enter image description here ^ Not to scale


  1. These worlds are engineered. So some aspects of the worlds can be manipulated.
  2. Equal Earth-sized planets.
  3. Tether attaching them is infinitely strong.
  4. Their separation is far enough that the Roche Limit doesn't come into effect https://en.wikipedia.org/wiki/Roche_limit
  5. They orbit around a sun-like our own.

What would the forces feel like on the planets? Would it be liveable?

  • $\begingroup$ Welcome to worldbuilding. Please take a good read at our help center to find what we do expect from questions asked here. To summarize: a focused worldbuilding question, answerable in a measurable way. At the moment you are asking more questions, and some are overly broad. Please rework it to fit our standards. $\endgroup$
    – L.Dutch
    Dec 30 '20 at 17:14
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    $\begingroup$ The information that you have provided is insufficient to answer the question you ask. Two equally sized planets can easily orbit around their common centre of mass with or without a tether. The real question is how close are they? Note for similarly sized objects the Roche limit is very small indeed. $\endgroup$
    – Slarty
    Dec 30 '20 at 19:32
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    $\begingroup$ "How would the forces affect <stuff>?": What forces are you thinking of? $\endgroup$
    – AlexP
    Dec 31 '20 at 0:55
  • $\begingroup$ @AlexP Centrifugal and centripetal forces I thought of. How does that work with the massive gravity of a planet? $\endgroup$
    – Whiski
    Dec 31 '20 at 2:57

...you don’t actually need a tether here. These planets are co-orbiting each other, and will naturally stay together indefinitely (barring interference from another planet or star.)

The main effect of this is tidal locking; each planet will always be facing the same side towards the other. This means tides will be affected only by the sun, as there is no moon moving around the planet, and will probably be much weaker. (Someone correct me if I’m wrong.) however, because the planets are tidally locked to each other, and not to the sun, they will both still have a day/night cycle, which will be the same on both planets.

If you’re wondering why tidal locking doesn’t happen on earth, it does; the moon is tidally locked to earth. This is why we never see the dark side of the moon. However, because earth is so much bigger than the moon, earth is not tidally locked to the moon.

Similarly, the earth is not tidally locked to the sun because the two are very far away. These planets are much closer together, so the near sides will be pulled observably harder than the far sides, causing tidal locking.

Also, Since these planets are larger that earth’s moon, eclipses will probably be bigger and more frequent.

  • $\begingroup$ "Much weaker": on Earth, the tidal forces due to the Sun are about one half of those due to the Moon. With no Moon to produce tides, the strength would be reduced to about one third of what we currently have. $\endgroup$
    – AlexP
    Dec 31 '20 at 5:11
  • $\begingroup$ Maybe they would co-orbit naturally... but what if the tether was placed then shortened. You might eventually get to the point where the planets begin to tear themselves apart. $\endgroup$
    – Monty Wild
    Dec 31 '20 at 7:08
  • $\begingroup$ @MontyWild OP already stated the planets are outside of each other’s Roche limit. This tether isn’t doing anything that gravity won’t do for free. $\endgroup$
    – Globin347
    Dec 31 '20 at 13:07
  • $\begingroup$ @Globin347 the tether is important for the story for my context. It's good to note without it, that nothing would change about their relative position to each other. If they rotated around their midpoint, would this create normalish day/night cycle? $\endgroup$
    – Whiski
    Dec 31 '20 at 19:40
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    $\begingroup$ Distance and mass simply change how long it takes for tidal locking. Given enough time all secondaries lock to their primaries and all primaries lock to their secondaries. Earth will eventually lock to the white dwarf the sun will leave behind (if it survives the red giant phase.) $\endgroup$ Jan 2 at 1:37

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