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Before picking specs, I need to ensure feasibility. Only one of the planets need be Earth-like; in fact, I would prefer if the other planet were rather small and dead, like a moon. The orbits will be eccentric enough to produce tectonic strain. Obviously they'll be tidally locked.

What would the tidal bulge on the home world be like? Would the planet take the shape of an egg? Is it reasonable to suppose that an immense volcanic plateau would form in the shadow of the sister world? Might there be major geological uplift?

I'm trying to create this plateau landform within scientific parameters. The only other necessary condition I can think of is the length of day-- it must approximate the 24-hour cycle (though with some room for variability, as a relatively long day/night cycle may also be an interesting feature of this world).

I basically want a normal world that gets weirder and more alien as you near the spot directly below the sister world. I'm thinking that plateau and/or surrounding environs will resemble Iceland's landscape. I imagine leading up to that you'd find a Patagonia-like desert strewn with what seems like a geographically far-flung "staircase" of strike-slip faults, escarpments, and terraces.

Is this scenario plausible, and, if so, under what conditions?

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    $\begingroup$ Why would they be tidally locked? $\endgroup$
    – Ash
    Commented Sep 14, 2018 at 19:11
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    $\begingroup$ Textually speaking? For the purposes of creating the specific kind of tidal bulge/plateau I described. Scientifically speaking? Because it's a binary planet system, and unless one of the planets was only recently captured in an orbit around the other, it's particularly likely that they'd be tidally locked to each other by now. $\endgroup$
    – Kyle Gravito
    Commented Sep 14, 2018 at 19:38
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    $\begingroup$ Have a look at spin-orbit resonance, tidal lock is not a physical necessity. $\endgroup$
    – Ash
    Commented Sep 14, 2018 at 20:03
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    $\begingroup$ It depends a lot on what youre going for. Things like the amount of deformation depend greatly on the composition of the various planets, their relative masses, atmospheric compositions, oceanic activity, etc. If you want scientifically rigorous answers, youll need more specific parameters. For the sake of the story, I dont see why it wouldnt be plausible. Deformation, gravitational heating, and climate zones are all well known enough to convince a reader of its plausibility. I say go for it :) $\endgroup$
    – Stephan
    Commented Sep 14, 2018 at 20:49
  • $\begingroup$ That's pretty much exactly what I wanted to know-- the approximate scientificity of the scenario. Thanks. $\endgroup$
    – Kyle Gravito
    Commented Sep 14, 2018 at 23:20

2 Answers 2

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Everything would more or less be the same as on earth in each planet, even the plateau.

Everything you'd want to know is in this video: "Double Planets and Rocheworlds" https://youtu.be/lgkqbHJczWs by Isaac Arthur. It even links to an orbital period calculator.

Per the video, the other planet would simply appear 'stationary' in space, while you still have your normal day/night cycles. I would imagine that you would have frequent eclipses. However, the video mentioned that the other planet would reflect sunlight and would be many times brighter than the moon! The animations in the video also illustrate how all parts of each planet would have regular day/night cycles.

Some exceptions:

  • If you bring the two planets close enough you can have a "bulge" in both planets. These are called Rocheworlds.
  • However, the closer you bring the planets together, the quicker the days must be.
  • Also, the bulges are not stable in geological time scales (millions of years I would imagine). They would eventually be drawn to each other and the planets would eventually merge into one (rather destructively, as the narrator mentions).
  • The bulges could probably lead to the oceans covering a good part of the planets with continents potentially arising on the opposite ends of the bulges.

The only other primary concern are tidal forces. But these can be minimized with mostly-circular orbits. If you need 24 hour periods then the planets must be far enough away to not have Roche bulges and would otherwise evolve like regular earth-like planets. However, the "dead" planet could have a mars-like or venus-like atmosphere that would explain its dead-like state. The 'weirdness' in the region might be more along the lines of wildlife that has adapted to the unique scenario of having frequent eclipses. Indigenous people would probably worship the dead planet as some kind of deity.

From the video description:

Orbital Period Calculator:

http://www.calctool.org/CALC/phys/astronomy/planet_orbit

Note: make sure to set units of both bodies to Earth Mass and period to hours or days if trying to look at double planet cases.

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[Assuming I read your question correctly, Planet A has a spot which always faces Planet B; B still rotates relative to A]

Planet A would certainly have a massive Mountain range or plateau on that spot, as well as having no tides.

The plateau would also be massively cooler since it's the spot that only gets light if the sun is in an angle and therefore always misses "noon"-sun. I can't say whether there would be Iceland-style volcanoes and seysers, since on one hand it's so far above the rest of the world, while on the other ALL tectonic plates strife there. [Also,if this constellation is older, the side facing away from B would have noticeably fewer land, if even any, due to all tectonic plates moving away]

B in contrast would be a nightmare to live on. Volcanoes everywhere, 24/7 earthquakes, and tides in the range of kilometers high (speculative), as well as horrible storms because of that.

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