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Contextualizing, I'm writing a book where the planet is tidally locked with their star and i want to have a reason for that. Whatever cause the planet to stop rotating have to also cause a mass extinction, not fully, but enough to send the humanity in a pre-civilization stage. I thought in a large meteor impact in the opposite direction of the planetary rotation, but looking more careful in my idea, I figured out that the impact doesn't transfer the angular momentum into the planet, transforming the kinetic energy of the impact in an explosion that distributes the energy equally in a spherical shock wave. For an impact to cause drastic changes in the rotation of a celestial body, the source has to be large enough to wipe out the life in the planet, and that isn't my goal.

Characteristics:

  1. The planet has a ring around it but no moon. (The ring could be the remains of a dead moon, that in some point, falls towards the planet, but the problem above persists).

  2. The planet is smaller than Earth.

  3. The planet is in the outer border of the habitable zone.

Somebody have and idea that could help me?

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    $\begingroup$ Venus actually spins backwards. The reason could help for your situation. $\endgroup$
    – Gillgamesh
    Jan 17, 2023 at 15:29
  • $\begingroup$ The meteor solution is worse than it sounds. Any impact great enough to rob (e.g.) Earth of its spin would also rip the planet apart. Tidal locking is usually a condition that comes about quite slowly (millions if not billions of years... assuming it wasn't the nature of the world upon formation). This might need the infamous Cosmic String to solve the problem. Throw away science and simply state that a very long, gravitationally dense cosmic string was pulled through the planet, robbing it of its momentum. It would cause merry havoc along the way. $\endgroup$
    – JBH
    Jan 17, 2023 at 16:45

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The explanation of "tidally locked" is in the name. Tides used up the energy of rotation. It usually also requires some help from collisions to start things off with a relatively low rotation rate, since it's a fairly slow process. The Earth's day gets longer by about 0.0016 seconds each century due to tidal effects.

So a collision could use up part of the rotational energy, leaving a planet with a very slow rotation. However, that's a real challenge to not being the total kill. Collisions happen at planetary orbit speeds, and a bit above. So typically in the range of 10 km/s up to a few 10's of km/s. At this speed, you get a lot of very hot stuff. Typically you get the colliding bodies mostly pulverized, then being largely molten. And you also get huge amounts of collision debris that falls back to the colliding bodies, producing additional chances for total kill. It is extremely unlikely for anything to live through that.

So generally, having the event that produced the tidal locking happening in any sense recently (from the standpoint of life evolving) is a difficult challenge. It takes a very long time for a planetary object to cool from molten-magma type temperatures to the range where liquid water can exist.

So the scenario might go like so. A collision used up most of the rotation about one billion years ago. This could have produced the ring. (Though you should check with somebody with astronomy knowledge if a ring can last that long.) Then a few tens of millions of years to cool a crust. The length of this time depends on how big the planet is, the bigger the longer. Then there could still be enough time for life to evolve. If these time scales work then that could be the explanation.

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Well well well, this is basically the inverse question to "Yes but can we make Venus spin ?"

The answer of course being, yes.

To add some context, the Angular Momentum of any planet is not actually that much. For example, if you wanted to spin up Venus to a 24 hr day night cycle, that would requiere about $1.88e29$ Joule of Energy. Now that sounds like a lot, because it is. But it also really isnt. Its the kind of Energy you can reasonable produce in a given period of time. For Venus, if you install 1400 Mass Drivers firing 100 Ton projectiles every 84 Minutes (So the Mass Drivers fire continously but each individual one only every 84 min), it would only take 300 Years to spin the planet fully up.

Now, the reason it would still take so long is because Planets are not solid. You need to apply a constant force over a long time, otherwise the Planet will absorb the energy. Hence why Tidallocking usually takes millions of years to happen. The Tidal Forces are not large to beginn with and they are not as constant as you would like. As the Orbital distance changes.

This is also why you cant just snap your fingers and make a planet spin or slow down. Even though the Rotational Energy of a planet (All things considered) is not super big, the Inertia is absolutly massiv. And if you dont apply this force evenly everywhere, the planet will turn into lava. It wont blow apart or anything, but the Energy will vaporise everything on the Surface due to friction.

So, for your planet there are 2 ways to get Tidal Locking which dont evolve Magic or Technology.

Either, the Planet never rotated that fast to beginn with and just came to a halt when a Civilisation arose. This might be your best option because if the Planet becomes tidally locked it will lose its Magnetosphere. Same the other way around btw, if we Spin up Venus she will regain her Magnetosphere as we are literally restarting geological processes. Meaning, your mass extinction could be in part duo to the planet losing its Magnetosphere. Which in combination with the heat, shutdown of major Geological activity etc might very well be enough for a Mass Extinction.

The other option is significantly more violent. In theory, your very unlucky planet could interact with another major planetary body. A body which cancels out the Angular Momentum over a given period of time. Needless to say, this is very unlikley but it gives you a bit more freedome in the timeline. But it would also probably kill every lifeform on the planet.

For the most part, these are your two Natrual options. Again Magic can do anything and there are technological ways that can slow the planet down significantly faster. But it really depends what your precise goal is. if you want a story where you explore the impacts of a planet suddenly stopping, well then Magic is your only real option. If you on the other hand want to explore how Evolution and slow processes adapt to this new Enviorment, you can just say the planet became locked from the Star.

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