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Some research papers, such as Arnold 2005, has investigated the possibility of using artificial megastructures transit signatures as long-range, long-lived and low-maintenance signals of unambiguously intelligent origin.

My question is the following:

What kind of N-body stable orbital pattern would be the most likely to be interpreted as a sign of intelligent intervention by people doing photometric (and associated) transit observations?

To provide some context, let us imagine a sentient ET civilization that has the capacity to rearrange the orbits within its own solar system and, for some reason, want to signal its existence into deep space. The technology employed doesn't allow to create fancy geometrical structures, such as toroidal planets, swarms or continuous rings, excluding clear cut transit signatures. However they can eventually channel asteroids into self-gravitating (spheroid) objects and modify planet masses through controlled collisions. The process is time consuming and incredibly difficult but in practice any N-bodies orbital configuration can be achieved (N is mainly constrained by solar system mass availability although there is also a minimal cross-section limit to consider for detectability, so 1 < N < 50 seems fair). Importantly, because of energy availability limitations, the new orbital configuration must be dynamically stable such that no future correction is needed to maintain the system in the long-range.

I am not necessarily looking for a precise configuration, but rather to explore categories of solutions. My impression is that there are two seemingly opposed approaches to the problem.

The first is to use tailored periodicities as a message carrier, in the spirit of our lighthouses, which could for instance mimic an atomic spectrum in frequency domain. The unlikeliness of this kind of orbital configuration would come from its high level of order but could also be overlooked, particularly by an automated detection procedure. Also long periods would necessitate even longer observation times to be spotted.

The second approach would be to use an overly complicated contraption of coupled orbits, triggering attention because it appears against any common sense (i.e. what is this thing?). Then, with deeper analysis from the observers, something must drive the cursor from strange to clearly unnatural. Maybe an underlying ordered signal. The risk for this kind of configuration is to be interpreted as a perturbed system in transient state.

Another formulation for this question is, what could SETI enthusiasts look for in the Kepler mission dataset?

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    $\begingroup$ "What kind of N-body stable orbital pattern would be the most likely to be interpreted as a sign of intelligent intervention ..." - obviously an unstable, which will not exist too long in a natural way. $\endgroup$ – MolbOrg Jul 7 '17 at 0:38
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    $\begingroup$ I would suggest your question a self-answering one. Arnold (2005) has already proposed several transit light signatures that are distinguishable from those of natural objects orbiting a star. Beacons using radio broadcasts or laser beams would be easier and better at sending messages. $\endgroup$ – a4android Jul 7 '17 at 2:32
  • $\begingroup$ @a4android , this question specifically consider only planet sized spheroid bodies, assuming that special geometrical shapes such as artificial megastructures cannot be created. Also radio or laser beams are technological devices that would probably need maintenance. The advantage of a stable "orbital beacon" is that it is physically self maintaining in the long run (imagine a hibernating species for instance). $\endgroup$ – Aderam Jul 7 '17 at 8:15
  • $\begingroup$ @MolbOrg , I mean stable in the range of hundreds of millions of years, and it is a constraint to the problem. i.e. among the set of all "hundreds of millions of years stable" configurations which would be selected ? $\endgroup$ – Aderam Jul 7 '17 at 8:28
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    $\begingroup$ Sorry to seem as if I was being perverse. I am aware of your question and its requirements. It seemed to me there were simpler, more practical solutions. $\endgroup$ – a4android Jul 7 '17 at 9:32
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The alien beacon makers could move some planets into orbits that are perpendicular to the plane of the ecliptic of their planetary system. If every second planet in the planetary system was moved in a perpendicular orbit, this would be so unnatural that anyone observing it would have a reasonable chance of concluding this was due to technological interference.

For example, if this was done in our solar system the planets in perpendicular orbits, in alternating order, would be Venus, Mars, Saturn, and Neptune. The planets mercury, Earth, Jupiter, and Uranus would remain orbiting in the plane of ecliptic.

The changes to the orbits of the planets moved in perpendicular orbits can be accomplished using gravity tractor technology. This obviates the need to move asteroids and other planetary matter to create new spheroidal planets.

This orbital configuration of alternating perpendicular orbiting planets should be sufficiently unusual to only be artificial in origin. This could be a stellar beacon.

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We see transits now, and use the period to determine orbital characteristics of the planets. Something like KIC 8462852 has people perplexed, and the hypothesis that it’s a megastructure has in fact been raised.

So, something like that, but make it unambiguous as intelligent: have the transit periods follow prime numbers rather than constant! Change it up, even send coded messages using the star as a semaphore lamp.

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