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An interesting question was raised about satellites being tide-locked (Does a satellite naturally turn in phase with its orbit, always facing Earth?).

It was a educational read for me, and in one part there is a comment that like our moon, there are other moons that are tide-locked. I began to wonder if in theory larger planets could also be tide-locked and if in practice we actually of know some specific examples of some?

My thoughts are that such a planet would be less hospitable as there would be a warm and cold side, unlike a rotating planet which would have more graduations in the temperatures.

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    $\begingroup$ Such a planet would not only have a warm and cold side but also very strong and continous winds flowing between both sides. $\endgroup$
    – Uwe
    Commented Mar 2, 2018 at 9:50
  • $\begingroup$ When I was in grade-school, Mercury used to be tide-locked to the Sun, with both its days and years being 88 earth-days long. And Pluto used to be planet. And continental drift was pseudo science, while brontosauruses were real. I wonder how much of what we know now will be obviously wrong in 50 years. $\endgroup$
    – Ray Butterworth
    Commented Apr 15, 2019 at 0:42

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Yes, any planet within a sufficient distance from the parent sun is assumed to be tidally locked. Proxima Centauri B, the nearest exo-planet to the Earth, is believed to be tidally locked. Pluto and its moon Charon, are believed to be tidally locked to each other.

Your thoughts are exactly correct - life would be tough and probably confined to the, ahem, twilight zones of the planet. Here's a decent paper on the subject.

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    $\begingroup$ So based on that paper it seems a tide locked planet normally is close to its sun, so that means the warm side will be hot indeed and quite inhospitable. How close would a planet have to be to our sun? $\endgroup$
    – miltonb
    Commented Mar 2, 2018 at 0:58
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    $\begingroup$ I don't know the exact number, but somewhere between Mercury and Venus. Mercury is locked, Venus is not. $\endgroup$
    – Chris B. Behrens
    Commented Mar 2, 2018 at 1:39
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    $\begingroup$ @ChrisB.Behrens Mercury's 3:2 spin-orbit resonance is not the same kind of "Lock" as the 1:1 resonance that the OP is talking about, having linked to Does a satellite naturally turn in phase with its orbit, always facing Earth? in the question. Mercury does not have a permanent "warm side". $\endgroup$
    – uhoh
    Commented Mar 2, 2018 at 5:45
  • $\begingroup$ @miltonb see comment above; there are types of locking besides the 1:1 situation you are asking about. $\endgroup$
    – uhoh
    Commented Mar 2, 2018 at 5:47
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    $\begingroup$ Different types of lock is interesting, not at all what I expected to learn about from the question. @uhoh do you have any reference explaining the different types of lock? $\endgroup$
    – miltonb
    Commented Mar 2, 2018 at 7:31
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Mercury is tidally locked to our sun, although in a 3:2 resonance, not 1:1 as you're probably imagining. There is therefore one day on Mercury every 2 years (3:2 from an exterior frame of reference; appears 2:1 from on Mercury). Kim Stanley Robinson wrote about a city built on train tracks on Mercury, that slowly traveled around so as never to face the sun, and if you walked fast enough, you could actually walk to stay ahead of the terminator.

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  • $\begingroup$ In effect, the planet rotates under the city. How does the city handle the inevitable changes in terrain it would encounter? $\endgroup$
    – Oscar Lanzi
    Commented Apr 14, 2019 at 22:29
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    $\begingroup$ @OscarLanzi Maybe so? $\endgroup$
    – peterh
    Commented Apr 14, 2019 at 23:59

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