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Some sufficiently advanced species has decided they will move planets.

Let's assume earth-like, life sustaining worlds. The only real way to do this as far as I know is using "gravity tugs" or messing with orbital trajectories (slingshot), again using gravity.

They want this done in a reasonable amount of time (not millennia).

So they build a ship, with the front being a giant ball of something sufficiently massive (and super dense to save space) to have the required gravity. So how small can we make a ship massive enough to move worlds?

You can address the feasibility, rarity of material, speeds and times, all you want but the Question at hand here is the size of the ship. I don't have any real specifics to share here. the galaxy is huge and they might want all kinds of planets from all over the place. Don't break the planet and get it there within the next thousand years or so is all that really matters.

Ideally the gravity of the ship is enough to select a planet to move without disrupting the entire solar system, so not as massive as a star obviously.

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  • $\begingroup$ related Q which includes some ideas for you. $\endgroup$ – JDługosz Nov 23 '17 at 0:32
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Nov 29 '17 at 4:47
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The shape of your device doesn't really matter. At a sufficient remove, the gravitational effect of a mass of any shape is the same as that of an equivalent point mass located at its center of gravity. So shape isn't really a concern, unless you're inside the bounding sphere of the tug, which seems extremely unlikely.

What is important is how close you must be to the planet. Gravitational tidal effects will rip apart the planet if your tug is too close, because the pull on the near side will be stronger than the pull on the far side. (See also the Roche limit.)

The other factor is how quickly you want to have an effect. You could have even a small tug maintaining a station on the outside of the earth for a few millennia, and its effect would slowly migrate the Earth to a higher orbit. (You'll want to be slightly behind the Earth in orbit, since primarily you'll be slowing it out of its existing orbit.)

Conversely, trying to move the Earth significantly out of its orbit in a day would almost certainly rip it apart.

Once you know those two answers, you can figure out how much mass you need--enough to exert the desired force, while staying far enough away not to rip the planet apart (or levitate its inhabitants, etc.).

Size of the tug then depends on the density of the material you use, plus a space for propulsion. (Tugs need to move under their own power, or they're called "planetoids".)

So the answer of "how big must the tug must be" depends on:

  1. what effect you want to achieve (where is the planet going?)
  2. how fast you want to achieve a particular effect
  3. how much disruption of the planet you're willing to inflict
  4. how dense a material you have access to

Without those answers, it's impossible to give any sort of quantitative answer.

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    $\begingroup$ 1. Anywhere. 2. Within a thousand years or so. Not in a day, not in a million years. 3. Lets not blow it up obviously. 4. You tell me? What's reasonable? Osmium? Iron? Whatever. $\endgroup$ – oxide7 Nov 21 '17 at 19:35
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I've been thinking about this all morning, I've even been running some numbers.

  1. You will kill everything on the planet and render it uninhabitable. I can't think of a way to preserve atmosphere, the cold of space will freeze everything, and moving the planet in a "reasonable amount of time" will require so many G's that it will flatten everyone and everything. Therefore, you must terraform every planet you move (if habitability is your intent).

  2. A "reasonable amount of time" will be measured in centuries unless you want to start ripping chunks of material off the planet. Sometimes, science sucks.

  3. Your ships must be remotely piloted. The gravity involved on the ship will kill microbes.

  4. The size of the "gravity sink" can be a golfball using the right material (something like Neutron star mass).

  5. The planet will become tidally locked to the tug. Restarting the rotation will require multiple tugs spining around the planet to get it up to speed.

  6. This process will affect whatever solar system you move the mass into. It will affect it tremendously, and I'm not at all sure the tugs can enter and leave the system in a way that will leave the system balanced. You'd almost need to "throw" the moved planet into position, leaving the tugs a long way away from the system. This would make getting the rotation right a neat trick, and would likely require millenia for the system to completely balance.

  7. Inertia isn't free. The gravity sink will attract the planet in the same way a magnet will attract metal, but you still need the 1027+ Newton energy supply to move said planet. Remember Newton's 2nd & 3rd laws. The energy needed to do this is unfathomable.

IMHO, a story involving planet movers is handwaving. That doesn't mean it can't be enjoyable. It only means our understanding of physics today doesn't allow it at all.

Frankly, this list and a fair number more relating to #7 is why megastructures liky Dyson-anythings have not, do not, and will not ever exist. The cost of performing the action far outweighs the value of doing it.

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  • $\begingroup$ Concerning the last paragraph, what's the better bet for big populations? just terraforming everything and/or living in "domes"? $\endgroup$ – oxide7 Nov 21 '17 at 18:51
  • $\begingroup$ @oxide7, as far as we understand physics today, I can't see a way to peserve life in any way on the planet. The Gs alone will kill them. Moving the planet slowly enough to perserve the population pushes the transfer time beyond the time limit you specified. I believe the only option is terraforming - and that's ignoring all the other problems. (Out of curiosity, how do you intend to explain the disruption of both the sending and receiving star systems?) $\endgroup$ – JBH Nov 21 '17 at 20:00
  • $\begingroup$ @JBH, that comment includes an excellent introduction to your excellent answer. Consider adapting it within your answer. $\endgroup$ – user535733 Nov 22 '17 at 14:35
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IMHO you are approaching the problem from the wrong vantage point.

In order to be able to act on a planetary mass from far enough to avoid tearing it to pieces (i.e.: to have a reasonable pull also on the "far part" of the planet, otherwise you risk ripping away a piece of planet) you need a comparable mass, something between Earth and Moon mass.

If you compress it to "manageable size" you risk ending up with a very high surface gravity which will just squash your tug-ship.

In any case your "tug" needs to have engines powerful enough to move that mass and the mass of the planet trying to fall on it, accelerating both fast enough to prevent the planet from "catching up", i.e.: with the same acceleration the planet have, so that distance remains constant (a difficult exercise, if you ever tried to "tug" a magnet with another one).

If you have engines that powerful it would probably be easier to apply them directly to the planet.

There's a problem in applying such a force on a planet without destroying it.

A very large floating mass (about 100km radius) could be pushed down in the Ocean for about 100m could provide reasonable push "gently" enough. Of course synchronizing push with planet rotation and gently enough to avoid disruptive tsunamis is another exercise left to the reader ;)

A more viable plan could be to mount the engines unto a (large) metallic asteroid (or an asteroid fleet) and have it (them) repeatedly approach planet and be deflected by planet gravity.

This schema trades a high pull (not easily obtainable without destroying the planet) for a series of smaller pulls, possibly less damaging.

Before you are able to do such a thing you need to "invent" a reactionless engine along with a huge energy generator.

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    $\begingroup$ "about 100km radius" mass will crush through the oceanic plate, no matter how gently we try to push. $\endgroup$ – Alexander Nov 21 '17 at 18:51
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If one can create a ship with mass enough to nudge a planet, the ship can move directly the planet: just attach the ship to its center, and tug.

The actual size of the ship is the size of its engines, plus fuel. The size of one of the smaller asteroids should do nicely.

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  • $\begingroup$ You’re begging the question, not answering it. $\endgroup$ – JDługosz Nov 23 '17 at 0:34

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