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Comment by maaaaattttt | Hacker News Reader

If the star is close enough I have a hard time imagining what is happening to its orbiting planets.

Did they "de-orbit" during the creation of the black hole? Or do the planets orbit the black hole as they were already orbiting the original star before that?

Does that mean you can have a mix of stars and planets orbiting another star? Could it even be possible that a star has planets and a star orbiting it AND the star orbiting it also has it's set of planets orbiting as well?

And what happens to the time on the planets once they orbit closer to the black hole?

So many (and apologies in advance if stupid) questions!

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> Did they "de-orbit" during the creation of the black hole? Or do the planets orbit the black hole as they were already orbiting the original star before that?

If the mass wouldn't change the orbit wouldn't chance much, they're still orbiting "same" mass.

But in most cases (AFAIK) creation of black hole involves supernova so, well, that ain't gonna be very healthy for the planet itself (lmao) and part of the mass would get ejected and orbits would get more elliptical

Death by neutrinos! https://what-if.xkcd.com/73/

We have moons orbiting our planets orbiting our sun orbiting around the center of the Milky Way orbiting a larger galaxy… turtles all the way down.

Let's assume the Star A and the Black Hole B hole orbited each other when the black hole was Star B. If Star B collapsed into a black hole of roughly the same mass, I don't think Star A & its planets would be affected much at all in terms or orbits. Maybe the Star A system would have been fried when Star B went supernova, but the orbits I don't think would change all that much. It's like how people say Earth would continue orbiting the center of our solar system if our sun turned into a black hole, as the center of gravity would be the same.

Someone try this w/ Universe Sandbox & report back please :) => https://universesandbox.com/

"Star A" is nearly the mass of our sun.

When our sun exhausts the hydrogen in the core, it will swell as a red giant, encompass the orbits of Mercury and Venus, and likely come very close to earth.

"Black hole B" is currently ten times the mass of our sun. The star that produced it was likely much more massive still. The red giant phase of such a star might have reached to Neptune, so there is no way that these two evolved together.

When the star in this pair exhausts its hydrogen, the black hole will drain the red giant that it becomes.

> When the star in this pair exhausts its hydrogen, the black hole will drain the red giant that it becomes.

I wonder if the accreting matter might undergo fusion.

Disclaimer: I can't make any calculation but... Wouldn't Star A keep orbiting Star B inside the surface of Red Giant Star B, which wouldn't be very dense at the radius of the orbit? There is drag but a lot of inertia. Star A would start from further away than now. It would probably get some extra mass in the process, absorbing the gas of the red giant.

Here is an interesting variant on this concept.

https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_obj...

The drag will lead to orbital decay.

Thanks. The time frame is only hundreds of years.

They're not sure how the star came to be around the blackhole but it likely wasn't around when the blackhole was formed.

The mass of a star required to form the black hole would have swallowed this smaller star in it's formation.

Either way, it's challenging the current conceptions of how binary systems form/work.

I don't know the composition of this system or how feasible this arrangement would be given however the system may have evolved, but any planets could co-orbit the star and the black hole around their combined center of mass. They'd probably have to be relatively far away from both to be stable.

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