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I'm currently reading a novel where the story happens on a planet orbiting Proxima Centaury. As the planet is very close to the star, which is a red dwarf, it is tidally locked to the star like our Moon is to Earth. This means that one revolution around its axis is the same length as orbit around the star, making one side of the planet permanently face the star and the other side - permanently turned away from the star.

I got curious: is it possible for a planet to not revolve around its axis at all? How would day/year relations work? If I understand this correctly, the diurnal cycle would be the same length as a year, and both day and night would be half-year long, right?

Would such a planet be habitable?

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    $\begingroup$ if it doesn't revolve, it doesn't have an axis - technically speaking. $\endgroup$
    – Franz Gleichmann
    Aug 8, 2022 at 9:41
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    $\begingroup$ Venus is almost a real-life example in our own solar system. It does spin, but very slowly and backwards. Whatever happened to bring it into this state, it could have worked equally well to stop its rotation altogether. $\endgroup$
    – AlexP
    Aug 8, 2022 at 12:02
  • $\begingroup$ @AlexP A natural event that makes a day exactly equal to a year is highly improbable. And it’s not really necessary either. As long as you’re not tidally locked, you have a day/night cycle, and anything even resembling a year will have the same overall effect. $\endgroup$
    – StephenS
    Aug 9, 2022 at 14:33

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It's possible but unstable. The planet will extract angular momentum through tidal forces, eventually beginning libration and tidal-locking. (This is due to the tidal force "dragging" across the surface of the planet as it revolves around the star, urging the planet to spin in the same direction as its orbit.)

There's no real compelling reason why it couldn't be habitable, apart from the usual arguments for/against tidally-locked worlds. In its pre-locked condition it basically amounts to a slowly-rotating (rotisserie?) world, and the potential for such planets has been explored. You can find a nice read on that here.

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  • $\begingroup$ Your link leads to an image. $\endgroup$
    – Krišjānis Liepiņš
    Aug 10, 2022 at 15:41
  • $\begingroup$ @KrišjānisLiepiņš Not for me, it doesn't. Did the page finish loading for you? If you're still having trouble, try searching the name of the article, "Rotation of planets influences habitability". $\endgroup$
    – BMF
    Aug 10, 2022 at 17:29
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    $\begingroup$ It's because the link had the .amp extension (something called Accelerated Mobile Pages), and went via google rather than directly - it's auto generated mobile-frendly content, but apparently, it doesn't work when you are accessing the link from a desktop browser. $\endgroup$
    – Filip Milovanović
    Aug 12, 2022 at 8:24
  • $\begingroup$ @FilipMilovanović Ah, I didn't know that! Makes sense, I visit this site primarily from my phone. Thanks for catching that $\endgroup$
    – BMF
    Aug 12, 2022 at 17:09
  • $\begingroup$ I don't know if anything has been changed, but now the link opens the article for me too. $\endgroup$
    – Krišjānis Liepiņš
    Aug 26, 2022 at 9:45

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