What would happen on a binary planet if they captured a new moon into orbit?

Question

I've been working on a world where the origin of the magic system is a catastrophic event that ripped the surface of the planet apart allowing magical energy from beneath the crust to escape to the surface, but minor enough to allow pockets of life to survive. I've been slogging through research the best I can and have come up with a handful of options, but the most appealing one at the moment is that this catastrophe occured because the planet captured a new moon. However, I've been struggling to figure out what this process would look like, and how I can justify my civilization surviving. I thought that perhaps if this was a binary planet instead of a single planet the competing gravity of the two bodies against the third might help to sheild the habitable planet from becoming completely uninhabitable, as well as slow the new body down enough to be captured in a stable orbit. I'm no scientist though and I'm struggling to figure out how this could work

Habitability: In my mind Planet A was habitable before the event and home to an advanced human race. The scientists on Planet A were able to predict the event and took several actions to try surviving it, including underground bunkers, technologically shielded cities and a group that went to hide on Planet B (or maybe another smaller moon) to wait out the destruction of Planet A. Exactly how destructive would this be for both planets? Is it safe to assume that if Planet A and Planet B are tidally locked the faces locked towards each would be the most stable?

Timeframe: How long should I expect it to take for Planet A to stabilize and become more habitable? Should I be thinking of this in terms of hundreds of years, or more like 100,000?

I honestly have a million more questions but I don't even know where to start haha. Again, I'm no scientist, but I hope this is enough to at least give you an idea of where I'm trying to go.

Answer 1

Start with a moon in a horseshoe orbit.

Perhaps the moon was once stably trapped at L4 or L5, but this isn't necessary. One way or the other it got into a less and less stable orbit until it was in a horseshoe orbit, which involves periodic close approaches to the massive planet.

Only in this case it is two planets, in close orbit around each other. When the period of the horseshoe orbit came into resonance with the period of the planets, it was nudged further and further, until it joined in with them at close range.

Note that this approach puts the moon into orbit absolutely as gently as possible, but some small vagary of a three-body problem easily allows for as bumpy of a landing as you need. I assume you don't have to cover the planet in lava flows - just enough disruption to create volcanic conduits from its "magical" (perhaps artificially built) interior.

Making the moon stick the landing is the least credible part - after all, as a rule, things leave at about the rate they came in. It could plausibly end up in an elliptical orbit that is just barely not a horseshoe orbit at first. Maybe there's a way to simulate an orbital resonance that explains how the moon went into an eccentric orbit around the two planets and had that dampened with every pass until it ended up at an L5-like point (one between the two planets rather than between the two-planet system and the sun). I think you'd actually have to write and run a simulation for a few million encounters to get that trajectory - I have no idea.

Answer 2

I feel you need to find a different basis for your cataclysm. Good thing that there are lots.

The tidal effect of object (comet, asteroid etc) large enough to survive to eventually form a moon on a planet large enough to capture it would be devastating. We are talking dinosaur extinction event level.

The invading object would have to be large enough that its gravity would be able to keep in one piece through the event, but nothing on its surface would survive.

The planet's surface would be pommeled by the debris field that would come with the invader, in addition to earthquakes that would split the crust. Shock waves would travel at the speed of sound through the planet, disrupting any existing tectonic plates. There would be loss of atmosphere, huge solar storms, cosmic radiation would penetrate the ozone layer (if there was one) etc etc.

If the slim odds of such a near miss event resulted in a binary object like our Earth and its moon it would be millions of years before stability was completely restored. And many more millions before life would redevelop.

All that said .. a really heavy meteor strike would get you what you need .. and the core of the meteor could be the magic material that you need for your story .. or the blast of its impact could open a hole deep enough that the "magic layer" inside the planet would be revealed.

OR tectonic activity could lead to a volcanic eruption that could lift magic material from deep in the crust to the surface, and then renew the MM deposits every so many thousands of years .. reference the formation of the Hawaiian island chain

hope this helps