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?

  • 5
    $\begingroup$ 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, 2016 at 23:25
  • 2
    $\begingroup$ @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, 2016 at 23:27
  • $\begingroup$ @Samuel That's an interesting thought. Expensive, but interesting. $\endgroup$
    – HDE 226868
    Feb 2, 2016 at 23:28
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    $\begingroup$ 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, 2016 at 23:41
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    $\begingroup$ Forgive me if this is just my ignorance showing, but wouldn’t those planets oribiting that sun become pitch black and bitterly cold since there’s no sunlight? Even at night, the moon reflects some of the sun’s light to earth, though I’m not sure how much that does for warmth. How would people see anything on these planets? A plethora of artificial lighting? And most importantly, what would they do for warmth? $\endgroup$
    – Kal Madda
    Apr 21, 2023 at 21:07

3 Answers 3


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, $\endgroup$ Feb 3, 2016 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, 2016 at 2:09

It depends on our technology and how long they've been out there.

Our telescopes are getting good

No one who's answered so far considered microlensing effects from the stealth system, but it looks like that gives a detection range of around 550 AU - well within the Oort clouds mentioned in the two current Answers. All-sky surveys of microlensing effects to pick up faint stars, wandering planets, and the like are current research projects. If your stealth system comes within a few hundred AU while one of the all-sky microlensing surveys is active, it will be detected.

Similarly, the James Webb Space Telescope is really good at seeing in infrared - the region of the spectrum which would be hardest to hide a star with current understanding of physics. To decrease the surface temperature of your stealthed star, you'd want to increase the radius of the Dyson sphere - probably to also enclose the habitable planets as well, but that means a larger patch of sky would be anomalously warm - even though it wouldn't be as warm as if there were no sphere. JWST is able to distinguish planets' temperatures from background for many nearby stars, and able to resolve the distances between the planets and their host stars - so the thermal signature from a Dyson sphere might be detectable out to tens of light years. The weakness of JWST is that it's not made to do rapid all-sky surveys, so a fast-moving Dyson sphere or one which happens to be in a direction where JWST doesn't look would be able to pass unnoticed.

How long is the stealth system in the vicinity of ours?

Comets kicked out of the Oort cloud could take centuries to fall close enough to the Sun for us to detect. If the stealthed system is just passing by, we might not notice any effect until the comet rate increases - and by then it might be very difficult to figure out the trajectory of the triggering mass.


If you want a "drive-by" interaction, you can probably pass within 1000 AU safely, provided your stealth system is moving very quickly compared to the sun's orbital speeds at that distance. There will be lots of disturbance in the Oort cloud, but if folks are invading / surveying Earth during the brief interaction, a Kardashev Type II ought to be able to get up close well before we notice the disturbance. If you want a hidden presence for thousands of years, you're going to have a hard time harding a star within 100 LY that won't be found by JWST, Roman, Hubble, or one of the big survey programs.


This answer is very short but as close as the middle of the Oort cloud
enter image description here

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! $\endgroup$ Feb 3, 2016 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. $\endgroup$
    – Loduwijk
    Jul 18, 2018 at 17:27

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