I am attempting to create a pair of planets, both capable of harboring life. Which happens to also be quite colossal in design. We are talking about 10.13x the size of Earth.

So the system I am attempting to design would have two planets and three satellites.

Each planet would orbit a singular barycenter, spinning about parallel to one another, while simultaneously being tidal locked to one another so that, in hopes, would mean they have a spot along their respective equators that allow someone to look up and just see the other planet in the sky directly above them.

The question at this point is: does this allow each of these spots to have a day/ecliptic-night/day/night cycle. In which the duration would be something along the lines, for simplicity, 15 hours of night, 4 to 4.5 hours of day, 6-7 hours of said ecliptic night, 4 to 4.5 hours of day again before finally repeating into the night.

Now, each planet would have its own moon, roughly in size to the equivalent the moon is to earth. These two planets would be far enough apart that these moons would never risk smashing into one another while orbiting their respective worlds(?).

The third moon/satellite, being far larger than the other two, would in hopes orbit the barycenter itself, at a distance that allows both planets to see the "moon" during certain parts of their year(?)

The barycenter itself would orbit the system's sun, and I would hope that the fact each planet is revolving around said barycenter along a set tilt, which is revolving around the sun, would allow for seasons to occur in some manner or another(?)

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    $\begingroup$ Hello @Bangerton, welcome to Worldbuilding. For future reference, please note that asking more than one question - no matter how related - is an actual reason to close a question. You are allowed only one question per post. I added the orbital-mechanics tag because that appears to be your primary question. Also, can you clarify if the third "moon" is a moon orbiting one planet, both planets, or a planetoid orbiting the barycenter like the other two planets? $\endgroup$
    – JBH
    Mar 5, 2023 at 18:59
  • $\begingroup$ Yes, the third moon would orbit the barycenter itself, simply circling the other two planets. $\endgroup$
    – Bangerton
    Mar 5, 2023 at 19:10
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    $\begingroup$ OK, so it's not really a moon, it's a planetoid in a three-body planetary grouping orbiting a sun. It's a small distinction, but important for working through the analysis. Thanks. $\endgroup$
    – JBH
    Mar 5, 2023 at 19:39
  • $\begingroup$ Just to add to what JBH said, you're more than welcome to ask other follow-up queries about this system! When you do, please be sure to crosslink so that respondents can look back at your first question and future worldbuilders can catch a glimpse at this absolutely delightful planetary system in all its glory! $\endgroup$
    – elemtilas
    Mar 6, 2023 at 4:50
  • $\begingroup$ You may find this video informative: youtu.be/lgkqbHJczWs (I referenced it in a previous answer: worldbuilding.stackexchange.com/a/128097/56286 ) $\endgroup$
    – stux
    Mar 22, 2023 at 20:37

1 Answer 1


Very very unlikely.

If the planets can have their own moons in stable orbits, the planets need to be quite far apart from each other. If the planets were too close to each other, the other planet would disturb the moon's orbit and the moon would either get ejected out of the system or it smashes into either planet.

Now, if the planets are of similar size and they are this far apart from each other, then the eclipses cannot create ecliptic nights that last a significant portion of the day, because the shadow will only cover a portion of the other planet even when the planets are fully aligned. The sun is so much bigger that a significant a portion of it will always remain visible somewhere on the planet behind the other.

The only way to have a true ecliptic night on one planet is if the other planet is much bigger than the other. Then the bigger one is able to cast a shadow that takes hours to pass through for the smaller one.


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