I've been interested in binary systems lately, and I was recently wondering what Earth would be like if it was in a binary orbit with another Earth-like planet.

Both planets exhibit the following characteristics.

  • Approximately equal size. (~12,000km diameter)
  • Same atmosphere composition, nitrogen (~78%), oxygen (~21%), and other various gases.
  • Population of sentient beings with ~21st century technology, and the spaceflight technologies of today. (Manned craft in Earth orbit, and probes across the solar system.)
  • No moon(s).

I have these two questions:

  • How would these beings communicate?
  • What would spaceflight be like?
  • 4
    $\begingroup$ You need to post one of those questions at a time; it's too broad as it is. $\endgroup$
    – Samuel
    Jul 29, 2015 at 4:59
  • $\begingroup$ As Samuel had pointed out you may like to revise your question based on this article! $\endgroup$
    – user6760
    Jul 29, 2015 at 6:06
  • $\begingroup$ I like your questions. As already pointed out, you should split them up, though. $\endgroup$
    – Burki
    Jul 29, 2015 at 6:50
  • 1
    $\begingroup$ Some of the sub-questions will still be too broad to answer except with random imagination unless the scenario is much more defined. You didn't even say how far apart the planets are, or what the history and nature of the people are. There are millions of possible answers to what people might do and how in any particular situation, for any given combination of people types (of which there are also millions). $\endgroup$
    – Dronz
    Jul 29, 2015 at 16:52

3 Answers 3


With both worlds having an oxygen atmosphere, life would have to exist on both. This would imply that the two worlds were fairly close together, enough so that meteor impact ejecta could carry microorganisms from one to the other.

It is unlikely that there would be naturally-occurring sentient, tool-using species on both at the same time, so any population of tool-using sentients would effectively have the option of colonising the sister world.

The presence of a neighbouring world would have encouraged the production of telescopes, and when life could be seen on the other world, it would have encouraged the chemical and physical sciences to the point where a trip from one world to the other could be contemplated and executed.

We could well have had DaVinci's Rocket Mission to Counter-Earth in the late fifteenth to early sixteenth century. It would have been a one-way trip for the explorers, who might have been as numerous as ten or so, but with the prospect of long-term survival and colonisation, the missions would have continued, carrying both men and women, and establishing a permanent human presence on the Counter-Earth.

Communication could be conducted initially by heliograph and a code similar to morse, as well as by laying out large letters in open spaces to be viewed by telescope, but with the invention of radio, this would have enabled two-way communication between the worlds.

It would probably have become possible for return trips to Earth in the seventeenth to eighteenth centuries, and we would probably be using our computers and writing Stackexchange answers on our PCs right now in the nineteenth century.

There would be regular trips between the worlds, though they would be very expensive and only used by VIPs and the wealthy. The likelihood is that the launch vehicles would involve eithersome sort of atomic rocket, or a ground-based power source, such as a laser launch system. A space elevator would probably already be in planning.

There would be no wars between the worlds save wars of words - and in our modern times, electronic wars conducted over the internet - since the cost of conducting physical warfare across the gap between gravity wells would be prohibitively expensive, out of proportion to any possible gain.


Well the interplanetary program would be much like our space race to the moon, except that the incentive to get there would be WAY larger. The first civilisation to reach the other planet could colonise the entire planet and then use their extra resources to keep everyone else OFF THEIR PLANET!

Was the other planet already populated? Even better, these beings would make excellent slaves, reducing the need for mass migration. Unless they can defend themselves, in which case diplomacy would become necessary. Alliances could be made with nations of your own planet to help dominate the other planet, or more likely, fickle alliances could be made with the natives of the opposing planet so that you can defend each other while you build up your colony defences.

For inspiration, you might look to the colonisation of the Americas, and what happened there, such as plagues, wars, colonies, alliances, genocides, etc.


What is the likelihood that both reach 21st century technology at the same time, without interference of some sort? That is beyond the scope of your question, but it really needs to be answered.

  • Assume that instead of the moon, you have an earthlike planet in the sky. Perhaps it appears slightly larger than Luna, but the same order of magnitude. It would be possible to see continents with the naked eye. (As a side effect, people would understand more clearly that they live on a globe.)
  • With the first powerful telescopes, man-made structures would become visible. At first there might be some arguments if they're really man-made or natural, but I'm sure that reason would prevail.
  • I don't share Monty's confidence that space travel would come early; getting out of the gravity well is hard. Consider how many boosters/stages a rocket or space shuttle needs to get into space. At first, all travel would be one-way. Planet A sends people into orbit and they land on planet B, but the return vehicle would have to be built and fueled on B. Either by people from B, or by a large expedition. That's where the colonisation mentioned by Lorry comes in.
  • Would the biospheres be compatible? Perhaps panspermia applies.

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