4 foot diameter black hole comes to rest in Kuiper belt - then what?

[eggchess]

I am a writer completing a science fiction novel involving a four foot diameter black hole (with approx 1.5 times the mass of Saturn) At one point in the story, this small black hole has sling shot past the sun, is headed outward toward the Kuiper belt & Oort Cloud. As it moves, the sun's gravity acts as a drag on it, slowing it's speed more and more. As it reaches the outer limits of the Kuiper Belt, the black hole slows to a stop. My question is how would it then behave? Would it be pulled straight back to the sun, into the sun? Would it assume the helix motion, the same the planets inscribe, and follow along as a new object in that region? Might it just get left behind? My thanks for any help you can provide.

I don't have a significant background in physics or math training past trigonometry. I have no equations.

I'd only be guessing. But I'd guess the black hole would assume some kind of helical motion and maybe move back toward the sun. Not sure.

 

[Vanadium 50]

As it reaches the outer limits of the Kuiper Belt, the black hole slows to a stop. My question is how would it then behave? Would it be pulled straight back to the sun, into the sun?

Yes.

It sounds, though, like you are going about this backwards. You have decided that your story involves a 4 foot black hole and are trying to figure out what it does. Doesn't it make more sense to decide what you want it to do to advance your story and then to then figure out how big it needs to be (or even whether it is a black hole at all) to do that?

 

[pbuk]

As it moves, the sun's gravity acts as a drag on it, slowing it's speed more and more.

But as it moved in from wherever it started, the Sun's gravity acted to accelerate it just as much (ignoring any increase in mass which we have already established is tiny).

As it reaches the outer limits of the Kuiper Belt, the black hole slows to a stop.

There is no reason it should slow to a stop given the above.

Would it assume the helix motion, the same the planets inscribe, and follow along as a new object in that region?

You do realise that when you read about planets following a helical path, that is intended to be relative to our Milky Way galaxy (and it is not really a helix)? Relative to the Oort Cloud anything orbiting the Sun follows an elliptical path.

Might it just get left behind?

No. Newton's second law does not 'leave things behind'.

If you want to write a believable science fiction story you need to learn some science. Answers to individual questions won't help because without some background understanding you don't know what questions to ask.

 

[DaveC426913]

this small black hole has sling shot past the sun, is headed outward toward the Kuiper belt & Oort Cloud. As it moves, the sun's gravity acts as a drag on it, slowing it's speed more and more. As it reaches the outer limits of the Kuiper Belt, the black hole slows to a stop.

Celestial trajectories don't work this way, and your readership will know it.

• If the BH came from outside the solar system, it will have enough velocity to escape the system again - no coming to a stop without some majorly contrived interventions. The details of - and exceptions to - this are too involved for a single post. As others have said, you've got some homework to do.
• As the shortest "get it done" scenario or your story, I'd suggest that the BH would have to interact with a major mass in order to be captured - Saturn itself, or better yet, Jupiter. It will send Saturn/Jupiter flying, possibly into a long elliptical orbit, possibly out of the solar system altogether.
• This has to happen because the total momentum of the solar system / incoming back hole must be conserved. Whatever energy the BH loses in order to be captured must be gained somewhere else in the solar system (but not the sun).
• Even at that though, the BH won't end up staying in the KB in any kind of circular orbit. At best, it will have a highly elliptical orbit with its aphelion in the KB and its perihelion likely at Jupiter's former (now vacant) orbital distance. To try to contrive it into a circular orbit would require a second major interaction with another body in the KB - which is reeeally stretching believability.
• BTW, it doesn't have to have its orbital plane aligned with the planets. It could be in a polar orbit, which would limit the chaos it would otherwise shower upon the stability of the rest of the system.

I wish I could stick around to see how this thread evolves but I'm AFK for a full week.

 

[DaveC426913]

No it won’t. It is significantly lighter than both.

Per the OP:

approx 1.5 times the mass of Saturn

The mass as specified by the OP is the significant property we'll be working with. Physical diameter - be it 4 feet or 4000 feet - is incidental and should be derived from the mass.

 

[eggchess]

Invaluable information that helps me finish this saga. Thanks to all respondents. You've all been generous.
Thank you. Ed G