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Okay. I have been piecing together the elements of another story for my school PDF newspaper (we publish one to a wiki page every two days). Yesterday I asked this question on whether a second habitable planet could evolve life in the solar system and what its location could be if we kept all of the inner planets. Ckersch pointed out in his answer that a planet could orbit stably in the L4 and L5 positions.

I have decided to make this second habitable planet have intelligent life. Technically, in my world the planet will come from out-of-system, meaning it has been a rogue planet for a long time. The intelligent creatures that lived on it have survived and when the planet began to pass our solar system, it entered and the aliens figured out where the stable L4 and L5 points are in the inner solar System. Since their cities and society above ground froze solid soon after exiting their own system, most of the infrastructure for the planet is still semi-viable (explaining why they don't simply jump ship to Earth or Mars).

The point is, they manually moved their whole planet into the L5 zone. My question is, how could one move an Earth-sized planet? Would one use rockets of some sort? Are there any feasible manners in which to do this, assuming my alien race recently discovered how to mass produce and use antimatter as a source of energy?

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  • $\begingroup$ The amount of energy to do this sort of thing is incredibly huge. Basically even with a lot of energy it will take a lot of time in order not to destroy the planet you are moving. And on top of it you have to be carefull not to break the equilibrium of the solar system. Look at this if you want to have a good laugh youtube.com/watch?v=G01NoaTM46o $\endgroup$ – Maxime Lucas Feb 25 '15 at 14:21
  • $\begingroup$ I mention this only because you are writing for publication, and probably don't have a professional copy editor to learn from like we had back in the days of print magazines. Add /its/it's/ to your checklist to use for self-proofing. $\endgroup$ – JDługosz Feb 25 '15 at 17:39
  • $\begingroup$ Would a civilization or even a species survive the millions or billions of years between leaving the home star and arriving? You'll have entirely new kingdoms of life and a different ecosystem. $\endgroup$ – JDługosz Feb 25 '15 at 18:12
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    $\begingroup$ The premise is flawed. If you stick an Earth-sized mass in one of the Earth-Sun lagrangian points you change the system entirely. The lagrangian points are stable for negligible masses, like small asteroids or satellites, not planets. $\endgroup$ – Samuel Feb 25 '15 at 18:24
  • $\begingroup$ As for how to move planets, Thucydides's answer is the most plausible one with technology that we know of (without having to sacrifice a gas giant). JDługosz's is also good. Both use methods that don't require manipulation of spacetime, therefore they don't need technology that would seem "magical" in our own real world. $\endgroup$ – Renan Jun 3 '16 at 21:24
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If the occupants of the exo-planet were technologically advanced enough to survive the permanent ice age which comes with their world's out-of-system rogue status, they might also have a handle on direct manipulation of gravity. Specifically, they would need to be able to bend the fabric of space without the conventional crutch of mass. They would need to be able to induce and dissipate intense gravity wells by application of energy alone. And as skysurf3000 suggested, they would need an incredibly huge amount of energy.

Now given the availability of stable orbit points at L4 and L5, and with respect for all of the gravitational relationships already in play in our solar system, I would suggest that there exists at least one perfect vector and speed upon which a rogue planet could enter our solar system and be captured into L4 or L5 orbit around our primary, without disrupting the other planets' orbits significantly. The planet would have to be coming from just the right point in exospace and would have to be travelling at just the right speed in all three dimensions, arriving at just the right moment... It would have to be perfect, but it could (and previously has) happened.

The trick would be to calculate the exact vector, speed and timing and then manipulate the rogue planet while it is still out in deep space until it conforms exactly to that course. The manipulation would involve creating multiple gravity wells into which the planet would plunge or from which it would climb, each time changing its course and speed until finally everything was just right.

The exoplanet aliens would probably also need a rather large space ship to shelter in off planet during some of the more radical preparatory course changes, but that is a rather small requirement compared to the equipment needed for planetary billiards.

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  • $\begingroup$ If they have a huge amount of energy, why would they want to stay near a star? $\endgroup$ – JDługosz Feb 25 '15 at 18:17
  • $\begingroup$ Because WE are here. They're lonely and want a nearby primitive planet to vacation on, with quaint natives to barter with. $\endgroup$ – Henry Taylor Feb 25 '15 at 18:38
  • $\begingroup$ So they might want to stick around, but not upset their environment by getting warm. Maybe they are parked far out, or have a cool sunshade (which could be a key piece of technology and feature in the plot) $\endgroup$ – JDługosz Feb 26 '15 at 8:19
  • $\begingroup$ Not really relevant, but we have the technology to survive a permanent ice age and have had since the invention of nuclear power. The scale of infrastructure would be immense and probably beyond us even with the motivation of impending extinction, but the technology of surviving cold is simple enough. So the ice age implies little about the technology level and a lot about the level of social organization they are capable of. Kind of like how irrigation kickstarted state building in ancient civilizations of Egypt and Sumer. $\endgroup$ – Ville Niemi Feb 26 '15 at 8:31
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A problem with moving planets is they have vast inertia, and trying to move it quickly into a new orbit would probably be very disruptive (think giant earthquakes, volcanic eruptions and other natural disasters).

You can move planets by shooting asteroids past them. The gravitational interaction will transfer energy from the planet to the asteroid (or vice versa, depending on how you choose the orbits and interactions), with the lane gaining or losing a small amount of energy with each flypast. Multiply this by millions of asteroids over thousands or millions of years, and you have a fairly stress free way of shifting your orbits. It is thought that something like this (on a grander scale) can explain the shifting of the orbits of the Gas Giant planets in our solar system since the creation of the system, or the strange orbits of "Hot Jupiters" orbiting close to the suns of other solar systems.

Of course this takes lots of time and resources. A rogue planet coming in from interstellar space is quite possibly moving far to fast to even be captured by the Sun, and will simply fly through the Solar System on a hyperbolic path and exit back into deep space.

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  • $\begingroup$ This method is the most plausible with our current understanding of physics. Manipulating space-time is fine and all, and possible, but we don't even know where to start to make that happen. Giving or taking energy through slingshot maneuvers like this, in a way that changes orbits, is something we know how to do. $\endgroup$ – Renan Jun 3 '16 at 21:20
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In Larry Niven's novel pictured below, you can gleem that moving planets is part of the plot.

A World Out of Time

He used a gravitational tether: move Uranus around and Earth will follow it after approaching closely. To move the ice giant (he probably thought it was a gas giant in composition) he had a thruster use the atmosphere for fuel and reaction mass. Ejecting a fraction of the planet will move the remainder.

So, you move one planet in a manner that would deatroy it or stress a rocky planet, slosh oceans around, etc. Then use that to more gently move the target planet.

Gravitational tethers have been studed seriously in later decades, for use on asteroids.

Also, we know that planets move (change their orbital parameters) naturally. Neptune and Uranus did not form anywhere close to their curren positions and were in the other order.


Btw, isn't rogue planet an oxymoron?

A story set in the future could use some evolved more mature nomenclature reflecting the known objects and ways to classify them used by people to effectively communicate within their fields of study.

A world-sized object with rich solid structuring and renewing surface, hydrosphere and atmosphere with dynamic behaviors worthy of study individually, is the thing being studied by an entire scientific discipline, so it ought to have a name. And they don't care about the location per-se, so Titan (not a planet), Earth, and the untethered world (not a planet) all are that, and parking the untethered world is intentionally not changing those characteristics.

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