This question is inspired by NASA’s recent TRAPPIST-1 announcement. Imagine two planets orbiting a star with orbits very close together. At the nearest point in their respective orbits they are only 1 million kilometers apart. Both planets lie in the habitable zone and both will evolve sentient life. Much like humans these two distinct species create structures, roads, and farms. They start fires, cut trees (or their flora equivalent), and divert waterways. As they progress technologically they create larger cities and farms and their effect on the world becomes more noticeable. So too does their ability to observe each other. Both species are curious, and naturally they train their telescopes on their clearly habitable neighbor. At what technological level will they most likely become aware of each other? What would be the first signs that would betray one species’ existence to the other?

I’m interested in varying technological disparities between the two species. If one species is neolithic at what point will the other species discover them? Will it require manned/robotic expeditions or could powerful telescopes detect them? What if both species are roughly on par technologically? What is the technological breakthrough that will reveal the other?

Bonus question: are there any plausible scenarios where the technologically inferior species could become aware of the other species’ existence first?

Assume for our purposes the atmospheres of both worlds do not obscure astronomical observation any more than the Earth’s does.

Note: this is distinguished from this earlier question by:

  • the separation is 4× greater
  • the civilizations may be at different levels

4 Answers 4


The answer depends on both civilizations' technological levels.

If one civilization has the use of electricity and it lights up its cities in the night, with the distances as close as 1 million kilometers those cities would be seen with unaided eye. At the distances of 20-40 million kilometers a simple telescope would be needed.

If one of civilization uses fossil fuel lighting (similar to XIX century Earth) and builds sizable cities, a more advanced telescope would be needed.

Other effects of civilization - like deforestation and large scale farming, can be easily detected, but it would take an advanced level of theorizing to prove that those are not natural phenomena.

Detecting neolithic civilization would be a tough task. Without actually sending a probe to another planet, advanced civilization can detect changes in vegetation, but it would be difficult to prove anything conclusively. However, if the distance between planets can be indeed as close as 1 million kilometers, even small settlements could be seen.

P.S. Answer to the bonus question - advanced civilization has not only advanced means of exploration, it has more ways to give itself away. A neolithic civilization can observe night time illumination of their more advanced neighbors, while those neighbors still don't know what kind of species inhabit another world.


I'm going to assume one planet has advanced to around our current level and place the other at different times.

  • Our neighbours are cavemen: Even with exoplanets we have looked at the composition of their atmosphere with an interest for discovering life. This would lead us to note an atmosphere prone to developing life and, as such a search for that life. Robotic craft would make it first, at the very least a satellite would be put in orbit in order to verify the existence of life.
  • Our neighbours are have discovered farming: The burning of large areas of land without much of a good reason for it to happen naturally would indicate the existence of life.
  • Our neighbours have tech a hundred years behind us: Radiowaves would be the easiest to get through, sending us a very clear sign that our neighbours are advanced. A hundred years ago we were just getting into sending long-range transmissions on a regular basis.

As for your bonus question: There is no real reason for an obsession with life elsewhere, it could be that the more advanced civilization has stopped looking or never was. The use of radio-waves will be an early point of contact across the world so one civilization may start detecting radio-waves of the other when they first discover the technology - the other world would expect a lot more noise in their signal from other emitters on their world and, perhaps, not think anything of it whereas the more backwards world would have a clear detection of some language being transmitted or at least a non-random signal where they expected to pick up nothing.

  • $\begingroup$ Why hundred years? Distance is only about million km, and that's like three seconds. We could listen to their radio comfortably. $\endgroup$
    – Mołot
    Feb 24, 2017 at 17:11
  • 1
    $\begingroup$ @Mołot Ah, I hadn't thought of it being read that way. I meant their tech is a hundred years behind us (so they have just started using radio - of course they wouldn't necessarily follow the same time-line). I'll edit to try to make this a little clearer. $\endgroup$ Feb 24, 2017 at 21:15

The telescopes weren't enough strong until around WW2 to detect a nearby civilization visually. Consider the long suspections about the Martian life, simply the valleys of the Mars were suspected as irrigation tunnels.

If the remote civilization has already radio communication, it would be detectable with our radio telescope many, MANY light years away. Our radio senders would be detectable in an around 80 light year big sphere around the Sun (only from not much more, because earlier we hadn't radio).

1million km isn't very much, it is only around 2.5 times of the Earth-Moon distance. An another Earth 1million km away would be higher as the Moon. We would see that it is green even with free eye since the ancient times and probably it would be clear for us that also there is life.

Note, so nearby independent orbits are impossible, the planets would too strongly affect eachother. Most probably, after some significant "perturbations", they would go into a more distant orbit. There is another possibility (much smaller chance) that they would go to orbit around eachother and become a dual planet. (The third possibility, their collision, is closed out: planets are much more distant as their size.)

The dual planet situation wouldn't be a catastrophic event for the life on them only in the case if it had happened before the existence of the life, and even their common orbit would make the life possible on them.

In this case, the planets would fast tidally lock. It would mean that the "other Earth" is always on the same point of the sky, and 1 day would be 1 month. It would be a much slower rotation as we currently have, thus life similar to ours is possible on such planets, only if the planets have a much denser atmosphere. Ideally, I would suggest a much ticker nitrogen atmosphere (some ten times as on the current Earth), with same oxygen content as we have.

On such planets, flying life forms (birds) would be much more common, probably a significant part of the whole biosphere would live in the air. Maybe flying plants would also exist. Intelligent lifeforms could be also flying ones (our birds can't develop because they have no place for a bigger brain).

If the intelligent race of the planets are like human, they could fly even with ancient technologies, like with ships.

Falling down from a high elevation wouldn't be dangerous, because the drag would avoid to accelerate to too high speeds. There would be much more strong winds.


According to Wikipedia's article on visual acuity, normal vision can resolve not quite 2 mm line pairs at 6 meters. That's 3000:1

So at a million km, the Eye Ball, Mark I would be able to see features 300 km across, if they were high contrast. This would give you the gross outlines of continents, major weather systems, but would not reveal cities.

A set of 10 power binoculars brings that down to 30 km. At 65 power you're down to 5 km. This is the standard field model 3.5" Questar field scope. Still dodgy for all but the biggest cities. However, the construction of large dams that impound lakes would be visible. (Not the dam, the lake.)

Once they start an industrial revolution you may be able to see ash plumes from industrial areas.

The first spectrometer on Earth was 1802. Once absorption lines were discovered, then the variation in gas composition from analyzing light skimming the planet would reveal industrial activity.

This answer suggests that .5 to 1 arc second is the atmospheric limit. That's 1/3600 of a degree or a target distance ratio of about 1:200,000. At 1 million km, we're talking 2.5 to 5 km again.

The Hubble can do about 20 times this, so now we're getting down to city block size.

Caution: The power of tides goes with M/r3. Two earth mass planets 1 mega-km apart will have 1/27 the tides due to distance, but 80 times the tide due to bigger mass compared to earth moon. Net result is about 3 times the present lunar tides.

Caution: Close orbits tend to be unstable. See Wikpedia article on orbital resonance. Orbits tend to work better with simple ratios. Play with orbital dynamics models.

TRAPPIST-1 planets are very close together, but also very close to the star.

Postulate that the close encounters keep the planets from being tidally locked to the primary, or that they get locked in resonance. (Mercury is locked in a 3:2 resonance for example. 3 'days' = 2 'years' Given the very short orbital periods of TRAPPIST-1 this sort of period would be fine.


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