16 '17 at 7:46
  • $\begingroup$ The planets won't automatically be tidally locked for millions, perhaps billions, of years. A civilization advanced enough to create spontaneous gravity wells likely would have no trouble tidally locking or unlocking either of the planets to or from the other. $\endgroup$ – MichaelS Aug 16 '17 at 21:48
  • $\begingroup$ @MichaelS Not sure that the rotational mechanics are that simple. $\endgroup$ – Ash Aug 20 '17 at 7:00

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