In my world, days last 9 years. The most realistic solution is for life to have evolved before the days were 9 years. My original idea was to have a passing body slow the day down, but the reality is that that is unrealistic. Every other solution suggested took multiple generations to complete, rendering the idea of an extinction level event pointless.
What (at most 10 year) event could both cause a mass extinction and increase the day length from 24 hours to 9 (Earth) years.
This is going to be a very unpleasant event, its chances of successful occurrence anywhere in any galaxy are less than 0.0001% (theoretically possible, though) and almost all times, if this scenario occurs, it would destroy the planet completely, or cause such major changes to it that it will no longer be able to host life at all. In fact any event which so drastically alters the rotation speed of a planet in such little time would have the same, or worse consequences for the planet.
You have been informed.
They are massive and they are shot out of a galaxy's core at unimaginable speeds. Enough said. If such a rogue star happens to pass near the target planet, it can create unprecedented changes in the orbital and axial spin speeds of the planet.
Most of the time, the target planet would be sterilized by the horrible high energy radiation by the rogue star. Furthermore, its orbit would be so ghastly modified that it would either be shot out of the solar system into interstellar medium or fall into the parent star.
However, if we replace the hypervelocity star with a hypervelocity black hole, we would solve the problem of the high energy radiation (provided that the interplanetary medium of the target solar system has no asteroid belts to feed the black hole).
If such a fasting hypervelocity black hole happens to pass at the right distance (I don't know that distance at the moment, but it can be calculated), at the right angle, at the right time of the year, it can create the desired changes to the planet in question. However, the immensity of the change would almost definitely wipe out all life from the planet. Unicellular life and some very primitive, benthic organisms might survive, but what for the major tectonic activity and what for the volcanic activity, most of the land-based life would be extinct within the first year.
This works the same way as a hypervelocity star, but at a lesser horrific scale. A large moon, when coming at the right angle and speed, can be gravitationally captured by the parent star. If the moon enters in an unstable orbit around the planet, it can either crash into the planet, or leave it forever and be shot out at high speeds (read this article for further detail). We would rather the second scenario where the moon shoots out of its orbit after putting extensive gravitational impact on the parent planet.
This too, will probably wipe out all complex, evolved life from the face of the planet, specially land-based life. Deep water creatures have a high chance of surviving.
Why not just let your planet slow down naturally? Who says that your story has to be set 443 million years after the first evolving fish crawled onto dry land. Why not set your story a couple billion years later, when, after innumerable ages and extinctions, an intelligent species has arisen.
The orbital mechanics would be a nightmare, but what about a rogue planetoid. Flies by planet in question, alters solar orbit and rotation. Probably need to have them orbit one another for a few years to slow the rotational period.
Then when it finally flies off again returning the planet to it's regularly scheduled life zone, minus lots of rotational speed. Two co-rotating planets in the same solar orbit are not going to have a good supportive biosphere while they are looping around, but once the interloper leaves things should settle down.
This gets a bit complicated. When you say the day is nine years, I presume what you actually mean is that the time between sunrises is nine Earth years. Is that correct? How long does this planet take to orbit its star?
If the orbit is more than nine Earth years, then the solution is simple: make the planet quite old. The rotation slows naturally, due to tides dissipating energy (this progresses faster with a large moon) but it hasn't reached tide-lock yet.
If the orbit is less than nine Earth years, your planet has a weird rotation like Venus. Getting there can't be done by tide-lock, you need a large impact, or a massive body flying by much too close. None of these kinds of events make it easy for life to survive.
