In the event that both planets of a binary planet system were life supporting, and both ended up developing a sapient, tool using species, at what technological level would they be able to start effectively talking to each other?

Now when I say "effectively communicate" I mean at what point would it be reasonable for meaningful massages to be send to the other world(the ability or the other to respond would be nice, but not needed immediately)? would this be possible prior to both species having developing Radio communications to send and receive with?

Obviously the technological levels of each species would vary due to countless reasons, and one will notice that there is a civilization on the world in the sky before they other(though cultural, religious, and other preconceptions/biases would cloud how they interpret this fact until they have really good observation technology to more closely/accurately look at their neighbors), but beyond doing things that are basically "hey! We exist! look at this giant structure we made to get your attention!" and "we know! here's visually identical one we build to acknowledge it!" which could be easily motivated by and misinterpreted through those previously mentioned biases and preconceptions of the universe, not to mention if it happened before a species had fully explored its own world(or at least the tidally locked side facing the other) then this could lead to an interplanetary game of telephone as civs build structures inspired by alien structures that were inspired by structures from another part of their own world(but that's going into the psychology and cultures of two alien races growing up next to each other, and how it effects each other over the course of their histories, which is a bit beyond what I'm asking here)

EDIT: Since someone asked for details on the worlds themselves here they are; the binary worlds are of course tidally locked to each other at a distance where they'd have an Earth-moon like view of each other, they are of similar "temperate" climates and changing seasons due to their similar position relative to their sun, though obviously the weather would be different due to the atmosphere and landscape of each world having differences(higher or shorter mountains, shallower or deeper oceans, how much surface area those oceans cover, different ratios of nitrogen, oxygen, carbon dioxide, and other gasses, and so on).

And to go into more detail on the species as well, since I have a vague idea of what I'd want them to be like, though I'm not sure if that's that relevant to the question of what tech they'd need to start properly talking to each other; the one living on the world with more oceans is a partially amphibious species, and the other is a "reptilian" species that endures the winters with a form of hibernation that induces a lucid dreaming like state of mind.

  • $\begingroup$ You should spend a paragraph on describing how your binary planet system looks/works. Maybe add a graphic as well? $\endgroup$
    – dot_Sp0T
    Commented Jun 2, 2017 at 17:41
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    $\begingroup$ Re: "Obviously the technological levels of each species would vary due to countless reasons": This seems like understatement. What are the odds that both planets develop intelligent life before either planet's intelligent life develops space travel? $\endgroup$
    – ruakh
    Commented Jun 3, 2017 at 0:27
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    $\begingroup$ The likelihood that both worlds develop a sapient species is actually unknowable, sense we have no data to judge the probability of this; it's true that only one sapient species currently lives on the only known life barring planet; but our analysis of the fossil record shows that there was once a wide multitude of hominin species(and not just human ancestors) that show human-like brains, and there are plenty who would argue the intelligence of many modern apes, dolphins, octopi, and some birds is much higher then we give credit. There are many evolutionary benefits to intelligence after all. $\endgroup$
    – Zak
    Commented Jun 3, 2017 at 4:08
  • $\begingroup$ Note: being able to send a message is one thing, being able to understand said message is however a completely different thing. Unless both societies evolved together first, they would have little in common, and would have to learn to understand each other first. Doing so at a distance sounds... incredibly difficult. And the more different the worlds/groups, the more difficult it will be, as they will have less and less common reference points. $\endgroup$ Commented Jun 3, 2017 at 12:22
  • $\begingroup$ @Zak: I didn't ask about the odds that both planets develop intelligent life -- I'm on board with that assumption -- but about the odds that both planets develop intelligent life on a close-enough time-scale for this question to be meaningful. Those other species you mention are nowhere near the point of communicating with a planet that's hundreds of thousands of miles away. $\endgroup$
    – ruakh
    Commented Jun 4, 2017 at 22:23

8 Answers 8


Well, you're on the right track by first considering that their technological and sociological evolution wouldn't parallel our own world's history (even if they somehow were both human races on earth-like planets) nor each other's.

Something to take into account is how close the binary planets are to one another. Are we talking a Rocheworld, where they're so close they even distort each other's shape due to gravity, and even share a tiny bit of connecting atmosphere, or are we talking more along the lines of Earth/moon distance? If the former, it's actually going to be pretty difficult for both planets to support much life, or either to support intelligence due to the numerous destructive forces a Rocheworld would experience.

If the latter, it depends on how you define meaningful communication. Again, you were on the right track by touching on "we can see each other, and build structures to show we're here". If they're close enough that a pre-industrial society with telescopes can see at a decent level of detail, then really, they don't need to be too much more advanced than our renaissance/expansionist periods of the 14th-18th centuries.

Certainly, communication permitted by radio or other high technologies would be far MORE meaningful and dense in information, but mariners in the ancient days had ways of communicating meaningfully across distances sound couldn't travel well. These included semaphores and precursors to Morse code communication by blinking lights.

This sort of system would be really easy to work out in a tidally locked binary planet system, where stations on both worlds where they face one another could be set up, with large reflective mirrors (which needn't be the type of perfect mirrors used for lasers and telescopes) and fire to provide the light.

It would be tricky to establish the initial communication - to develop a mutually intelligible code, but with enough trial and error and a basic grasp of maths, that could be achieved.

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    $\begingroup$ I read the question and was trying to boil down what I'd need and came up with something not too far off from "fire and a large mirror." You might even be able to dispense with the "fire" by using the parent star's light at the right time of day. $\endgroup$ Commented Jun 2, 2017 at 17:56
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    $\begingroup$ Yeah, that's true. Of course, it depends on the type of star, and the dynamic with the planets. Tidally-locked would mean it may be more work to channel the parent star's light a lot of the time. But at the same time, perhaps not. The other problem with tidally-locked worlds, of course, is that many in the academic community suspect only the day/night terminator, the "twilight zone" would be very easy to live in, especially for a pre-industrial culture. I agree, we need more information on how this binary system works, to say anything more concrete. $\endgroup$
    – Cereza
    Commented Jun 2, 2017 at 17:58
  • $\begingroup$ dang, my answer too. I think the mariner semaphore system with reflected sunlight would work; basically louvers one can manually shut and open with a handle, to blink. They used Morse code, but the two "characters" in Morse code (long on, short on) can be used to represent counting, relay prime numbers, and then eventually pictures (like the movie Contact imdb.com/title/tt0118884 ). They would need tech about like 2000 years ago, Archimedes could have figured all this out. Decent mathematics and geometry, stuff I took in the 7th grade. $\endgroup$
    – Amadeus
    Commented Jun 2, 2017 at 18:15
  • $\begingroup$ yeah, it was the "to establish the initial communication - to develop a mutually intelligible code" part that I was having trouble with in my head, sense not only would this be hard to do between two species, but the earlier such contact is established, the more factions each world will have and the more "mixed signals" they'd be sending to each other with no idea that these other factions from their own world are "saying" to the other, as well as the confusing mess of the different alien attepts at communication. $\endgroup$
    – Zak
    Commented Jun 2, 2017 at 18:22
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    $\begingroup$ Take into account how language, especially transmitted, written language evolved here. It didn't form as the modern (or well-known ancient) writing systems we're familiar with offhand. It started as pictographs representing concrete ideas, which then came to represent concepts that sounded similar in the language in question, and so on. So, establishing a way to transmit basic ideas first could just evolve on its own into a standard language for them both, independent of the natively-spoken languages of either world after enough time. $\endgroup$
    – Cereza
    Commented Jun 2, 2017 at 18:40

Agriculture-level, plus some rudimentary telescopes, should be enough.

If you create massive tulip fields, I mean.

Tulips are awesome.

Tulips can be used to effectively paint the landscape. If your races are dedicated enough and have some big, wide open areas visible to each other, with suitable flowers, they can paint the surface of their planets to make drawings to each other.

This will consume time and space, of course, but hey - if you draw something on your super garden, and the other planet nearby does a similar super garden, you wouldn't be amazed by it and invest more time to it?

They will probably need some sort of telescopes to check the gardens of each other, but the sheer cuteness of space aliens talking to each other by gardening already makes me want to write this story!

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    $\begingroup$ @Cereza If you use plants that glow like fireflies (we are talking about alien things, why not?) you can reach some pretty amazing imagery. $\endgroup$
    – Mermaker
    Commented Jun 2, 2017 at 20:55
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    $\begingroup$ These massive tulip fields? $\endgroup$ Commented Jun 2, 2017 at 21:00
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    $\begingroup$ while the idea is sound in terms of being possible with low tech levels, it brings up another issue; unless the plants producing these colors are also food crops(be it for the species themselves or livestock) and they're able/willing to alter their diets based on what images they want to display to their alien neighbors, I doubt this method would ever be used(at least not for long) $\endgroup$
    – Zak
    Commented Jun 2, 2017 at 22:56
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    $\begingroup$ to add to my previuos comment(due to my having posted it while it was incomplete and then the editing being interrupted by an error); the use of large agriculture to send messages would only be practical if that land wasn't better used for farming or other immediately useful purposes that benefited the people living there; and while this is true for any thing being done to contact the other world's inhabitants, options like bonfire/lighthouse signaling are less space consuming and can be buildings used for other porpoises when not being a message sending device. $\endgroup$
    – Zak
    Commented Jun 2, 2017 at 23:13
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    $\begingroup$ @Zak "Damn, I don't want to eat Krishnok again. Can't we smuggle some Potros in somewhere in a corner? " "Sorry Bud, the orders are clear. It's Krishnok, Radis and Applan this season. Remember '87? It was 'just a few pearos', and we all know how that ended up." $\endgroup$
    – JFBM
    Commented Jun 3, 2017 at 13:06

The techs they would need:

Astronomy (to know that there's another planet that might have people on it).

Beginning radio would do it if they throw enough power into it (cubed root drop off).

They might even be able to do this without astronomy if they both have radio and are pumping out enough wattage. That's unlikely however. Why put that much juice into a signal if you don't know there's someone out there to receive it. I just mentioned this because their tech development order does not have to mirror ours completely. It just seem likely that astronomy will occur first.

If they learn enough to make parabolic reflectors, they can aim more of the signal at the target and use less power to make the transmission. the parabolic reflectors also allow them to pick up weaker signals. So only one of the pair needs them. That will make communication easier.

It will become easier still when they figure out masers.


First I'd like to talk a bit about statistics. Our planet has existed for some 4 billion years-ish, and support human-like life for perhaps half that, if im being generous.

Of those two billion years, humanity has been intelligent enough to looks at the stars in a scholarly fashion for perhaps 3000, again if i'm really generous. Lets say it takes us another 1000 to set up a bus to go to the moon and back.

That's a time frame of 2 000 000 000 years that a planet has stupid life, compared to 4 000 years of intelligent life. Meanwhile, with such a fertile planet that close, humanity would spread across it's surface like mice in a barn. If the moon were fertile and of even remotely suitable atmosphere, you can be pretty sure we'd have a colony there already. Statistically speaking, either intelligent species will colonize the other planet long before a species can evolve there to be about as smart.

To make your story believable, I hope you got something to remedy these statistics.

As for communication, I think radio is the first method available. With a good telescope, you can figure out there's life on the other rock a couple hundred years early, basically as soon as you get higher quality glass working for lenses. However, how would you agree to any communication protocol ? You probably can't. Every communication protocol we use, both sides have an understanding of how it works, we encode and decode the information. This was a consideration for the old voyager drones - part of what was done was engraving pictures of humans in a plate of gold, as well as an attempt to codify a small message in a sort of language we hope is easy to understand, but all our perspectives are human, not alien. Your races would have likewise problems.

It's not like talking to other humans. It's like talking to animals. With a lot of training, we can for example teach sign language to certain types of monkeys. However, establishing any mutual understanding that way requires close contact. Even if you found geeks on either planet who talk by radio and attempt to make sense of what the other one's trying to say, chances they'll reach some understanding in any short amount of time is really slim.


Once they have radio, communication is clearly possible. You could send a message to another planet with pulsed lasers but that's a more complex technology than radio.

But they could at least theoretically communicate well before that. They might, for example, build giant bonfires that would create enough light to be seen with telescopes from the other world. Once you have something that is detectable, you can vary it in one way or another to create meaningful messages. Like create a string of fires in some pattern that conveys a message.

The big question is, how big a bonfire? I found a formula that says that the maximum resolution of a telescope, in radians, is R=w/D, where w is the wavelength of light and D is the diameter of the telescope. (I have no idea where this formula comes from or how to prove it.) If we assume visible light is about 600 nM and the telescope is 10 m, that gives R=60e-9. The smallest object we could then see would be x=sin(R)*s, where s is the distance between the planets. Let's say they're as far apart as Earth and Mars, around 60e9 km at closest approach. So sin(60e-9)*60e9 = 3600 m, or about 3 1/2 kilometers.

You can distinguish light versus darkness much more easily than resolving an object in general, so I think a bonfire much smaller than this would be visible. Anybody know how to calculate that? I guess it's still at least hundreds of meters. So, big project, but doesn't seem impossible.

Of course just sending a signal and actually working out a comprehensible message are two different things. That's another whole dimension. If aliens could send us a letter with printed text, could we figure it out? It's hard enough to decipher ancient languages, which are of course written by fellow humans. What would it take to decipher a message from aliens?


I think it all boils down to the aliens' sensationary capabilities. Do they both have vision? Maybe they're colorblind? What if only one of them does, and the other has evolved bat-like sonar?
I'd say the question needs some clarification from the OP on that part.
However, as soon as they get technologically developed enough (like 20th century) and they're both on the same techological level, they could use radio-waves for communication. But are they? The OP should also specify that.
In the end, it seems to me extremelly improbable to learn o feach-other's existence, barring space-shuttle visits to each other. Just imagine, humans on one planet and dolphins on the other...


Size limit you could see on Earth

Bluring from the atmosphere limited best angular resolution in about 1 arcsec which is 1/3600 of a degree. This mean that 10 kilometers field could be recognized from distance 2,061,100 km and 1km field from 206,105 km. For example, average distance from the Moon is 384,402 km and L2 point is about 1,500,000 km. Thus, 1km objects could NOT be seen from the Moon.

I suppose that paired planets should have distance between them more than distance from the Moon and less than L2 point.

Astronomy development

In our 17th century people could see mountains on the doubled planet, compute distance from it and estimate how high is this mountains. Although estimations was rough and even wrong, this could lead to early attempts to communicate through flashes and square-of-fields signs. But these attempts should be very naive since people could not estimate how bright flash and how big square should be.

In 19th century people have precise values for distance from the planet and size of its continents/mountains. The clouds restrict ability to observe the same area for a long time. This time correct estimations could lead to success attempts to show signs of squares/circles. Most likely it would be 10km*10km fields covered by cloth of different color. Then colors should be changed after some weeks. This is very expansive though.

In the late 1890 first radio was invented. The first radioastronomy observations was made 40 years later, in early 1930. After that radio is the main factor to find out neighbors.

Gas analysis through it spectrum came in 20th century and after radio invented so this way is not useful.

I doubt that single mediveal field could span 10 km long. So my estimation they could know about each other in 20th century.


You could use light semaphore and telescopes, depending on how close they are together, you could have that they are very close at one point in their rotations.

Obviously this would have been after they had been using more primitive methods to communicate.

  • $\begingroup$ Umm, dunno why this was marked down, this was a method the Romans had used by around 0AD. It was still in use into the 19th century in certain parts of the world. As long as a baseline for communication standard could be reached then communication (as long as it is visible) would be relatively easy. Getting to that point not so much, but starting with simple yes/no and working up from there should work. $\endgroup$
    – Slipoch
    Commented Jun 5, 2017 at 14:23

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