Start with a powerful space-faring civilization. They are Kardashev Type II, having harnessed the total energy output of their sun.

To do this, they have encapsulated their star with a tight-fitting Dyson Sphere which is totally dedicated to energy capture. None of their star's light or radiation penetrates the sphere.

The planets of their system still orbit the imprisoned star, but they do so in almost absolute darkness. The inner planets are all populated, but none of them still have their original atmospheres. Instead, they have each been encased in a defensive shell, then terraformed into planet-wide urban cities.

There is no wild plant life, no living oceans and no undomesticated animals. All of those things have been abandoned on their quest for the stars.

Now as a Type II civilization, they obviously know a lot more about the universe than we do. One of the most important examples of that greater knowledge is a real understanding of how dangerous the universe is. There are lots of militant Type II and imperial Type III civilizations out there and any one of them would be thrilled to conquer and enslave these peaceful people.

So they hide.

It is for concealment that they have blacked out their sun and worlds with opaque shells. Similarly, they use only line-of-sight lasers for inter-system communications.

The Question

How close could this civilization's planetary system be to our own, without our noticing the effects of its gravity on objects that we can see?

  • $\begingroup$ The one issue here is that Dyson spheres will still radiate in the infrared, so it would still be detectable by some telescopes. $\endgroup$ – HDE 226868 Feb 2 '16 at 23:25
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    $\begingroup$ @HDE226868 You mean as a blackbody? They could use handwavy-lasers to beam excess energy into empty space and lower the shell to background radiation levels. $\endgroup$ – Samuel Feb 2 '16 at 23:27
  • $\begingroup$ @Samuel That's an interesting thought. Expensive, but interesting. $\endgroup$ – HDE 226868 Feb 2 '16 at 23:28
  • $\begingroup$ The problem is the efficiency of most lasing techniques is very poor (something under 20%). So handwavy-lasers to beam excess energy makes your problem worse. LED lasers can have efficiencies as high as 90%. But you are converting a useful form of energy (e.g. electricity) into a laser beam and heat. You still need to get rid of that heat. Besides, if you have energy in a useful form like electricity, why waste it by beaming energy into space? $\endgroup$ – Jim2B Feb 2 '16 at 23:41

This is a tough question to answer because any planetary system with more than one planet is tough to analyze. When two planetary systems collide, the result is overwhelmingly difficult to analyze, even for computers. It's safe to say that we would notice effects before the planets even got near each other. Perturbations of comets in the Oort Cloud would be evidence enough that something funky is going on. This would be hard to figure out at first, but eventually, we'd figure it out.

I'll use the Hill sphere approximation to make a really rough lower bound on the distance. For this, I'm assuming that the other star and Dyson sphere have a much greater mass than that of the Sun. Therefore, the radius of the Sun's Hill sphere is $$r=a\sqrt[3]{\frac{m_\odot}{3m_{\text{star}}+3m_{\text{sphere}}}}$$ where $a$ is the distance between the two bodies. Inverting this, we have $$a=r\left(\frac{m_\odot}{3m_{\text{star}}+3m_{\text{sphere}}}\right)^{-1/3}$$ Let's set $r$ to the minimum outer radius of the Oort Cloud, 50,000 AU. If we set the masses of the star and the Sun equal, and assume that $m_{\text{sphere}}\approx9.15\times10^{-5}m_\odot$, then we have $$a\approx62,500\text{ AU}$$ This sets a lower bound on the distance the star would have to get to. The star would have to be relatively close to perturb these comets enough to start to attract them to itself. But we'd notice some fishy effects a bit sooner. Just how soon depends on how good our detection methods are.

  • $\begingroup$ +1 and Thanks! That answer confirms my hope that the home of the little grey men in one of my stories could be less than 5 light years away. Your answer allows me to put them 1 light year away, but that seems a little too close. At 5, they are far enough away that they wouldn't have already colonized us, but close enough that they worry when we started calling attention to ourselves, broadcasting radio waves and setting off nukes, $\endgroup$ – Henry Taylor Feb 3 '16 at 2:07
  • $\begingroup$ @HenryTaylor Five light-years? You're definitely fine. That's farther away than the Alpha Centauri/Proxima Centauri system, and they certainly don't bother us! $\endgroup$ – HDE 226868 Feb 3 '16 at 2:09

This answer is very short but as close as the middle of the Oort cloud
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The only way we would notice it is aperiodic and missing comets.

  • $\begingroup$ +1 and Thanks! Both of today's answers confirmed that my story will work. And even more amazing, you agreed with each other, approximately. With our solar system having an oort cloud out to 2 light years (which I'm estimating from your picture) and the stealth solar system also needing about that much space, I think 4 to 5 light years between primaries is a functional separation that keeps them from messing with each other's satellites. Thanks! $\endgroup$ – Henry Taylor Feb 3 '16 at 2:15
  • $\begingroup$ @HenryTaylor Just note that the situation you just described, if it also includes objects reaching out that far from the other star system, would result in the outer "clouds" of the two systems to interact. However, note that if your other system is mobile then it likely would not be dragging such an outer cloud around with it, as that would get left behind, leaving that system with only the objects that it wants to bring along. This does not break your story; it just gives you another thing you need to be aware of. $\endgroup$ – Aaron Jul 18 '18 at 17:27

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