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Say you've got a hypothetical human-colonized planet planet roughly the size and composition of Earth orbiting around an M-class star a third the size of the sun at 0.2 AUs. Due to the proximity, this planet is tidally-locked. There's a ~518 mile wide terminator of moderate, human-habitable temperature and the other ~96% of the planet is either boiling ocean/desert or tundra/ice. A tremendous storm at the substellar point blows precipitation and heat to the rest of the planet. The population of this planet is ~1 billion, the vast majority living in the terminator.

And then some astronomic-sized magic event such as a comet (sure let's go with that) knocks its rotation speed out of the normal (assume it doesn't mess with the orbit somehow, only the rotation) and the planet starts rotating out of sync with its orbit--the tidal-lock is broken. One study says this may have happened with Mercury, although I'm interested in the effects on a planet with a human and animal population: http://www.space.com/13889-mercury-spin-asteroid-collision-tidal-locking.html

(Magic and iffy science acceptable, this is for a story where magic is a thing.)

Edit: Took out the comet thing which was just intended to be an example.

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  • $\begingroup$ I think Olaf Stapledon did something similar to this in Star Maker, though it was a bit footnote-y and cursory and took up maybe like a paragraph. Basically things froze over very quickly and everything died. $\endgroup$ – drunkBrain Jul 13 '15 at 20:26
  • $\begingroup$ Yeah, a bit of further thought and I realized that it would be pretty dang cataclysmic. Climate effects aside, the flora and fauna would have evolved for the dark/twilight/light sections of the planet and would likely all die off pretty quickly at the very least. $\endgroup$ – Respheal Jul 13 '15 at 20:31
  • $\begingroup$ 1. What is the new length of the day? Rather hours or months? 2. What is the luminosity of the sun? Wikipedia gives possible values between 0.015% and 7.2%. (4% with distance 0.2 AU would give the same apparent magnitude as our Sun from Earth) $\endgroup$ – BartekChom Jul 13 '15 at 21:27
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    $\begingroup$ How could a comet change a planet out of tidal-locked rotation? Sounds like "made for SyFy movie" writing. $\endgroup$ – JDługosz Jul 13 '15 at 21:37
  • $\begingroup$ There is a LOT of energy in the rotation of planets. No wimpy comet is going to do much to influence the rotation of the planet. I doubt Mercury's impactor knocked it into a synchronous rotation period but if it did, realize that impact was so cataclysmic that it left mountain ranges on the opposite side of the planet from the impact. An event of that magnitude is going to wipe out the life on the planet so there's no need to worry about what the change in rotation would do it. $\endgroup$ – Jim2B Jul 14 '15 at 1:51
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Given that the inertia of a planet is pretty darn huge, the sheer amount of energy needed to break out of synchronous rotation would have pretty cataclysmic effects on the geology, geography, ecology and every other aspect of the planet. While you might not end up with a glowing ball of lava, you certainly will be experiencing violent earthquakes, volcanic eruptions, extreme dislocations of the hydraulic and atmospheric cycles and so on. It would be really unpleasant to live there, to say the least.

Since you didn't specify how fast the planet will be rotating afterwards, I will assume that it will be rotating rather slowly (Only a "modest" amount of energy would be needed for this, maybe a passing neutron star or something). Since the gravitational forces operating on the planet from the sun remain constant, and active over the entire lifespan of the planet and star, it seems reasonable to assume that the system will settle down to some other periodic cycle, perhaps like Mercury with it's strangely resonant rotation/orbital period. The tidal forces that locked the planet in synchronous rotation in the first place will still be in operation.

As for any life on the planet, anything which evolved for a sedentary lifestyle based on its location relative to the "hot pole" will most likely become extinct, and most of the larger life forms which grazed on any vegetation that filled that ecological niche, and all the predators, parasites, symbiots etc. that partake of that part of the ecological web will also become extinct as well. Your planet will most likely be reduced to the equivalent of lichen and moss for aeons while the hydraulic and atmospheric cycles are reestablished in a new equilibrium and the ecology can stabilize.

As an aside, given the planet's close proximity to the star, the release of enough energy to break the synchronous rotation of the planet will most likely be distributed to the star as well, so in addition to everything else, the star might have much higher activity and sterilize the planet with violent flares, rendering a lot of what was written above moot.

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  • $\begingroup$ Yeah, it would be slowly. The goal of getting the world out of its normal rotation is to dispel the storm at the substellar point, so it doesn't have to be rotating all that fast. Life as they know it on that planet would definitely get wiped out almost entirely. I originally was thinking of making this the protagonist's goal, but now that I know the side effect I'm thinking this might be better left to the antagonist...Thanks for the help! $\endgroup$ – Respheal Jul 14 '15 at 12:08
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The first thing that happens is that life on the planet is wiped out.

The planet you describe has a rotational period of roughly 4,800,000 seconds, and a rotational kinetic energy of 1.4*10^26 joules. Changing that by even 1% requires three times as much energy as was released by the Chicxulub impact -- and given the inefficiency of energy transfer from an impact, your proposed comet will need to be at least an order of magnitude larger than that.

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    $\begingroup$ Don't forget the need to import angular momentum, not just kenetic energy. What's the orbital angular momentum of a comet? $\endgroup$ – JDługosz Jul 14 '15 at 6:28
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Try having an inner asteroid belt made up of small asteroids graze your planet. Most impacts will be on the unpopulated starward face at an angle that will induce retrograde rotation from their higher orbital velocity. If that's not enough, put in an outer belt to add more angular momentum. Perhaps the asteroids were perturbed out of their usual orbits by a rogue planet. Or someone redirecting them. Or magic.

Also postulate that your planet is very, very nearly symmetrical so that the locking forces are small, or you could get rocking.

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  • $\begingroup$ OP does not ask how to break it, he asks what would happen after. $\endgroup$ – Mołot May 17 '17 at 19:01

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