Technology won! Humans managed to advance technology fast enough that they circumvented any global warming apocalypse and by some sheer miracle (maybe God's intervention) they got their population under control and all agreed to stop killing each other.

So they are now in the familiar post-scarcity era and everyone's settled down nicely. There is a new problem, the sun's battery is running out. Let's set the scene several thousand years from now. I know the sun has a very long time left comparatively, but you know what humans are like, they worry.

It was decided that sending small groups of humans off to nearby habitable planets (which have been 100% confirmed) would be logistically difficult, favor some humans over others, unlikely to succeed and be hard to populate the planets for a myriad of reasons. "How could we take the whole of Earth to a new star?" they asked. "We could build the biggest Solar Sail ever?" suggested one naive proto-scientist.

OK enough of the theatricals. Is it possible to build a giant solar sail, big enough that the sun propels it and its gravity (and/or other forces) counteracts the propulsion so that it pulls the whole solar system with it? Basically we are trying to turn the whole solar system into a ship.

Imagining we have all the resources of the solar system, what would be the logistical problems in achieving this? Can we make it to another star, let's say 10 light years away before our Sun's "battery" runs out? Would it be possible to detach the sail and aim Earth so it starts orbiting the new star safely? Answers should be science-based but do not necessarily need citations.

If we cannot take the solar system with us, is there a way we could incubate the Earth while it travelled alone by solar sail to the new star? The majority of life would have to be present when we arrived.

  • $\begingroup$ My first instinct is to reject both ideas on science grounds, but how futuristic is the technological scene? $\endgroup$ – Frostfyre Sep 17 '15 at 13:47
  • $\begingroup$ Anytime between now and when the sun blows up. As advanced as you want, providing you explain what advancements allow us to achieve it. $\endgroup$ – Varrick Sep 17 '15 at 13:53
  • $\begingroup$ Also, when you say "reject both ideas on science grounds", explaining why it's impossible is also a valid answer! $\endgroup$ – Varrick Sep 17 '15 at 13:54
  • $\begingroup$ I'm not an astrophysicist to be able to provide a good, coherent argument why it can't be done. In truth, most of my mathematical understanding of planetary physics comes from WB. $\endgroup$ – Frostfyre Sep 17 '15 at 14:14
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    $\begingroup$ My first thought was not possible but the top answer on the above link explains how it might be possible. $\endgroup$ – bowlturner Sep 17 '15 at 15:20

Is it possible to build a giant solar sail, big enough that the sun propells it and it's gravity (and/or other forces) counteracts the propulsion so it pulls the whole solar system with it?

I haven't done the math, but I am willing to bet that the answer is no for a very simple reason: the sun makes up the lion's share of the solar system mass (we are talking somewhere on the order of 99% of the total mass of the solar system being concentrated in the sun), and it seems unlikely that we could devise a scheme in which the sun effectively moves itself.

Remember that movement is always relative to something else (you can play games and make that "something else" an inertial reference frame, but that doesn't help you in practice here because your target will also be moving in that reference frame), and also remember Newton's third law of motion: all forces between two objects exist in equal magnitude and opposite direction. Note that this is force, and as we know, a force of a given magnitude has different effects depending on the mass of the objects involved. An apple falls toward the Earth more than the Earth falls toward the apple because the Earth is so very much more massive than the apple, but the apple still exerts the same force on the Earth as the Earth does on the apple. Our proto-scientist would face a similar dilemma with the solar sail.

Even if we could somehow concentrate all the mass of the solar system except the sun into a single object, and turn that object into a solar sail, and attach that solar sail to the Earth somehow, the sun would still be about a hundred times more massive. The gravitational pull of the sun on this object would be very much greater than the graviational pull of this object on the sun, let alone anything that could be imparted by the solar wind, and hence, it would simply remain in orbit of the sun.

To add insult to injury, much of the material in the solar system is woefully inadequate for constructing a solar sail. Much of what you've got to work with is helium, hydrogen and other gaseous elements.

This answer to What could cause Earth to slowly fall out of orbit? also has some numbers for the problem of moving simply the Earth. What you are proposing isn't a hundred times harder; it is far worse than that. (The Earth's mass is about 5.97e24 kg; the Sun's mass is about 1.99e30 kg, or some 333,000 times more massive. For our purposes, we can approximate the solar system mass as the sun's mass.)

You may also be interested in the question How could a sentient life form move an exoplanet the mass of the Earth? and its associated answers.

"How could we take the whole of Earth to a new star?" they asked.

Even assuming that you could pull the above trick off (let's play the handwave game for a moment here), how are you going to transfer the Earth from an orbit around the sun into an orbit around the new star? It's hard enough to push a small rocket into an orbit around something that is not the Earth, even starting in an orbit around the Earth (and thus out of the worst depths of Earth's gravity well); transferring the Earth from orbiting one star to orbiting another would be many orders of magnitude more difficult.

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  • $\begingroup$ I like your answer but you should maybe correct the big in bold. The gravitational pull of the sun will be equal and opposite to the gravitational pull of the sail as you pointed out in the paragraph before :) $\endgroup$ – Varrick Sep 17 '15 at 14:26
  • $\begingroup$ @Varrick Actually, not quite. The force exerted must (unless we hand-wave away Newton's third law, which seems a bit much...) be identical and opposite, but the gravitational pull isn't related only to the force exerted by the objects on each other. In orbit, there is also a forward momentum, which is where the vast majority of the force ends up in a system with two bodies with a large difference in mass. Remember, it isn't hard to get into orbit because it is high up, it's hard to get into orbit because you have to go so fast. Here, space == orbit. $\endgroup$ – user Sep 17 '15 at 14:30
  • $\begingroup$ The pull will still be the same though surely, just the resultant vector may favor the Sun. $\endgroup$ – Varrick Sep 17 '15 at 14:36
  • $\begingroup$ @Varrick Good point. I will see if I can come up with a better way to phrase that. $\endgroup$ – user Sep 17 '15 at 14:39

You don't want to bring the solar system with you because it will disrupt the orbits of any habitable planet you approach. Even if you could move the sun and all the planets to get you to another habitable solar system, by bringing the sun with you, you'd destroy the orbits of new planets unless you were incredibly careful.

The orbits of the planets in our solar system are stable because they've had a long time to settle down into their present orbits. Introducing a strong gravitational influence like another star will throw all those orbits into disarray. If you bring the Sol system with you, what you thought was a nice habitable planet in this new solar system is now an ice ball, fire ball or lonely planet (depending on how the gravitational interactions work out.)

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