# What would be the effects of an existing planet becoming tidally locked?

Let's assume this planet has had a normal lifecycle up to this point, becoming an Earth-like world with around the same climates, size, landmass, day/night cycle, and has a single moon of similar size.

I'm trying to figure out the effects of becoming tidally locked with a day side and night side.

Due to the interference of a god-like entity, the force and energy of the event that would've affected life and topography were transferred to to another plane; therefore, major damage to all life on this planet was averted.

EDIT: Sorry I wasn't clear on that, I had already figured that portion out, that's why I had said, "discounting the energy and cataclysm of the event."

This story is a mix of science fiction and fantasy.

• How rapidly are you assuming the planet's rotation stops? Over the geological time periods it normally takes, life will adapt, but it's hard to predict how. If you want it to happen suddenly due to some external event, such as a large collision, then while you're wearing a "science-based" tag, the answer is that the nature of the external event is of primary importance. The Earth has a lot of rotational kinetic energy, and many things that would stop it spinning would also melt the crust, ending life quite comprehensively. – John Dallman Jul 25 '16 at 18:24
• "Discounting the energy and cataclysm of the disaster that would cause this event" Unfortunately, that's a bit like saying "discounting the absence of life, and that the planet just turned into a bomb, how long does it take to boil an egg?". In fact, this reminds me of an old highschool math/physics assignment: Given this impact angle and velocity of a meteor at one end of a bridge, what force must be applied to the other end for the bridge to remain intact? (I might remember that slightly wrong, but that was the gist.) It's ignoring the giant white elephant unicorn in the china store. – a CVn Jul 25 '16 at 18:57
• Ooookay, so going to edit my question. You bring up valid points. – Crimzonkat Jul 26 '16 at 18:51

I consider the primary effect to be the incredibly limited habitable zone on the planet. This is now a thin strip where permanent dusk or dawn exists. To either side is a frozen wasteland and a desert wasteland.

Unless of course the inhabitants have done something about it. Given Earth, if this were to happen slowly, we could potentially create light-directing and diffusion structures. Combined with the right greenhouse gases, we could potentially normalize the temperature on the planet to some degree. This would have to occur over very long timescales.

Remedy or no, one side will be eternally night and the other eternally day. You'll have a number of strong weather phenomenon for sure in the beginning. Once wind starts being driven by a single source and direction of convection, large weather system will likely dominate. There's a possibility of heavy cyclonic activity in the habitable zone due to pressure and temperature contrast.

Subterranean water, oils and gasses will explode outward into the atmosphere as the desert side gets progressively baked. Meanwhile the night side's oceans will freeze, the atmosphere will get thinner as more than usual condensation falls from the sky and any rivers or oceans will follow the same fate. Inside the planet there will be a region that was previously too hot now entering a habitable scale of temperature, but its likely covered by tons of ice or rock.

Depending on the gasses spewing out of geysers on the day side, you might get a runaway greenhouse effect or maybe not.

Worst of all, if the planet becomes tidally locked its magnetosphere is very probably going to be screwed. Without a spinning iron/heavy metal core to generated this shell of protection, the atmosphere is going to be more easily blown away and cosmic ray radiation will start pouring in.

• Valid points, sorry, I wasn't clear, therefore I edited the question. – Crimzonkat Jul 26 '16 at 19:03
• @Crimzonkat no prob, i'll leave the second paragraph but you could imagine that, in a situation where this occurred suddenly, the population might begin to create light diffusion structures to salvage more of the planet and extend the habitable zone. of course they'll first have to start migrating as much as possible in that direction and saving as much tech as they can. – Garet Claborn Jul 27 '16 at 6:25

The time taken for a planet to become tidally locked is very sensitive to the orbital distance and size of the satellite.

$$t_{\text{lock}} \approx \frac{\omega a^6 I Q}{3 G m_p^2 k_2 R^5}$$

What this means is that either a satellite will become locked in a relatively short time (if the orbital distance, a, is small) or remain unlocked for a period of time greater than the life span of their star. The situation in which tidal locking occurs late in the life of a planet is rare and exceptional.

If, in the exceptional case in which tidal locking occurs after 5 billion years, the planet has already experienced very slow rotation for the las billion years or so. The time it takes a single species to evolve is much quicker than the time it takes for the rotation to stop.

So what I'm saying is that the there is no great difference between a rotation rate of 1 every million years, and true tidal locking. Whatever problems the changes happen too slowly for them to affect anyone.

• Thanks for your response! I wasn't clear on my question so I edited it. – Crimzonkat Jul 26 '16 at 19:04