I am currently writing a story that involves the never-ending-theme of alien invasions. Assume Humans are roughly at a Type I civilization at this point. Humans have unfortunately wandered into a middle of a galactic battle, and because we always make great decisions, we chose a side to join, let's say the Galactic Empire.

The other side, the First Order, is not very happy about this and decides to completely annihilate Earth. Their planet-destroying weapon is going to lock on Earth and destroy it. However, its preparation time is long and needs a very accurate lock-on of the target. Therefore, in defense, Earth has came up of the idea of a Decoy Earth:

  • The Decoy Earth would be an obloid spheroid in the same dimensions and the shape of the Earth, except that it is actually completely hollow and only consists of surface light emitting panels.
  • The Decoy Earth would be placed on the Earth Orbit at the point directly opposite the current Earth, and be given a rotational velocity exactly equal to that of Earth's, so that the two Earths are always directly opposite each other.
  • The light emitting panels of the Decoy Earth emit light of the same spectrum as the true Earth.

The purpose of the Decoy Earth is for the First Order to not know which Earth is the real one and thus have a $1/2$ chance of locking on the wrong planet. Assuming the weapon takes an extremely long time to recharge, this would give humans more breathing time for us to figure out a plan.

Now my questions are:

  • How well can this Decoy Earth actually serve as a Decoy from distances of $\sim 1000$ light years? From my (very basic) understanding of astrophysics, the Decoy Earth would seem to do pretty well under direct imaging and the transit method. Clearly the mass of the two objects are vastly different, so gravitational microlensing can potentially reveal the truth, but is microlensing capable of discovering Earth-like planets at that distance?
  • (Optional) What would be the minimal distance in which this Decoy is effective, assuming current human technology in telescopes? You are welcome to assume that all current telescopes in full construction (e.g. JWST) as usable.
  • $\begingroup$ Welcome to worldbuilding, please take the tour and visit the help center to better understand our community. Since we prefer to have 1 question per post, I would ask you to rework your post to fit it in. Also, you need to better define the capabilities of the humans in your world. Else we cannot have a starting point. $\endgroup$
    – L.Dutch
    Commented Mar 3, 2019 at 5:27
  • $\begingroup$ Given that the decoy does not have an atmosphere and the real Earth does, spectral analysis will give the game up from far away. Also, I don't really understand this business of "locking"; the position of the Earth can easily be computed with great accuracy a very long time in advace: why would the kinetic impactor be fooled by a sort-of similar looking target 300 million kilometers out of position? $\endgroup$
    – AlexP
    Commented Mar 3, 2019 at 5:36
  • $\begingroup$ @AlexP Earth was never on the radar of the First Order before so they don't know the exact position. I assume the spectrum of light emitted from Earth is the same as on the Decoy. $\endgroup$
    – Michael Li
    Commented Mar 3, 2019 at 5:37
  • $\begingroup$ "The spectrum of light emitted from Earth is the same as on the Decoy:" that's new physics. As for "they don't know the exact position" -- what keeps them from finding it out during that long preparation time? $\endgroup$
    – AlexP
    Commented Mar 3, 2019 at 6:14
  • $\begingroup$ I hate to be a spoilsport, but a planet-destroying weapon that requires such long preparation times would be extremely ineffective. Besides what's to stop the First Order from having more than one super-weapon? This says nothing of your Decoy Earth. I suspect you need multiple Decoy Earths to really fool the enemy, not just one. $\endgroup$
    – a4android
    Commented Mar 3, 2019 at 6:29

4 Answers 4


How well can this Decoy Earth actually serve as a Decoy from distances of 1000 light years?

Well nothing travels as fast as light so it's 1000 years before they detect the fake Earth and at least 1000 years for the weapon to arrive. If they miss, another 1000 years before they know and another 1000 years before shot two arrives.

In reality, the fake will make no difference. We currently detect planets by the wobble the star has which is due to the mass of the planet.

The fake doesn't have the mass and therefore won't affect the sun so a fake won't fool anyone and depending on how they target, they may not even notice the fake.



Just kidding, what a stupid idea. To the point, as nobody knows the tech behind our attackers' weapon we cannot know how effective the decoy would be, what we can ascertain from this question is that your humans are super advanced. So this gives me an idea that doesn't directly answer your exact question but I believe could be useful to you.

What if instead of a literal decoy you make the enemy ships and databases simply believe there is a decoy? But now we have the issue of if their computers would detect this. If not great but otherwise we could beam light from a single point, at their ships sensors that looks like there is another planet from their perspective. Either one you do I believe has great potential in saving earth. Just think, with either of these methods you could create many more than one decoy at an incredibly nominal portion of the cost and effort of your initial idea. Hope this is helpful :)


Unless enemy is using extreamly large telescopes(or star's gravitational lensing) he cannot resolve any details, so we need to simulate only spectrum and most basic time patterns.

If decoy emits a ray with divergence about second of arc(at 1000 ly it would be some 10 000 of AU) then it requires very little power - about 100 kW.

So decoy could be quite cheap and in this case it would make sense to make thousands and even millions of them.

  • 1
    $\begingroup$ Yeah - if one is good, how about a bunch? I am a little concerned that an orbital path is determined by the mass and velocity of the orbiter and an inflatable fake earth would need to move much more slowly than the regular earth to stay in orbit. But only a little concerned. $\endgroup$
    – Willk
    Commented Mar 3, 2019 at 18:42

Fake is not really practical.

First consideration is the amount of time you had to create the fake before the aliens started aiming. If the fake was ready N years ahead of time the aliens will not even notice if they are further than N light years away.

And if they are close enough to see the fake, they will see two planets in the same orbit and will take steps to verify which they want to shoot. The same steps they are taking to avoid shooting Mars or Venus. So unfortunately they are already forced to take the exact countermeasures needed to make the fake useless anyway by the existence of those other planets.

Your fake is not going to be better at fooling them than an actual planet. Unless you assume a REALLY dumb targeting system.

Which you totally can. The classic trope is that the aliens have placed some sort of a targeting beacon on the targeted planet and you simply move that beacon somewhere else. Only real issue is that this has been used enough that it is kind of a cliché. Otherwise it is a perfectly fine way to solve this, it is a plausible way to exploit a plausible weakness of a plausible way to aim a super-weapon.

Otherwise... If I was allied with a bunch of super advanced aliens I'd just ask their help to colonize the asteroid belt and other such small targets within the solar system. Then move there with all the people I like. This could be funded by selling everything we own on Earth to people I do NOT like. I mean, we cannot stop the planet from being destroyed so there would be no real reason to cause an useless panic. Let people enjoy their final moments and the wheels of economy to turn normally until everything has been sold and all the supplies have been bought.

That said, generally in fiction such super weapons pretty much exist to serve as targets for daring attacks. You should have that at least considered. A super weapon capable of destroying planets is at the very minimum worth the same as an actual planet as a target, usually it is worth entire star systems. And it is generally much easier to destroy and is generally less morally questionable than destroying the equal value in inhabited planets. In all ways an excellent target. Except for the minor fact that the enemy also knows this and, yes, it will be a trap. But for a story that is just a bonus.


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