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The title really says it all.

I know that there are planets like Earth that rotate around their axis and also around their star. I know that there are tidally locked planets which always have the same side turned towards their star. And, of course, there are rogue planets that don't orbit a star at all.

But what about a scenario where a planet is in a stable orbit around a star, yet does not revolve around its own axis? From the point of view of someone on the planet, all the stars stay almost unmoving in the sky, but the sun seems to slowly orbit the planet. Half a year there is light, and half a year there is darkness.

Is this realistic? Or would such a planet start rotating around its axis anyway due to whatever forces?

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    $\begingroup$ Do you consider very slowly rotating planets to be not rotating? $\endgroup$
    – Daddy Kropotkin
    Nov 7, 2021 at 18:52
  • $\begingroup$ I feel this would be highly unlikely due to conservation of angular momentum during the formation of planets ... or during collisions of said planets with other bodies $\endgroup$
    – Tosic
    Nov 7, 2021 at 19:33
  • $\begingroup$ Do you consider tidally locked planets to not be rotating, and if you also don't consider slowly rotation planets to be not rotating, I suspect what you are asking about is a set of measure zero. $\endgroup$
    – David Hammen
    Nov 7, 2021 at 19:54
  • $\begingroup$ Even if a planet is tidally locked to its star, it does rotate—just in the same period it orbits its star. Any and all movement in the universe is with respect to something else. If you don’t move relatively to me, well, it can’t be said you’re motionless; we’re both moving around the Sun… And the list goes on… So NO planet “does not rotate”; it’s all a matter of relatively to what? $\endgroup$
    – Pierre Paquette
    Nov 8, 2021 at 3:30

1 Answer 1

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There is nothing specifically preventing it, but there is no mechanism that can create or maintain this situation.

Tidal locking occurs as a transfer of energy between rotation and orbit. This transfer of energy stops when the rotation rate of the planet equals its orbital time. So planets will tend to evolve towards this state of tidal locking.

Now the formation of planets in protoplanetary discs will naturally give them some angular momentum. They will tend to spin the same way. It is possible events (such as collisions) will occur that will change the rotation, even reverse it. It is, I suppose, possible that events will conspire to give a planet very close to zero rotation. Unlike tidal locking, having no rotation is not self-sustaining, so tidal effects will tend to cause a planet having no rotation to evolve towards tidal lock.

I suppose further that a planet that happened to have a slow backwards rotation could, as a result of tidal locking, have it's rotation slowed and eventually reversed. At some point it would not have been rotating at all. And since these changes happen slowly, there might be a period during which the planet was not rotating for much longer than the lifespan of any creatures on it.

There are so many ifs here, which is why although there is nothing to prevent this happening, it would be almost infinitely rare.

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